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Concept of a Soil

The term soil as been derived from Latin word of solum which means ground or earth material in which plants grow.

Definition of Soil.

Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life on the Earth.  OR

Soil is the thin surface layer of the Earth, known as regolith which formed by break down of the rocks in various ways and various processes. Its nature depends on parent rock from which it develops.

The scientific study of a soil, its origin and characteristics is called pedology and the person who studies a soil called pedologists.

Pedology is the scientific study of a soil, its origin and characteristics.

AGENTS OF SOIL EROSION

a. Water

This is the most important agent of soil erosion

Erosion by water involves:

Splash erosion caused by rain drops

Sheet erosion which involves the removal of the maximum cover of soil by surface water

Rill erosion which leads to the formation of small channels called rills on the surface

Gully erosion that leads to the formation of deep troughs called gullies due to severe under cutting river erosion that takes place in the specific channels called river valleys.

b. Wind

This is another agent of soil erosion. It takes place in arid and semi-arid areas or where the soil is loose

c. Ice

It is also another agent of soil erosion. It takes place in cold areas where there is the formation of ice

d.Gravity

 This leads to the gradual movement of weathered material down the slope without involving transporting agents.

IMPORTANCE OF A SOIL

• It’s a medium for plant growth.

Human being/organisms are dependent on plant to fulfill his appetite and absorb oxygen which is the lifeline of the human body. So it is implicit that without soil it will be tough for human being/organisms to survive.

• Helps rain water to get drain/infiltrate and thus forming a ‘AQUIFER’ (source of drinking water apart from lakes) and also helps to recharge underground water.
• Building materials soil is used directly in making of bricks, tiles and white wash, The materials are the used in building of houses, bridges and other structures.
• Plays vital role in maintaining phosphorous cycle.

Phosphorous cycle is the process by which phosphorous moves through the lithosphere, hydrosphere, and biosphere. Phosphorus is essential for plant and animal growth, as well as the health of microbes inhabiting the soil, but is gradually depleted from the soil over time.

• It is one of the major sources of almost all the minerals and precious rocks apart from the ocean.
• It acts as Habitat for organisms such as burrowing rodent, earthworms and termite.

Around 25% of everything alive on the Earth uses soil as a habitat. Some animals live on top of the soil (in leaf litter or other organic matter), and others live below the surface. Some things live in the soil for their entire lives, and others live there for just a part of their lives.

• Soil is the sources of food, air and water for plants.

SOILS AND ROCKS

Rocks and soil are all around us, in all shapes and sizes, in all colors and forms. The earth’s crust is made primarily up of these two things which were formed from inside the earth. Rocks eventually break down to become soil.

QN.

Differentiate Soils from Rocks.

ANSWER

Rocks are naturally occurring materials which composed of one of more minerals while Soils is a mixture of one or more rocks materials, mixture of minerals, gases, liquids etc.

SOIL COMPONENTS.

The soil is made up of the following main components:

a. Mineral matters

b. Organic matters

c. Water

d. Air

A. Mineral matters

The mineral constituents of the soil are derived from the parental rocks or regolith. They may be found in the form of particles of different sizes; from clay (0.002 mm or less in diameter) to large pebbles and gravels. The minerals represent about 45% of the total components of the soil. Important elements which are found in compound state are Oxygen, Si, Fe, Al, N, P, K, Ca, Mg, C, H, etc. In soil, nitrogen comes from atmosphere in the form of nitrogen salts.

Pedologists categorized soils into different type according to the following diameter size

• Gravel 20mm-200m.
• Fine gravel 02mm-20mm.
• Course sandy 0.2mm-02mm.
• Fine sandy 0.2mm-0.02mm.
• Clay less than 0.002mm.

B. Organic matters

Organic matter derived either from dead remains of plants and animals or through metabolic activities of living organisms are present in the soil. When the plants and animals die, their dead remains are acted upon by a number of microorganisms and are finally degraded or decomposed into simple organic compounds. A product of this microbial decomposition is humus which is a dark coloured, jelly-like amorphous substance composed of residual organic matters not readily decomposed by soil microorganisms. The process of humus formation is called humification.

Organic matter forms 5% of the total components of the soil.

Humus percentage in the soil is affected by climatic and biological factors. It is less in arid soils and very high in humid soils. In the top layer of the soil, humus quantity is greater than in the deep layers.

In dark humid areas which are thickly covered with vegetation, the humus may be found in the following three stages of degradation:

• The top floor is covered with dead organic parts showing low degree of decomposition. These poorly decayed dead parts of plants form litter.
• Below the litter may be found a layer of partially decomposed organic matter which is known as duff layer.
• When the duff is decomposed completely into organic substances, the decomposition products, generally called leaf moulds, are accumulated below duff layer.

Sometimes under anaerobic conditions, the dead remains are not at all acted upon by the microorganisms. Accumulation of such un-decomposed organic remains is termed as peat.

Humus plays many important roles in the soil, such as:

• It makes the soil fertile.
• It provides nutrients to the plants and microorganisms.
• On complete decomposition, it forms several organic acids which serve as solvents for soil materials. Thus humus increases the availability of minerals in dissolved state to plants.
• Because it is porous, it has got high capacity for retaining water.
• Humus makes the soil porous, thus increases the aeration and percolation which make the soil more suitable for the plant growth.
• It also acts as weak cement thus binds the sand particles.
• Presence of humus in the soil increases the rate of absorption in plants.

The factors which influence the rate of humifications are outlined below:

• Nature of plants, animals or soil organisms.
• Rate of decomposition.
• Temperature (increase in temperature up to a certain limit increases the rate of humification).
• Aeration and moisture.

These increase the rate of humification.

Soil organisms

Organisms present in the soils are called soil organisms. Soil organisms categorized into micro and macro organisms, micro organisms are organisms that cannot  seen by naked eyes unless microscope is used example Protozoa while macro organisms are organisms that can be seen by naked eyes example worms.

Important group of soil organisms are

Soil flora	Soil fauna
BacteriaSoil FungiAntinomycetesAlgaeRoot, rhizoid and rhizomes of higher plants.	ProtozoaNematodes Insects and mites Rodents and earthwormsBurrowing vertebrates

QN

What are the importance of Organic matter and living organism in the soil?

ANSWER

1. Organic matter influence soil moisture by retaining water in the soil.
2. Influence various physical, chemical and biological processes taking place in the soil body.
3. Organic matters help the process of soil aggregation to bind soil particles together.
4. Organic matters reduce the plasticity of the soil.
5. It adds more plants nutrients to a soil body released from tissues of died plants.
6. The remains of died organism provide good habitable environment.
7. Organic matter help to regulate the chemical reaction of the soil through the released of minerals from their broken tissues.

C. Soil Water

Soil water plays very important role in the plant growth. Plants absorb a small quantity of ram water and dew directly from their surfaces but most of water absorbed by them comes from the soil. Soil water maintains the soil texture, arrangement and compactness of soil particles. It is good solvent for minerals and it makes the concentration of nutrients low so that nutrients may be absorbed by plants easily. Amount of water present in the soil is of 25% of the total components of the soil.

Water in the soil comes mainly through infiltration of precipitated water (rain, sleet, snow and hail) and irrigation whereas it is lost from the soil chiefly through evaporation, percolation stream flow and transpiration. ‘The quantity of water available in the soil varies from place to place. The amount also depends upon the quality of soil. In loamy, silty and clay soils, the amount of water is greater than that in coarse sandy soil.

Water is held in the soil in the following forms:

• Gravitation water

After complete water saturation of soils the excess water displaces air from the pore spaces between soil particles and percolates downwardly under gravitation influence and finally it is accumulated in the pore spaces. This excess water is called gravitational water. The amount of water held in the soil, when all pores are filled and when drainage is restricted is maximum water holding capacity.

When the gravitational water percolates down and reaches to the level of parental rock it is called ground water.

• Capillary water

The amount of water present around the soil particles at saturation stage, when gravitational water has drained away through capillaries or channels, is called capillary capacity or field capacity and the water which is held by surface tension and attraction force of water molecules as thin film around soil particles in the capillary spaces is called capillary water. It moves in the direction where capillary tension is more.

• Hygroscopic water

Water which is adsorbed on the soil particles and held on the surface of soil particles by forces of attraction and cohesion of its molecules is called hygroscopic water.

• Water vapour

This is the water present in the soil atmosphere in the vapour form.

• Combined water

It is water of chemical compounds held by chemical forces of molecules (as for example, CuSO4.5H2O). It can be driven out from the compounds only at bright red heating.

Water requirement of plants varies from individual to individual. Some absorb large quantity, while some others require very small quantities of water for their normal growth. A major fraction of total water absorbed from the soil is transpired by the plants and only a small quantity of it enters the composition of protoplast.

QN

What are the importances of water in the soil body?

ANSWER

1. Water act as a solvent of various minerals in the soil body.
2. It fastens the process of weathering.
3.  It is used by plants to manufacture their own food by process called photosynthesis.
4. Water regulates soil temperature.
5. It needed to the activities of soil organisms to decompose the remains of died organisms.

D. Soil Air

Soil air is the gases occupying the free pore space in soil. The spaces between soil particles and soil organisms are called pore spaces. These are filled with moisture and air in varying quantities which account for approximately 25% of soil. In dry soils, percentage of moisture is lesser than that in wet soils.

Composition of Soil Air

The soil atmosphere contains three main gases, namely oxygen, carbon dioxide and nitrogen. In soil atmosphere, oxygen is 20.25%, nitrogen is approximately 79.5% and carbon dioxide is 0.25% by volume. In the cultivated land, percentage of CO2 is much higher than that of atmospheric CO2, but oxygen content in such soil is poorer than the percentage of oxygen in atmospheric air.

Factors Affecting the Composition of Soil Air are

Nature and condition of soil

The quantity of oxygen in soil air is less than that in atmospheric air. The amount of oxygen also depends upon the soil depth. The oxygen content of the air in lower layer is usually less than that of the surface soil. This is possibly due to more readily diffusion of the oxygen from the atmosphere into the surface soil than in the subsoil. Light texture soil or sandy soil contains much higher percentage than heavy soil. The concentration of CO2 is usually greater in subsoil probably due to more sluggish aeration in lower layer than in the surface soil.

Microbial activity

The microorganisms in soil require oxygen for respiration and they take it from the soil air and thus deplete its concentration in the soil air. Decomposition of organic matter produces CO2 because of increased microbial activity. Hence, soils rich in organic matter contain higher percentage of CO2.

Seasonal variation

The quantity of oxygen is usually higher in dry season than during the monsoon. Because soils are normally drier during the summer months, opportunity for gaseous exchange is greater during this period. This results in relatively high O2 and low CO2 levels. Temperature also influences the CO2 content in the soil air. High temperature during summer season encourages microorganism activity which results in higher production of CO2.

Type of crop

Plants roots require oxygen, which they take from the soil air and deplete the concentration of oxygen in the soil air. Soils on which crops are grown contain more CO2 than fallow lands. The amount of CO2 is usually much greater near the roots of plants than further away. It may be due to respiration by roots.

QN

What are the importances of Air in the soil?

ANSWER

1. Air in the soil used by plants to manufacture their own food by process called photosynthesis.
2. Air in the soil used by living organisms for respiration.

SOIL FORMATION

All soils initially come from rocks; this is termed the ‘parent material’. The Parent Material may be directly below the soil, or great distances away if wind, water or glaciers have transported the soil.

 Soil formation is also dependent upon other prevailing processes affecting soil formation. The soil formation process is termed ‘pedogenesis’. Climatic conditions are important factors affecting both the form and rate of physical and chemical weathering of the parent material.

The formation of soils can be seen as a combination of the products of weathering, of structural development of the soil, of differentiation of that structure into horizons or layers, and lastly of its movement or translocation. In fact there are many ways in which soil may be transported away from the location where it was first formed.

Processes of Soil formation

Processes of Soil formation categorized

a. Simple processes and

b. Complex processes

A. Simple processes

Simple processes are process which organize on their own and play a particular function in soil formation.

Simple processes of Soil formation are as follow

i. Weathering

ii. Leaching

iii. Huminification

iv. Organic sorting

v. Mineralization

vi. Cheluviation

vii. Capillary action.

I. Weathering

Weathering is the name given to the process by which rocks are broken down to form soils. Rocks and geological sediments are the main parent materials of soils (the materials from which soils have formed). There is a very wide variety of rocks in the world, some acidic, some alkaline, some coarse-textured like sands, and some fine-textured and clayey. It is from the rocks and sediments that soils inherit their particular texture. When you see rocks in the landscape it is easy to appreciate how long the process of breaking down rocks to form soil takes. In fact, it can take over 500 years to form just one centimetre of soil from some of the harder rocks. Fortunately, in some respects at least, huge amounts of rocks were broken down during the Ice Age over 10,000 years ago and converted into clays, sands or gravels, from which state it was easier to form soils.

There are three main types of weathering namely;

• Physical weathering
• Chemical weathering and
• Biological weathering

Physical weathering

Physical weathering is the influence of processes such as freezing and thawing, wetting and drying, and shrinking and swelling on rocks and other sediments, leading to their breakdown into finer and finer particles of the soil.

Chemical weathering

Chemical weathering is the decomposition of rocks through a series of chemical processes such as acidification, dissolution and oxidation. Some minerals, while stable within solid rock, become less stable on being more exposed to the atmosphere and so begin to alter in the rocks near the surface, destabilizing the rocks.

Biological weathering

Biological weathering is the effect of living organisms on the breakdown of rock. This involves, for example, the effects of plant roots and soil organisms. Respiration of carbon dioxide by plant roots can lead to the formation of carbonic acid which can chemically attack rocks and sediments and help to turn them into soils.

II. Leaching

Leaching, refer to the loss of soluble substances and colloids from the top layer of soil by percolating precipitation. The materials lost are carried downward (eluviated) and are generally re deposited (illuviated) in a lower layer. This transport results in a porous and open top layer and a dense, compact lower layer. The rate of leaching increases with the amount of rainfall, high temperatures, and the removal of protective vegetation. In areas of extensive leaching, many plant nutrients are lost, leaving quartz and hydroxides of iron, manganese, and aluminum. This remainder forms a distinctive type of soil, called laterite, or latosol, and may result in deposits of bauxite. In such areas rapid bacterial action results in the absence of humus in the soil, because fallen plant material is completely oxidized and the products are leached away. Accumulations of residual minerals and of those re deposited in lower layers may coalesce to form continuous, tough, impermeable layers called duricrusts.

III. Huminification

Huminification is the process whereby fresh organic material is converted to more stable humus. This normally takes place under cool, moist climatic conditions and gives rise to horizons with high humus content. The soil is well-drained, as under water-saturated conditions, accumulation of non-humidified soil will take place by preference.

Accumulation of organic material takes place under poorly drained and/or cool (almost cold) conditions.. Under these conditions, microbe activity is slow and less organic material is humidified than that which was added and therefore it accumulates. Accumulation of organic material takes place faster in wetter climatic regions, while humidification is predominant in cooler climatic regions and acidic soils. Accumulation may also take place in extremely acidic soils. Under these conditions, there are insufficient basic nutritional materials for the micro-organisms and therefore the organic material is not oxidized.

IV. Organic sorting

This is separation of materials, usually of different sizes, through organic influence. It involves changes of sizes of soil particles, enrichment of the soil with organic matters such as humus and movement of mineral elements in solution that is leaching.

V. Mineralization

Mineralization is the decomposition or oxidation of the chemical compounds in organic matter releasing the nutrients contained in those compounds into soluble inorganic forms that may be plant-accessible.

The mineralization process results in increased bioavailability of the nutrients that were contained in the organic compounds being decomposed, most notably (because of their quantities) nitrogen, phosphorus and sulphur. Whether the decomposition of an organic compound will result in mineralization is dependent on its concentration relative to that of carbon. If the concentration of the element in question exceeds the needs of the decomposer for biosynthesis or storage it will be mineralized.

VI. Cheluviation

This is downward movement of materials in the soil which is very similar to leaching. However cheluviation occur through the influence of organic agents which are also referred to as chelating agents. The process involves plant acids rather than mere water as the case with leaching.

VII. Capillary action

It is an upward movement of water to a surface and may cause some materials deposited to form layers depending on the nature of parent rocks from which the solution has been derived.

B. Complex processes

Complex processes are processes which involves the combination of different activities in the soil formation.

Complex processes of Soil formation are as follow

i. Podzolization

ii. Laterization

iii. Calcification

iv. Salinization and

v. Gleization

I. Podzolization

It is a process of soil formation resulting in the formation of Podzols and Podzolic soils. In many respects, podzolization is the negative of calcification. The calcification process tends to concentrate calcium in the lower part of the B horizon, whereas podzolization leaches the entire solum of calcium carbonates.

Apart from calcium, the other bases are also removed and the whole soil becomes distinctly acidic. In fact, the process is essentially one of acid leaching. The process operates under favorable combination of the following environments.

• Climate: A cold and humid climate is most favorable for podzolization

• Parent material: Siliceous (Sandy) material, having poor reserves of weather able minerals, favor the operation of podzolization as it helps in easy percolation of water.

• Vegetation: Acid producing vegetation such as coniferous pines is essential

• Leaching and Translocation of Sesquioxide:

In the process of decomposition of organic matter various organic acids are produced. The organic acids thus formed act with Sesquioxide and the remaining clay minerals, forming organic- Sesquioxide and organic clay complexes, which are soluble and move with the percolating water to the lower horizons (Bh, Bs). Aluminium ions in a water solution hydrolyze and make the soil solution very acidic.

 2Al +6H2O à2 Al(OH)3 + 6H+

As iron and aluminium move about, the A horizon gives a bleached grey or ashy appearance. The Russians used the term Podzols (pod means under the Zola means ash like i.e. ash-like horizon appearing beneath the surface horizon) for such soils.

II. Laterization

The term laterite is derived from the word later meaning brick or tile and was originally applied to a group of high clay Indian soils found in Malabar hills of Kerala, Tamil Nadu, Karnataka and Maharashtra.

It refers specifically to a particular cemented horizon in certain soils which when dried, become very hard, like a brick. Such soils (in tropics) when massively impregnated with Sesquioxide (iron and aluminium oxides) to extent of 70 to 80 per cent of the total mass, are called laterite or latosols (Oxisols). The soil forming process is called Laterization or Latozation.

Laterization is the process that removes silica, instead of sesquioxides from the upper layers and thereby leaving sesquioxides to concentrate in the solum. The process operates under the following conditions.

• Climate: Unlike podzolization, the process of laterization operates most favorable in warm and humid (tropical) climate with 2000 to 2500 mm rainfall and continuous high temperature (25°C) throughout the year.

• Natural vegetation: The rain forests of tropical areas are favorable for the process.

• Parent Material: Basic parent materials, having sufficient iron bearing ferromagnesian minerals (Pyroxene, amphiboles, biotite and chlorite), which on weathering release iron, are congenial for the development of laterites.

III.Calcification

It is the process of precipitation and accumulation of calcium carbonate (CaCO3) in some part of the profile. The accumulation of CaCO3 may result in the development of a calcic horizon.  Calcium is readily soluble in acid soil water and/or when CO2 concentration is high in root zone as:

CO2 + H2O –> H2CO3

 H2CO3 + Ca –> Ca (HCO3)2 (soluble)

                            Temp.

 Ca (HCO3)2 –> CaCO3 + H2O + CO2 (precipitates)

                  CO2

 The process of precipitation after mobilization under these conditions is called calcification and the resulting illuviated horizon of carbonates.

IV. Salinization

It is the process of accumulation of salts, such as sulphates and chlorides of calcium, magnesium, sodium and potassium, in soils in the form of a salty (salic) horizon. It is quite common in arid and semi arid regions. It may also take place through capillary rise of saline ground water and by inundation with seawater in marine and coastal soils. Salt accumulation may also result from irrigation or seepage in areas of impeded drainage.

. Gleization

The term glei is of Russian origin means blue, grey or green clay.

 The Gleization is a process of soil formation resulting in the development of a glei (or gley horizon) in the lower part of the soil profile above the parent material due to poor drainage condition (lack of oxygen) and where waterlogged conditions prevail. Such soils are called hydro orphic soils.

The process is not particularly dependent on climate (high rainfall as in humid regions) but often on drainage conditions.

The poor drainage conditions result from:

• Lower topographic position, such as depression land, where water stands continuously at or close to the surface.
• Impervious soil parent material, and.
• Lack of aeration.

Under such conditions, iron compounds are reduced to soluble ferrous forms. The reduction of iron is primarily biological and requires both organic matter and microorganisms capable of respiring an aerobically. The solubility of Ca, Mg, Fe, and Mn is increased and most of the iron exists as Fe++ organo complexes in solution or as mixed precipitate of ferric and ferrous hydroxides.

This is responsible for the production of typical bluish to grayish horizon with mottling of yellow and or reddish brown colors.

FACTORS THAT INFLUENCE SOIL FORMATION

Soils form from the interplay of five main factors namely

a. Parent material

b. Time

c. Climate

d. Relief  

e. Organisms.

And summarized as follow

Soil = f(PCROT) Where by

f=Function

P= Parent material

C = Climate

R= Relief  

O= Organisms

T=Time

A. Parent material

This refers to the mineral material, or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material.

B. Time

 Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually they may change from one soil type to another.

C. Climate

 This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. Warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area.

D. Relief

This refers to the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile.

E. Organisms

 The source and richness of organic matter is down to the living things (plants and animals) that live on and in the soils. Plants in particular, provide lots of vegetative residue that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help breakdown organic matter and aid decomposition. Animal droppings, dead insects and animals result in more decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

SOIL CLASSIFICATION

Soil classification is a way of describing a given plot of soil. We classify soil according to

a. According to textures and

b. According to orders

A. According to textures

We classify soil according to textures as shown below

Sandy Soils

Sandy soils are free draining, with the largest, but fine and hard particles. It has a gritty feel. It does not bind very well. It is poor in holding water and easily warms up in the spring season. Sandy soils are very low in nutrients, as they are usually washed away. Its degree of aeration depends on the sizes of the particles, which vary a lot in size.

 It is usually formed from the weathering or disintegration of bedrock such as shale, limestone, granite and quartz.

Silty Soils

 This kind is finer, smoother in texture and hold water better than sandy soils. It also holds up nutrients and makes it better for crop cultivation. Silty soils are heavier than sandy soils, and almost midway between the properties of sandy and clay soils.

 It is formed when fine sediments (dust, organic matter and debris) are carried by water or ice and deposited. When silt is deposited and cemented with time, it forms siltstone. Silt particles are so small and not easily seen by the eyes. It leaves a bit of residue after you touch them.

Clay Soils

 The particles that make up clay are the finest and they bind very well. It has very little air spaces. Clay very sticky when wet, and can be molded into any shape and form. When they dry, they are rock hard. Clay soils do not drain very well. Clay is believed to form in places where rock is in contact with water, air or steam. Example, sediments on sea or lake bottoms may become clay soils with time.

Loamy Soils

 This soil is a mixture of sand, clay and silt particles and has the ability to retain water. It is high in calcium, aeration and ideal for most crops and vegetables. It is the soil all farmers dream of, as it is full of nutrients from decomposed organic material. It is soft and easy to cultivate.

Peaty Soils

 Peaty soils are acidic and as a result, do not support decomposition very well. It is dark in color, rich in organic material, although contains less nutrients than loamy soils. It retains water very well.

Chalky Soils

Chalky soils are alkaline with a pH of about 7.5. It is not acidic and often stony with chalk or limestone bedrock. It is free draining because of its coarse and stony nature. Not the best for crops to grow in as they lack manganese and iron.

B. Accordingto orders

We classify soil according to orders as shown below

• Zonal soils
• Intrazonal soils and
• Azonal soils.

Zonal soils

Zonal Soils are formed on normal sites from ordinary siliceous rocks and show clearly the impress of climate and vegetation. In short, these are formed under conditions of good soil drainage through the prolonged action of climate and vegetation, e.g. chestnut soils.

This kind of soils have following types –

• Tundra Soils
• Podzols
• Brown Forest Soils
• Laterite Soils/Latosols/Ferralsols
• Chernozem/Prairie/Steppe
• Grumusol/Reddish Brown Soils
• Desert (Seirozems and Red Desert) Soils

Intrazonal Soils

The intrazonal soils include the soils from less common parent materials and those influenced by high ground water or  under conditions of very poor drainage (such as in bogs, flood-plain meadows, or in the playa lake basins of the deserts) or upon limestone. Depending on the role played by water, presence of calcium in the parent material and the location.

intra-zonal soils may be –

• Hydromorphic
• Calcimorphic
• Halomorphic

Azonal Soils

The azonal soils are youthful, owing to recent renewal by sedimentation or erosion. This has no well-developed profile characteristics. These soils are common where the parent material is being continuously eroded and deposited, e.g. alluvial Soils (newer or younger Khadar and older Bhangar soils) or lithosols (those at high altitudes on resistant parent material).

These soils have poorly developed horizons due to three reasons:

• Lack of Time For instance, in new flood plains alluvium is being continually eroded and deposited.
• Parent Material Azonal soils like ‘regosols’ result from loose sand and loess.
• Geomorphology ‘Lithosols’ result on steep slopes where soil is eroded as soon as it is deposited.

SOIL PROFILE

If one could dig a massive trench (hole), about 50-100 ft vertically downwards into the ground, you will notice that you would have cut through various layers of soil types. A look at the layers from a distance gives one a cross-section view of the ground (beneath the surface) and the kind of soils and rocks it is made up of. This cross section view is called a Soil Profile.

The profile is made up of layers, running parallel to the surface, called Soil Horizons.

Each horizon may be slightly or very different from the other above or below it. Each horizon tells a story about the makeup, age, texture and characteristics of that layer.

Therefore Soil Profile defined as the vertical section of the soil body from its top part to the bottom.

Most soils have six horizons. These are

• O-Horizon
• A- Horizon
• E – Horizon
• B -Horizon
• C- Horizon and
• R-Horizon.

SOIL PROFILE DIAGRAM

The O-Horizon

The O horizon is very common in many surfaces with lots of vegetative cover. It is the layer made up of organic materials such as dead leaves and surface organisms, twigs and fallen trees. It has about 20% organic matter. It is possible to see various levels of decomposition occurring here (minimal, moderately, highly and completely decomposed organic matter). This horizon is often black or dark brown in color, because of its organic content. It is the layer in which the roots of small grass are found.

The A-Horizon

 The A horizon may be seen in the absence of the O horizon, usually known as the topsoil. It is the top layer soils for many grasslands and agricultural lands. Typically, they are made of sand, silt and clay with high amounts of organic matter. This layer is most vulnerable to wind and water erosion. It is also known as the root zone.

The E-Horizon

 The E horizon is usually lighter in color, often below the O and A horizons. It is often rich in nutrients that are leached from the top A and O horizons. It has lower clay content and is common in forested lands or areas with high quality O and A horizons.

The B-Horizon

 The B-horizon has some similarities with the E-horizon. This horizon is formed below the O, A and E horizons and may contain high concentrations of silicate clay, iron, aluminum and carbonates. It is also called the illuviation zone because of the accumulation of minerals. It is the layer in which the roots of big trees end.

The C-Horizon

 The C horizon lacks all the properties of the layers above it. It is mainly made up of broken bedrock and no organic material. It has cemented sediment and geologic material. There is little activity here although additions and losses of soluble materials may occur. The C horizon is also known as saprolite.

The R-Horizon

 The R horizon is bedrock, material, compacted and cemented by the weight of the overlying horizons. It is the unweathered parent material. Rock types found here include granite, basalt and limestone.

QN

What are the importances of Soil profile?

ANSWER

1. The depth of the top soil is important in agriculture because plants normally grow in the top soil.
2. Soil profile influence drainage in the soil.
3. Soil profile influence aeration in the soil body
4. Soil profile determines water holding capacity.
5. Soil profile has ideal influence on soil fertility.

SOIL PROPERTIES

Soil properties are categorized into three groups as follow

a. Physical soil properties

b. Chemical soil properties

c.Biological  soil properties

A. Physical soil properties

Physical soil properties include soil texture, soil structure, soil color, soil temperature, soil porosity, soil density, soil depth.

Soil texture

Soil texture is defined as the feelness or coarseness or fineness of a soil determined by the relative proportional of soil particles of different diameter. The size of soil particles and their spacing determine how much water can flow through the soil. The larger the spacing, or pore size, the greater the infiltration rate. Thus, sandy soils will have high infiltration rates because pore sizes are large and there are no finer materials to block the pores.

Measuring soil texture

Methods used to determine Soil Texture are

a. Texture by feel method and

b. Laboratory method.

A. Texture by feel method

The texture by feel method involves taking a small sample of soil and making a ribbon. A ribbon can be made by taking a ball of soil and pushing the soil between your thumb and forefinger, squeezing it upward into a ribbon. Allow the ribbon to emerge and extend over the forefinger, breaking from its own weight. Measuring the length of the ribbon can help determine the amount of clay in the sample. After making a ribbon, excessively wet a small pinch of soil in the palm of your hand and rub in with your forefinger to determine the amount of sand in the sample. Soils that have a high percentage of sand, such as sandy loam, or sandy clay, have a gritty texture. Soils that have a high percentage of silt, such as silty loam or silty clay, feel smooth. Soils that have a high percentage of clay, such as clay loam, have a sticky feel. Although the texture by feel method takes practice, it is a useful way to determine soil texture, especially in the field.

B. Laboratory method

A laboratory determination of soil texture gives a more detailed and reliable measure of the relative amounts of sand, silt and clay particles in a soil. The common term for measuring soil texture in the laboratory is particle size analysis (PSA). Particle size analysis determines particle size distribution (PSD) of a soil and while field texture is closely related to the PSD (McKenzie et al., 2004), texture classes assigned from field texture and PSA are not always equivalent. For example, sodic soils have a heavier field texture than is suggested by the laboratory determined Particle size analysis.

Classification based on field texturing of soils

SOIL	DIAMETER (mm)
Clay   Silt   Fine sand   Course sand   Fine gravel   Gravel	Less than 0.002mm   0.002- 0.02mm   0.02 – 0.2mm   0.2 – 2mm   2-20mm   20-200mm

QN

What are the importances’s of Soil texture?

ANSWER

1. Determine the relative resistance penetration to plants root into the soil.
2. Determine the infiltration rate of water into the soil.
3. Influence the rate of surface water runoff.
4. Influence soil resistance to erosion.
5. Influence soil fertility as it determines the ability of a soil to hold nutrients and water for plants use.
6. Influence other physical soil properties like soil permeability, compaction, structure, porosity, and water retention capacity.

Soil Structure

Soil structure is simply referring to the arrangement of these soil particles into larger aggregates of different sizes and shapes and the pore spaces that are left between them. It is into these spaces that root hairs will grow and extract the water and oxygen from the soil.

Formation of soil structure

Soil particles may be present either as single individual grains or as aggregate i.e. group of particles bound together into granules or compound particles. These granules or compound particles are known as secondary particles. A majority of particles in a sandy or silty soil are present as single individual grains while in clayey soil they are present in granulated condition. The individual particles are usually solid, while the aggregates are not solid but they possess a porous or spongy character. Most soils are mixture of single grain and compound particle. Soils, which predominate with single grains are said to be structure less, while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure.

Organic matter also plays an important role in forming soil aggregates as shown below

• During decomposition, cellulose substances produce a sticky material very much resembling mucus or mucilage. The sticky properly may be due to the presence of humic or humic acid or related compounds produced.
• Certain polysaccharides formed during decomposition.
• Some fungi and bacteria have cementing effect probably due to the presence of slimes and gums on the surface of the living organisms produced as a result of the microbial activity

Classification of Soil Structure

The primary particles sand, silt and clay usually occur grouped together in the form of aggregates.

Natural aggregates are called peds where as clod is an artificially formed soil mass.

Structure is studied in the field under natural conditions and it is described under three categories

• Type – Shape or form and arrangement pattern of peds.
• Class – Size of Peds.
• Grade – Degree of distinctness of peds.

Types of Soil Structure

There are four principal forms of soil structure which are

a. Platy aggregates structure (Plate-like)

b. Prism aggregates structure (Prism-like)

c. Blocky aggregates structure (Block-like)

d. Sphere aggregates structure (Sphere like)

A. Platy aggregates structure (Plate-like)

In this type, the aggregates are arranged in relatively thin horizontal plates or leaflets. The horizontal axis or dimensions are larger than the vertical axis. When the layers are thick they are called platy. When they are thin then it is laminar.

Platy structure is most noticeable in the surface layers of virgin soils but may be present in the subsoil. This type is inherited from the parent material, especially by the action of water or ice.

B. Prism aggregates structure (Prism-like)

The vertical axis is more developed than horizontal, giving a pillar like shape. Vary in length from 1- 10 cm. commonly occur in sub soil horizons of Arid and Semi arid regions. When the tops are rounded, the structure is termed as columnar when the tops are flat / plane, level and clear cut prismatic.

C. Blocky aggregates structure (Block-like)

 All three dimensions are about the same size. The aggregates have been reduced to blocks. Irregularly six faced with their three dimensions more or less equal.

When the faces are flat and distinct and the edges are sharp angular, the structure is named as angular blocky. When the faces and edges are mainly rounded it is called sub angular blocky. These types usually are confined to the sub soil and characteristics have much to do with soil drainage, aeration and root penetration.

D. Sphere aggregates structure (Sphere like)

All rounded aggregates (peds) may be placed in this category. Not exceeding an inch in diameter. These rounded complexes usually loosely arranged and readily separated. When wetted, the intervening spaces generally are not closed so readily by swelling as may be the case with a blocky structural condition.

Therefore in sphere like structure, infiltration, percolation and aeration are not affected by wetting of soil.

 The aggregates of this group are usually termed as granular which are relatively less porous. When the granules are very porous, it is termed as crumb. This is specific to surface soil particularly high in organic matter/ grass land soils.

QN

What are the importances’s of Soil Structure?

ANSWER

1. It has an influence on soil aeration.
2. It has an influence on soil drainage..
3. It has an influence on seed emergence.
4.  It has an influence on plant growth by influencing the roots penetration and water retention.
5. It has an influence on cultivation process.
6. It is a good indicator of soil fertility.

Soil Colour

Soil color is determined by the materials and the mineralogical composition from which the soil is derived and organic matter content. It varies from one place to another. Soil color can be black, grey, dark brown, cinnamon, yellow, orange, red, reddish brown, yellow brown, white and whitish grey.

Causes of Soil Colour

1. The amount of organic matter present in the soil body.
2. Mineralogical composition of soil.
3. Leaching process. and
4. Climate

Significance of soil colours.

1. Soil colour tells about the productivity of soil for crops cultivation.
2. Soil colour tells about the relative amount of moisture present in the soil.
3. Soil colour tells about kinds of minerals present in the soil body that is red coloured soil gives an impression that the soil as hydrated iron minerals.

Soil Porosity

Soil porosity refers to the amount of pore, or open space between soil particles. Pore spaces may be formed due to the movement of roots, worms, and insects; expanding gases trapped within these spaces by groundwater; and/or the dissolution of the soil parent material.

Types of Pore Space in a Soil

The pore space in a soil represents that part of the soil volume which has been occupied by air or water.

Pore space in a soil may be of the following two types

a. Macro pores

b. Micro pores

A. Macro pores

Macropores are large pores in the soil that helps aerate the soil and allow moisture to drain. They occur between the various aggregates that comprise the soil. They are necessary for rapid drainage of the soil.

B. Micro pores

Micro pores are small in size, Water is held within the micro-pores tightly enough to be unavailable for plant growth.

Factors Affecting Pore Space Per cent of Soils

(i) Texture

Total and Micro-pore spaces of soils increase when the clay percentage increases i.e. their texture becomes finer. The microspores ices of clayey soils are much more than those of the sandy soils, because clay particles unite to form soil aggregate within which micro-pore space occur.

(ii) Organic matter

Organic matter is decomposed by the soil microorganisms to form humus, which usually binds the soil particles to form aggregates. Besides this, humus also stimulates the growth of soil microorganisms, which mechanically bind the soil particles to form aggregates within which micro-pore spaces occur. Hence the micro and total pore spaces increase.

(iii) Nature of crops and cultivation

Excessive cultivation of soils increases the oxidation of humus. Consequently, total and micro-pore spaces of soils are decreased if soils are more and more cultivated. Less cultivation is required for growing grasses, the roots of which bind the soil particles to form soil aggregates.

Crops like potato, maize etc. require the maximum amount of soil cultivation for soil aeration which increases the oxidation of humus. Hence grasses increase the percentage of pore space of the soil, whereas arable crops like maize, potato etc. decreases it.

(iv)Soil depth

Since plant roots and organic matter occur more in surface soil then in sub-soil, so pore space per cent of the surface soil is usually much more than that of the sub-soil.

QN

What are the importance’s of Soil Porosity?

ANSWER

1. Soil Porosity as an ideal effect on drainage and water holding capacity.
2. It influences aeration in the soil body.
3. It influences soil fertility as it makes ability of a soil in holding water and nutrients for plants use.

Soil Temperature

Soil Temperature is the degree of coldness and hotness of a soil body. Soil Temperature tends to vary considerably from place to place due to the variation of climatic conditions.

Factors Affecting the Soil Temperature

1.  Solar radiation

The amount of heat from the Sun that reaches the earth is 2.0 cal/cm2 min -1 the amount of radiation received by the soil depends on angles with which the soil faces the Sun.

1. Condensation

Whenever water vapour from soil depths or atmosphere condenses in the soil, its heat increases noticeably.

1. Evaporation

The greater the rate of evaporation, the more the soil is cooled.

1. Rainfall

Rainfall cools down the soil.

• Vegetation

A bare soil quickly absorbs heat and becomes very hot during the summer and become very cold during the winter.  Vegetation acts as an insulating agent.  It does not allow the soil to become either too hot during the summer and two cold during the winter.

•  Colour of the soil

Black colored soils absorbs more heat than light closured soils Hence black color soils are warmer than light colored soils.

• Moisture content 

A soil with higher moisture content is cooler than dry soil.

• Tillage

The cultivated soil has greater temperature amplitude as compared to the uncultivated soil.

Soil texture

Soil textures affect the thermal conductivity of soil. Thermal conductivity decreases with reduction in particle size.

• Organic matter content

Organic matter reduces the heat capacity and thermal conductivity of soil, increases its water holding capacity and has a dark color, which increases its heat absorbability.

•  Slope of land

Solar radiation that reaches the land surface at an angle is scattered over a wider area than the same amount of solar radiation reaching the surface of the land at right angles.  Therefore, the amount of solar radiation reaching per unit area of the land surface decreases as the slope of the land is increases.

QN

What are the significances of Soil Temperature?

ANSWER

1. It determines the existence of soil organisms.
2. It controls the bio chemical processes taking place in the soil body.
3. It controls amount of moisture in the soil body.
4. It influences occurrence of some horizons in the soil body like the horizons of calcite deposits and salt crystals.

Soil Density

The bulk density of soil depends greatly on the mineral make up of soil and the degree of compaction. Soil density is expressed in

Particle Density=Weight of soil solid/volume of soil solids

Bulk Density of a soil sample= Weight of soil/ volume of soil

B. Chemical soil properties

Soils have considerable chemical properties. The most pronounced chemical soil properties include the following

• Soil reaction
• Leaching
• Caution exchange
• Soil colloids
• Soil nutrients.

SOIL REACTION

The degree of acidity or alkalinity of a soil is called soil reaction. It is an indicator of the acidity or alkalinity and is measured in pH scale. It is the most outstanding characteristics for plant growth factors because it determines the availability of plant nutrients.

Therefore Soil Reaction defined as the degree of acidity, alkalinity and neutrality.

DETERMINATION OF SOIL PH

The PH value of the soil is determined by finding out the concentration of hydrogen ions (H+) in the soil solution. This can be done by using one of the following methods

• The electrometric method
• The colorimetric method

The electrometric method

By the electrometric method, soil reaction is determined by a means of PH meter. When using PH meter, the hydrogen ions concentration of the soil solution is balanced against a standard hydrogen electrode then reading is made.

If a reading is about below 7, the soil is in acidic condition. If a reading is above 7, the soil is in alkaline condition, and if a reading is 7, the soil is in neutrality. The PH meter runs from 1-14.

PH range	Description
3.5-4.0   4.0-5.0   6.0-6.9       7   7.1-8.0   8.0-9.0   9.0-10.0   10.1-11.0	Very strong acidic   strong acidic   Moderately acidic   Neutral   Slightly alkaline   Moderately alkaline   Strong alkaline   Very strong alkaline

The colorimetric method

It is done in the laboratory by using dyes. A dye is poured into a container with a solution. Dyes sink slowly into the soil solution; then develop a certain colour depending the state of soil solution. The colour developed is by then compared to a standard colour chart with PH description.

Causes of soil acidity

1. Leaching of bases of calcium, magnesium, potassium and sodium.
2. Microbial activities and decomposition of organic matter.
3. The use of the acidic farming fertilizer like ammonium sulphate.

Causes of soil alkalinity

1. Soil alkalinity is mostly caused by the presence of the basic oxides in the soil.
2. Insufficient amount of rains, the condition which do not cause the severe removal of bases.

Agronomical significance of soil PH

1. Determine suitability of the medium for plant growth and micro organisms in the soil.
2. Determine extent of organic matter decomposition in the soil body.
3. Plants vary in their tolerance levels to acidity and alkalinity.
4. Soil reaction affects the availability of plants nutrients.

Example Nitrogen, calcium, phosphorus and potassium are mostly available at PH scale value from 6.5 to 7.5 as the condition favours a lot the decomposition of organic matter.

• Soil PH being an indicator of degree of acidity and alkalinity, gives an estimate amount of amendments to be done to a soil so as to bring favorable condition for plants growth.

Amendments of soil PH

The growth of plants and decomposition of materials in the soil body by micro organism, largely depend on favorable soil reaction. If soil is too acidic or alkaline does not pave away for good plants growth or decomposition of materials in the soil body. It is therefore for important for good methods to be taken, depends on the prevailing soil conditions.

Lowering of the soil acidity

Since soil acidity results from the scarcity of exchangeable cations, the best thing to be done is to add to the soil materials which contain metallic cations. The materials are known as lines and the process is called liming. The common materials for liming are of oxides and carbonate of calcium and magnesium. When these substances added to the soil, they add the amount of exchangeable bases as a result the soil acidity reduced.

Acidification of the soil

If the soil is too alkaline, something as to be done to raise acidity. This can be done by using the methods of eradication and conversion.

• Eradication is the process of removing sodium salts and other basic oxides from the overlying soil by passing a lot of water to cause leaching.
• Conversion is the process of adding materials to the soil which may cause acidity as they react with the basic oxides present in the soil. These materials can be of like gypsum and sulphur. As for example when sulphur added to the soil react with water to form sulphuric acid. Then the resultant sulphuric acid react with calcium carbonate. The sodium sulphate is soluble then leached away.

LEACHING

Leaching is a washing out of materials more particularly the minerals in solution or suspension down wards the soil body when water is percolating.

Leaching process is influenced by the following factors

• Soil texture and soil structure
• Climatic condition of an area.

Effects of soil Leaching

1. Causes the decline of soil fertility due to the fact that most of the plant nutrients are washed away in solution from the plants roots zone.
2. Increases soil acidity as the exchangeable bases which could cause the soil be alkaline, leached away.
3. Influence soil colour.
4. Cause the occurrence of horizons in the soil body by the process of eluviations and illuviations.
5. Decrease microbial activities due to the increase of soil acidity and decreases of microbial activities.
6. Affects proper plant growth due to the increase of soil acidity and decreases of microbial activities.

CATIONS EXCHANGE

It is the process in which the cations (positively charged particles) of like calcium (ca), magnesium (mg), potassium (K) and sodium (Na) replace hydrogen ions in the soil. The cations exchange can be between soil particles and soil solution or soil solution and plants roots. Usually the cations exchange in the soil body is influenced by the following factors

• Concentration of ions i.e ions move from high concentration to low concentration.
• Reactivity of ions

More reactive ions usually displace less reactive ions.

SOIL COLLOIDS

 These are substances which when dissolved remain dispersed in liquid. they include both mineral and organic colloids.

Properties of soil colloids

They are negatively charged

 This makes many positively cared ions called cations be attracted and absorbed around each colloidal particle.

They exhibit ion exchange

Ion exchange is reversible process were by cations or opinions are exchanged between solid and solid or between solid and liquid phases.

SOIL NUTRIENTS

These are chemical elements found in the soil which are essential for plants growth and the maintenance of the soil fertility.

The sources nutrients in the soil include the following

• The weathering of rocks from which mineral derived
• The decomposition of organic matter in the soil
• Application of the artificial fertilizers to the soil
• Rain water

C. Biological soil properties

Biological soil properties, largely considered on

i. Organic matter and

ii. Soil organism

I. Organic matter

Organic matter refers to the remains of died plants and animals which have been fully partially decomposed and mixed with soil mass.

Sources of organic matter

Origin sources of organic matter in the soil include the following

• Organic manure applied to a soil trout e agronomical practices.

Organic manure includes compost manure, green manure and farm yard manure.

• Plants remains

Plants remains of tree roots, shoots and Plants remains and grasses are important origin sources of organic matter in the soil.

• Industrial waste products

Industrial waste products which when added to a soil undergo rapid decomposition. But it is basically to organic industrial waste products.

• Died animals

These normally undergo decomposition and mix with the soil mass.

Factors that influence organic content in the soil

• The degree of aeration
• Soil moisture and temperature
• Topography
• Fertility status of the soil
• Soil reaction
• Ecological system

Importance of organic matter in soil

1. Organic matter influence soil moisture by retaining water in the soil.
2. Organic matters help the process of soil aggregation to bind soil particles together.
3. Organic matters reduce the plasticity of the soil.
4. It adds more plants nutrients to a soil body released from tissues of died plants.
5. The remains of died organism provide good habitable environment.
6. Organic matter help to regulate the chemical reaction of the soil through the released of minerals from their broken tissues.

SOIL ORGANISM

Soil body as natural system which supports the life of organism. The organism is the soil vary in size from smaller ones to larger ones and all these inhabitants find their food in the soil. They carry out a number biochemical activities. The name of plant kingdom is known as flora, while that of animals kingdom is known as fauna.

Both plants and animals are categorized into micro and macro organisms depending on their varied size. Micro organisms includes bacteria, algae, protozoa , fungi, virus etc. while macro organism includes earthworms, millipede, woodlice, spring tile etc.

Importance of microbial activities

• Nutrients supply
• Nitrogen balance in the soil.
• Improvements of soil structure
• They make symbioses with higher organisms
• Supply more organic matter by breaking down the residues.
• The activities make fixation.

SOIL EROSION

Soil erosion is the wearing away, detachment and removal of soil material from one place to another place through the agents like water, wind and ice.

TYPES OF SOIL EROSION

The soil erosions are of two types

1. Geological erosion
2. Accelerated soil erosion

1. Geological erosion

It is the wide spread type of erosion that occurs wherever there is a natural flow of energy and matter on the earth’s surface without man’s influence. It is normally very slow and so infectious to the soil cover of the world.

2. Accelerated soil erosion

Accelerated soil erosion is due to active interference of man and animals in disturbing the equilibrium in between the rate of soil formation and rate of soil loss and catalyzed by eroded agent of wind and water.

Wind erosion

It is the removal of top soils mainly caused by the blow of winds in dry areas in which the dry unconsolidated materials are easily removed by wind force.

Water erosion

It is the removal of materials by the water action. It is sub divided according to the appearance of the affected land and includes the following varied forms.

• Splash erosion

Splash erosion is the first stage of the erosion process. It occurs when raindrops hit bare soil.

The explosive impact breaks up soil aggregates so that individual soil particles are ‘splashed’ onto the soil surface.

The splashed particles can rise as high 60cm above the ground and move up to 1.5 metres from the point of impact.

• Sheet Erosion

Sheet erosion is the removal of soil in thin layers by raindrop impact and shallow surface flow.

~It occurs fairly evenly over an area.

~It is so subtle that it might not even be noticed until much of the valuable, nutrient-rich topsoil has already been washed away.

~If accumulation of soil and crop residue at one end of fields it may be sheet erosion.

• Rill Erosion

Rill erosion is erosion that results in small, short-lived and well-defined streams.

~Rills are shallow drainage lines less than 30cm deep.

~They develop when surface water concentrates in depressions or low points through paddocks and erodes the soil

~When rainfall does not soak into the soil, it can gather on the surface and runs downhill, forming small channels of water called rills.

~Rill erosion is often described as the intermediate stage between sheet erosion and gully erosion.

• Gully Erosion

Gully erosion can be thought of as advanced rill erosion. In fact, if rills are not addressed, they will grow into larger gullies.

~Gullies are channels deeper than 30cm that cannot be removed by normal cultivation.

~Gully erosion over time actually lose less soil than sheet and rill erosion

FACTORS INFLUENCING SOIL EROSION

A. Climate

B. Topography

C. Vegetation cover

D. Human activities

A. Climate

Climate is the most forceful factor causing erosion through rainfall and wind. Where there is heavy rainfall erosion tends to be severe while where there is low rainfall erosion is low.

The speed and duration of the wind have a direct relationship to the extent of soil erosion. Soil moisture levels are very low at the surface of excessively drained soils or during periods of drought, thus releasing the particles for transport by wind. This effect also occurs in freeze-drying of the soil surface during winter months. Accumulation of soil on the leeward side of barriers such as fence rows, trees or buildings, or snow cover that has a brown colour during winter are indicators of wind erosion.

B. Topography

On steep slopes soil erosion can be fast while on gentle slopes the rate of erosion tends to be low.

C. Vegetation cover

The presence of vegetation ground cover retards erosion. Forests and grasses are more effective in providing cover than cultivated crops. Vegetation intercepts the erosive beating action of falling raindrops retards the amount and velocity of surface fun off, permits more water flow into the soil and creates more storage capacity in the soil. It is the lack of vegetation that creates erosion permitting condition.

D. Human activities

Some of human activities may result into soil erosion example of human activities are deforestation, overgrazing, mining, monoculture etc. 

CAUSES OF SOIL EROSION

Overgrazing

Overgrazing is a practice of keeping large number of animals than the range land carrying capacity.

Overgrazing is one of the causes for erosion. Overgrazing reduces the usefulness, productivity of the land. The livestock press the subsoil into fine soil which can be carried easily by wind and water. Reduced soil depth, soil organic matter, and soil fertility affects the land’s future productivity.

Mono culture

The above farming practice leads to exhaustion of certain minerals from the soil making it infertile and bare leading to its erosion. Mono culture leads to loosening of soil particles thereby encouraging soil erosion.

Mining

If the mining practices are not well controlled, it can cause removing of ground cover hence soil erosion

Burning

Burning destroys micro-organisms which are essential for the formation of humus which binds soil particles together. It destroys vegetable matter that protects the soil against erosion hence less protection. Burning destroys the nitrogen fixing bacteria making the soil less fertile and therefore few plants and less protection of the soil. Burning loosens the soil making it susceptible to erosion or leaching which drains away soluble nutrients.

• Deforestation

Deforestation simply means that removal of forest crop from an area. On the other hand we also knows that trees are works as a soil binder by their roots and when a tree removed from an area the soil particles detached with each other and then it increases the condition of soil erosion.

• Ploughing to follow the slope

This practice accelerates much erosion to occur because soil erosion hazard is more severe on sloped land.

• Engineering works

Engineering like transport construction influence soil erosion.

EFFECTS OF SOIL EROSION

1. Loss of soil fertility
2. Decrease forest land
3. Difficult to cultivate the land.
4. Soil erosion hinders navigation.
5. It causes floods

CONTROLS OF SOIL EROSION

1. Crop rotation
2. Contour farming
3. Terracing
4. Planting trees
5.   Mulching
6. Ill side ditching
7. Cover cropping
8. green manuring
9. controlled grazing

SOIL FERTILITY

Soil fertility refers to the ability of a soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. A fertile soil has the following properties:

• The ability to supply essential plant nutrients and water in adequate amounts and proportions for plant growth and reproduction; and
• The absence of toxic substances which may inhibit plant growth.

FACTORS INFLUENCING SOIL FERTILITY

1. Soil texture
2. Dept of the soil profile
3. Position of ground water table
4. Soil structure
5. Soil reaction (PH)
6. Organic matter
7. Composition of parent materials

DECLINE OF SOIL FERTILITY

The following are some ways through which soil fertility may be lost

Leaching

This is common with nutrients that are highly soluble such as nitrogen, these nutrients are carried to lower far from beyond the reach of many plants roots, soil with many leached nutrients are infertility.

Soil capping

This is when the soil is covered (capped) with an impervious material which prevents the penetration of rainwater into the soil, this leads to surface run – off. This denies the soil adequate moisture and exposes the soil to erosion

Soil erosion

This is the carrying away of the top fertile soil by moving water and wind. Erosion leads to loss of the fertile top soil and plant nutrients, this makes the soil infertility.

Mono cropping

Mono cropping is the practice of growing one type of gropes on a piece of land for a long time. The gropes grown uses only those nutrients it needs while other nutrients remain unused, this leads to exhaustion of some nutrients and eventually to their deficiency in the following years.

There is also likelihood of buildup of pests and disease, the same pest and disease are passed on from the residue of previous crop, this leads to low yield

Accumulation of salt

Soil water contains dissolved minerals salts from the parent rock; some of the salt comes from decomposition of organic matter.

Under normal condition, the salts are washed away by rain water, thereby keeping their concentration in the soil low. However in arid and semi-arid areas the rainfall is irregular and is not enough to remove the salt from the soil.

This together with the high evaporation rate and poor drainage, leads to accumulation of salt on or below of the soil surface. The salt cause deficiency of water in plants as water moves out of the root in the soil under the osmotic pressure of the salt solution.

•  Change in the pH

Inappropriate use of fertilizers may change the soil pH, for example, the use of acidic fertilizer over a long period of time can make the soil acidic.

Change in pH affects the activity of the soil microorganisms and the availability of some nutrients. This, in run, affects the fertility of the soil.

• Burning of vegetation

When vegetation is burned, organic matter is destroyed; this affects the activities of micro organisms such as nitrogen fixation and decomposition of organic matter.

Accumulation of the resulting ash also causes imbalance of nutrients in the soil. Burning of vegetation also exposes the soil to agents of erosion such as wind and water.

METHODS OF MAINTAINING SOIL FERTILITY

Cover cropping

This method involves the planting of crops that provide shield or covering for soil resources against depletion. Their presence reduces evaporation of water present in soil. They help check against soil erosion which might wash nutrients away.

Crop rotations

This practice offers protection to the land from soil erosion and good chance to cover its original fertility crop rotation makes it possible to have the land occupied with crops most of the year.

Mulching

Mulching act as a huge sponge which absorbs the water that fall on to it and releases it slowly and harmlessly to the underlying soil if there is no protective cover over a wide area erosion may occur rapidly.

green manuring

It is a practice of ploughing in the green plant tissues grown in the field or adding green plants with tender twigs or leaves from outside and incorporating them into the soil for improving the physical structure as well as fertility of the soil.

SOIL CONSERVATION

Soil conservation is the preventing of soil loss from erosion or reduced fertility caused by over usage, acidification, salinization or other chemical soil contamination.

METHODS OF SOIL CONSERVATION

Crop rotations

This practice offers protection to the land from soil erosion and good chance to cover its original fertility crop rotation makes it possible to have the land occupied with crops most of the year. In addition the loss of crop most of the year. In addition the loss of nutrient elements by leaching is minimized and losses from erosion are greatly reduced Erosion hazard are n important factor in determining the kind and sequence of crops to be grown in a rotation of a particular piece of land n area where erosion can easily occur due to either slope or soil characteristic, permanent crops such as trees or pasture should be planted rotation will not provide erosion protection on steep slopes.

• Contour farming

Contour farming is ploughing, planting and cultivating across the slope following the contours, generally on gently sloping land each contour row can be viewed as a small dam that checks the speed of non-off water and reduce erosion on well drained soil. Contour farming is simple and easier of all the supplemental soil conservation

• strip cropping

This is a system in which crops are grown in strips that are arranged across the general slope or at right angle to the path of the prevailing wind .The strip don not necessary have to follow contours.

• Terracing

A terraced is an embankment of earth or stone or other suitable materials or combination of these materials made across the slope for the purpose of controlling run-off. Terrace decrease the length of the slope thus reducing erosion and run-off .

 There are two types of terraces

1. Level terrace

 Is a ridge built generally on sandy soil with little or no grade it is designed to hold water in the field until absorbed it adopted in areas where rainfall and soil characteristics are such that there is only slight danger of water accumulating on the soil and breaking the soil surface.

2. Channel terraces

 Consist that are cut across the slope these channel carry the excess rain water from the fields but at a low speed thus minimizing erosion .they are commonly constructed in regions that receive heavy rainfall.

• Planting of trees and grasses

 Trees and grasses can act as wind breakers and can also control water erosion .In controlling erosion caused by wind trees or grasses may be planted in strips so that soil particles carried by wind may be deposited on or near the grass strip.

• Controlled grazing

Overgrazing can be dangerous as most or all the vegetation can be removed with resultant exposure of the land to erosion rotational grazing with the optimum number of animals in one area can help to maintain the vegetation cover.

• Mulching

 Mulching act as a huge sponge which absorbs the water that fall on to it and releases it slowly and harmlessly to the underlying soil if there is no protective cover over wide area erosion may occur rapidly.

THE END

HAVE A NICE STUDY

 Weather can be defined as the physical condition or state of the atmosphere at a particular time and place.

• Weather takes into consideration the conditions over a few days to a week.
• All the short-term day-to-day condition of the atmosphere involving a description of rainfall, humidity, pressure, temperature, cloud cover and winds.
• Weather may change from day to day and is usually expressed with descriptive data.

Climate is defined as generalized or average condition of weather of a place or region. Or it is a composite or generalized of the variety of day to day weather conditions.

• The long-term atmospheric characteristics of a specified area. The characteristics are usually represented using numerical data on meteorological elements such as rainfall, temperature, wind, pressure and humidity.
• It refers to the average as well as the highest and lowest rate of temperature, pressure, rainfall, cloud cover, etc. over a period of at least thirty years.
• The climate of a place is a description of the average weather conditions that occur there, measured over a long period of time, usually thirty years.

Difference between weather and climate

S/N	Weather	Climate
1	Instantaneous physical state of atmosphere at particular place.	Normal physical state or generated condition of atmosphere or long term average condition of a place.
2	Weather changes refer to specific instant of time( day or week)	It is generalized over a longer span of time and for a longer area.
3	It is expressed in terms of numerical values of meteorological elements.	It is expressed in terms of time averages and area averages of meteorological elements.
4	Weather is measured in observatory. So the observatory must at a place for which weather is to be described.	This is derived information on regional basis. So scripts of observatories extending over a region are necessary.
5	No statistical treatment is applied to the meteorological elements. They are used as observed and hence always changing.	Application of statistical method over a longer period I done. It is more or less stable with few random changes.
6	It provides meteorological information.	It constitutes geographical information in respect of weather.
7	Weather of two places having same numerical value must be same.	Climate of the two places having the same averages of weather cannot be same, because their distribution over the years may be different.
8	Weather can be categorized as fair, unfair, excellent etc.	Climate is classified as desert climate, marine climate, tropical climate etc.
9	Weather decides the success or failure of a crop in a particular season.	Climate decides the type of crop suitable for a region, while introducing new crops climate is considered.
10	Adverse weather results into crop failure or loss and warrants short term contingent planning.	Climate is considered in long terms agricultural planning.

FACTORS INFLUENCING WEATHER AND CLIMATE

Relief and Altitude

Relief refers to the variations in elevations and slope of a particular area on the earth’s surface. For example, when we say an area is flat, gently sloping or mountainous (hilly) we are describing the relief of the land. The relief of an area can influence the weather and climate of that area.  Generally, air temperature decreases with altitude because air at higher altitudes is less dense and cooler. Temperature decreases with height on average 10oC/km.

Latitude

Latitude is one of the most influential factors of temperature. Latitudes are imaginary lines drawn around the earth parallel to the equator. 

Two factors affect the temperature are the angle of the overhead sun and the thickness of the atmosphere. At the equator the over-head sun is high in the sky, as result, high intensity of insolation is received.  On the other hand, at the poles, the sun is low in the sky so less energy or heat is received resulting in cooler temperatures.

Secondly, the thickness of the atmosphere affects temperature. At higher latitudes reduced sunrays strike the earth’s surface. The heat is spread over a larger area and is diffused resulting in lower temperatures in these areas.

Distance from the Sea

Land heats and cools more quickly than water; this affects the temperatures of coastal and inland areas. Places nearer to the sea will have fewer variations in temperature than places further inland. The sea moderates the temperatures near the coastal areas.  Water takes up heat and emits it much slower than land. This is known as maritime influence. The sea has little influence on the interior of continents. Here the temperatures are more extreme. These areas are under continental influence.

Land and Sea Breeze

Land and sea does not heat up at the same rate. The earth’s surface heats up faster than the ocean as a result; there is a lower pressure over the land than the sea. Wind blows from high pressure to low pressure. Therefore during the day, the wind blows from the sea to the land lowering the temperature. On the other hand, during the night, the land cools and the seas stay warm. The low pressure is now over the sea causing the breeze to blow from the land towards the sea.

ATMOSPHERE

Atmosphere is the thin layer of gases held on the earth by gravitation attraction.

– It composed by abiotic (non-living matter) and biotic (living organism).

– Non-living matter found in the atmosphere includes mixture of gases, water vapor and dust particles. Atmosphere consists of different gases such as carbon dioxide, oxygen, hydrogen, nitrogen and other gases.

– The living organism includes the smallest or microscopic organisms like bacteria.

CHARACTERISTICS OF ATMOSPHERE

Characteristics of atmosphere categorized into two groups as follow 

a. According to its composition.

b. According to its vertical structure from the ground level into interplanetary space.

COMPOSITION OF ATMOSPHERE

Atmosphere categorized into biotic and abiotic matters. Biotic matters include the life in atmosphere above 30m from the ground and abiotic matters include materials with no life found in air which is gases, water and solid matters.

ATMOSPHERIC GASES

The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (0.009%) and carbon dioxide (0.03%) and other gases include neon, helium, Krypton, xenon.

WATER VAPOUR IN THE ATMOSPHERE

The presence of water vapor in atmosphere plays a large role in determining the weather. Clouds and precipitation occur as a result of the phase change that occurs when water vapor condenses into liquid water.

Sources of water vapour in the atmosphere

The sources of water vapour in the atmosphere include the following

• Evaporation of water bodies.
• Evaporation of hot springs.
• Evaporation of water from the soil.
• Plants transpiration.
• Evapo- transpiration by plants.
• Volcanic eruption.

SOLID PARTICLES IN THE ATMOSPHERE

The air seems to be completely clear; it is full of atmospheric particles which are invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, soot, sea salt particles etc.

STRUCTURE OF ATMOSPHERE

Structure of Atmosphere is viewed on its vertical layers from the Earth’s surface into the interplanetary space, in its vertical dimension, atmosphere has varied conditions in temperature and chemical composition and these have enabled the meteorologists to divide it into a number or layers.

Atmosphere in its vertical dimension is broadly divided according to

a. Contrasting temperature conditions in it with altitude from ground level into further space.

b. Contrasting chemical composition with altitude.

A. Structure of Atmosphere according to Contrasting temperature conditions

In this group, Atmosphere categorized in terms of altitude

The main atmospheric layers according to contrasting temperature conditions in it with altitude from ground level are

i. Troposphere.

ii. Stratosphere.

iii.Mesosphere.

iv. Thermosphere.

I. Troposphere

The troposphere is the lowest layer of Earth’s atmosphere. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Most types of clouds are found in the troposphere, and almost all weather occurs within this layer.

The bottom of the troposphere is at Earth’s surface. The troposphere extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level. The height of the top of the troposphere varies with latitude (it is lowest over the poles and highest at the equator) and by season (it is lower in winter and higher in summer). It can be as high as 20 km (12 miles or 65,000 feet) near the equator, and as low as 7 km (4 miles or 23,000 feet) over the poles in winter.

Air is warmest at the bottom of the troposphere near ground level. Air gets colder as one rises through the troposphere. That’s why the peaks of tall mountains can be snow-covered even in the summertime.

II. Stratosphere

The stratosphere is a layer of Earth’s atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.

III. Mesosphere

The mesosphere is a layer of Earth’s atmosphere. The mesosphere is directly above the stratosphere and below the thermosphere. It extends from about 50 to 85 km (31 to 53 miles) above our planet.

Temperature decreases with height throughout the mesosphere. The coldest temperatures in Earth’s atmosphere, about -90° C (-130° F), are found near the top of this layer.

The boundary between the mesosphere and the thermosphere above it is called the mesopause. At the bottom of the mesosphere is the stratopause, the boundary between the mesosphere and the stratosphere below.

The mesosphere is difficult to study, so less is known about this layer of the atmosphere than other layers. Weather balloons and other aircraft cannot fly high enough to reach the mesosphere. Satellites orbit above the mesosphere and cannot directly measure traits of this layer. Scientists use instruments on sounding rockets to sample the mesosphere directly, but such flights are brief and infrequent. Since it is difficult to take measurements of the mesosphere directly using instruments, much about the mesosphere is still mysterious.

IV. Thermosphere

The thermosphere is a layer of Earth’s atmosphere. The thermosphere is directly above the mesosphere and below the exosphere. It extends from about 90 km (56 miles) to between 500 and 1,000 km (311 to 621 miles) above our planet.

Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C (360° F) hotter in the daytime than at night, and roughly 500° C (900° F) hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher.

The boundary between the thermosphere and the exosphere above it is called the thermopause. At the bottom of the thermosphere is the mesopause, the boundary between the thermosphere and the mesosphere below.

Although the thermosphere is considered part of Earth’s atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km (62 miles), slightly above the mesopause at the bottom of the thermosphere. The space shuttle and the International Space Station both orbit Earth within the thermosphere!

Below the thermosphere, gases made of different types of atoms and molecules are thoroughly mixed together by turbulence in the atmosphere. Air in the lower atmosphere is mainly composed of the familiar blend of about 80% nitrogen molecules (N2) and about 20% oxygen molecules (O2). In the thermosphere and above, gas particles collide so infrequently that the gases become somewhat separated based on the types of chemical elements they contain. Energetic ultraviolet and X-ray photons from the Sun also break apart molecules in the thermosphere. In the upper thermosphere, atomic oxygen (O), atomic nitrogen (N), and helium (He) are the main components of air.

B.Structure of Atmosphere according to contrasting chemical composition with altitude

The main atmospheric layers according to contrasting chemical composition with altitude are.

1. Homosphere
2. Heterosphere

1. Homosphere

The homosphere is the lower of the two and the location in which turbulent mixing dominates the molecular diffusion of gases. In this region, which occurs below 100 km (about 60 miles) or so, the composition of the atmosphere tends to be in

2. Heterosphere

Above 100 km, in the zone called the heterosphere, various atmospheric gases are separated by molecular mass, with the lighter gases being concentrated in the highest layers. Above 1,000 km (about 600 miles), helium and hydrogen are the dominant species. Diatomic nitrogen (N2), a relatively heavy gas, drops off rapidly with height and exists in only trace amounts at 500 km (300 miles) and above. This decrease in the concentration of heavier gases with height is largest during periods of low Sun activity, when temperatures within the heterosphere are relatively low. The transition zone, located at a height of around 100 km between the homosphere and heterosphere, is called the turbopause.

IMPACTS OF ATMOSPHERE ON LIFE

Positive Impacts

1. Some atmospheric layers particularly troposphere consists of much of useful gases to living organisms.
2. Atmosphere is associated with the weather making processes such clouds formation which result into precipitation.
3. Atmosphere act as protective shield to Earth.
4. Atmosphere allows air communication. It makes transmission of radio, telephone and television.

Negative Impacts

1. Carbon dioxide gas present in the Atmosphere causes rate of temperature increase as it absorbs long wave radiation from the earth surface contributing to the problem of green house effect.
2. Atmosphere has some constituents which cause the air borne diseases to people.
3. Pollutant gases present in the Atmosphere contribute to a problem of acidic rain occurrence.
4. The depletion of ozone layer, results into the following problems

      -Melting of Ice due to rise in temperature.

      -Rise in sea level because of Ice melting on the landscape.

      -Skin cancer diseases to people.

      -Death and disappearance of some plant and animal species because of the adverse atmospheric changes.

DIAGRAM OF VERTICAL STRUCTURE OF THE ATMOSPHERE

Picture from http://teachertech.rice.edu/Participants/louviere/struct.html

TEMPERATURE

Temperature is the degree of hotness and coldness of a body.

The interaction of insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature. While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is.

DISTRIBUTION OF TEMPERATURE

The global distribution of temperature can well be understood by studying the temperature distribution in January and July. The temperature distribution is generally shown on the map with the help of isotherms.

 Isotherms are lines joining places having equal temperature.

Factors Influencing Temperature Distribution

The temperature of air at any place is influenced by the following

• the latitude of the place;
• the altitude of the place;
• distance from the sea,  
• Air-mass and Ocean currents
• Continental influence
• Cloud cover
• Aspects.

The latitude

The temperature of a place depends on the insolation received. Insolation varies according to the latitude hence the temperature also varies accordingly.

The altitude

The atmosphere is indirectly heated by terrestrial radiation from below. Therefore, the places near the sea-level record higher temperature than the places situated at higher elevations. In other words, the temperature generally decreases with increasing height. The rate of decrease of temperature with height is termed as the normal lapse rate. It is 0.6°C per 100 m.

Distance from the sea

Another factor that influences the temperature is the location of a place with respect to the sea. Compared to land, the sea gets heated slowly and loses heat slowly. Land heats up and cools down quickly. Therefore, the variation in temperature over the sea is less compared to land. The places situated near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.

Air-mass and Ocean currents

 Like the land and sea breezes, the passage of air masses also affects the temperature. The places, which come under the influence of warm air-masses experience higher temperature and the places that come under the influence of cold air- masses experience low temperature. Similarly, the places located on the coast where the warm ocean currents flow record higher temperature than the places located on the coast where the cold currents flow.

Continental influence

Located in the interior of large continents or land masses are under continental influence, that is the sea does not an effect on them as they are too far in temperature. As land heats up rapidly, inland locations tend to have hotter summers than areas near the coast in similar latitudes.

Cloud cover

Heavy cloud cover will moderate both day and night temperatures.

In hot deserts,absence of clouds results in high day temperatures and very low night temperatures.Very humid air absorbs heat during the day and retains it during the night.

Clouds reduce the amount of solar radiation reaching the earth’s surface and the amount of radiation leaving.

Aspect

Aspect can have a strong influence on temperature. This is because of the angle of the sun in the northern and southern hemispheres which is less than 90 degrees or directly overhead. In the northern hemisphere, the north side of slopes is often shaded, while the southern side receives more solar radiation for a given surface area insolation because the slope is tilted toward the sun and isn’t shaded directly by the earth itself. The further north or south you are and closer to winter solstice the more pronounced the effects of aspect of this are, and on steeper slopes the effect is greater, with no energy received on slopes with an angle greater than 22.5° at 40° north on December 22 (winter solstice).

SOLAR RADIATION

The earth’s surface receives most of its energy in short wavelengths called electromagnetic spectrum. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.

Insolation is the solar radiation that reaches the earth’s surface. It is measured by the amount of solar energy received per square centimetre per minute.

The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun. During its revolution around the sun, the earth is farthest from the sun (152 million km) on 4th July. This position of the earth is called aphelion. On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion. Therefore, the annual insolation received by the earth on 3^rd January is slightly more than the amount received on 4th July. However, the effect of this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation. Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.

Variability of Insolation at the Surface of the Earth

The amount and the intensity of insolation vary during a day, in a season and in a year.

The factors that cause these variations in insolation are

• the rotation of earth on its axis;
• the angle of inclination of the sun’s rays;
•  Distance between the earth and the sun
• the transparency of the atmosphere;
• Duration of sunshine
• Solar constant

The rotation of earth on its axis

The fact that the earth’s axis makes an angle of 66.5 with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes.

The angle of inclination of the sun’s rays

This depends on the latitude of a place. The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.

Distance between the earth and the sun

Since the earth revolves around the sun in an elliptical orbit, the distance varies during the course of a year. The mean distance between the earth and sun is about 149,000,000 kilometers.

Each year, on about January 3, the earth comes closer to the sun (distance 147 million kilometers). This position is known as perihelion. On about July 4, the earth is a little farther from the sun when the distance becomes about 152 million kilometers. This position is called aphelion.

Although the amount of incoming solar radiation received at the outer boundary of the atmosphere is a little greater (7 percent) in January than in July, there are other major factors, such as the angle of incidence and the duration of sunshine that more than offset its effect on seasonal temperature variations.

It may be interesting to note that the earth is relatively closer to the sun during the northern hemisphere winter.

Transparency of the atmosphere

Transparency of the atmosphere is an important control on the amount of insolation which reaches the earth’s surface. Reflection from dust, salt, and smoke particles in the air is an important mechanism for returning shortwave solar radiation to space.

Similarly, reflection from cloud tops also depletes the amount of solar radiation that would otherwise be available to the earth. The effect of certain gases, water vapour, and dust particles on reflection, scattering, and absorption is well-known.

Obviously, areas with heavy cloudiness and turbid atmosphere will receive lesser amount of radiant energy at the surface. But the transparency of the atmosphere varies with time and place.

Transparency of the atmosphere is closely related to the latitude. In the higher latitudes the sun’s rays are more oblique, so that they have to pass through relatively thicker layers of the atmosphere than at lower latitudes. In winter when the altitude of the sun is relatively lower, there is greater loss of incoming solar radiation than in summer.

Duration of sunshine

The duration of sunlight hours determines the length of the day, which also affects the amount of solar radiation received at the surface. Undoubtedly, the longer period of sunshine ensures larger supply of radiation which a particular area of the earth will receive.

Obviously, the latitudes exercise the most dominant control over the duration of sunshine and thereby the length of the day.

Solar constant

As the energy emitted by the sun varies, the amount of insolation received at the surface also changes. But the percentage of change in the solar constant is rather negligible. The variations in the solar constant are caused by periodic disturbances and explosions in the solar surface.

The sun-spot studies that have been carried so far establish that when the sun-spots appear in larger numbers, the intensity of the solar radiation received at the surface is increased. Naturally, therefore, as the number of sunspots decreases, the quantity of radiation received at the earth’s surface declines.

The scientists are of the opinion that the number of sunspots increases or decreases on a regular basis, creating a cycle of 11 years. However, there is little doubt that the magnitude of the effect of the varying amount of the solar constant on the amount of solar radiation received here on earth seems to be too small.

HEATING AND COOLING OF ATMOSPHERE

There are different ways of heating and cooling of the atmosphere. The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form. The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction.

Conduction takes place when two bodies of unequal temperature are in contact with one another, there is a flow of energy from the warmer to cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken. Conduction is important in heating the lower layers of the atmosphere. The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere. This process of vertical heating of the atmosphere is known as convection.

The convective transfer of energy is confined only to the troposphere. The transfer of heat through horizontal movement of air is called advection. Horizontal movement of the air is relatively more important than the vertical movement. In middle latitudes, most of diurnal (day and night) variation in daily weather are caused by advection alone.

In tropical regions particularly in northern India during summer season local winds called ‘loo’ is the outcome of advection process.

TERRESTRIAL RADIATION

The insolation received by the earth is in short waves forms and heats up its surface. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form. This energy heats up the atmosphere from below. This process is known as terrestrial radiation.

The long wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and the other green house gases. Thus, the atmosphere is indirectly heated by the earth’s radiation.

The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth’s surface and in the atmosphere.

ATMOSPHERIC HEAT BUDGET

The earth as a whole does not accumulate or loose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.

Consider that the insolation received at the top of the atmosphere is 100 per cent. While passing through the atmosphere some amount of energy is reflected, scattered and absorbed. Only the remaining part reaches the earth surface. Roughly 35 units are reflected back to space even before reaching the earth’s surface. Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth. The reflected amount of radiation is called the albedo of the earth.

The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation. Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere (6 units absorbed directly by the atmosphere, 9 units through convection and turbulence and 19 units through latent heat of condensation). 48 units absorbed by the atmosphere (14 units from insolation +34 units from terrestrial radiation) are also radiated back into space. Thus, the total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balance the total of 65 units received from the sun. This is termed the heat budget or heat balance of the earth.

This explains why the earth neither warms up nor cools down despite the huge transfer of heat that takes place.

Variation in the Net Heat Budget at the Earth’s Surface

As explained earlier, there are variations in the amount of radiation received at the earth’s surface. Some part of the earth has surplus radiation balance while the other part has deficit.

LAPSE RATE

Lapse rate is a rate at which temperature decreases with height.

TYPES OF LAPSE RATE

Two types of lapse rates are

       a) Environmental Lapse Rate -ELR

       b) Adiabatic Lapse Rate -ALR

A. Environmental Lapse Rate

Environmental Lapse Rate is a normal rate at which temperature of an air in surrounding decreases for about 0.6 °C per 100m.

B. Adiabatic Lapse Rate

Adiabatic Lapse Rate is the amount of temperature change by decreasing, in which when air is forced to rise, expands and cools at the rate which either below or above the normal lapse rate depending on the condition of air weather saturated or dry.

When air is forced to rise up in the atmosphere, the pressure reduces with height. For a given volume of gas, the pressure divided by the temperature remains constant (Boyle’s Law). Therefore, as the air pressure reduces, so does the temperature.

If no heat is exchanged with the surrounding air during this process, which is called “adiabatic cooling”, the rate at which the air cools, the Adiabatic Lapse Rate (ALR) is a constant.

For unsaturated air, the lapse rate is 3°C per 1000 feet; this is called the Dry Adiabatic Lapse Rate (DALR). However, when the parcel of air reaches the Dew point and becomes saturated, water vapour condenses, latent heat is released during the condensation process, which warms the air, and the lapse rate reduces.

The Saturated Adiabatic Lapse Rate (SALR) is therefore the rate at which saturated air cools with height and is, at low levels and latitudes, 1.5°C per thousand feet.

At higher altitudes and latitudes, where there is generally less water content in the air, and therefore less latent heat to release, the SALR is closer to 3°C per thousand feet.

The ELR (Environmental Lapse Rate) is the actual rate at which the ambient temperature changes with height.

Considering the parcel of air as before and utilizing the DALR and SALR for that parcel of air in contrast to the surrounding air:

If the ELR is greater than the ALR, rising air will be warmer than the surrounding air and therefore keep rising; the atmosphere is then said to be unstable. If ELR is greater than SALR, the air is said to be absolutely unstable, since the air, whether saturated or unsaturated, will always have a higher temperature than it surroundings.

When the ELR is less than the SALR and greater than the DALR, then the air is considered conditionally unstable: the condition being whether the air is saturated or not.

If the ELR is less than the ALR, then the rising air will be cooler than the surrounding air and will sink – the atmosphere is said to be stable.

If the ELR is less than the DALR, the air is said to be absolutely stable, since the air, whether saturated or unsaturated, will always be cooler than the surrounding air.

QN

What are the effects of lapse rate?

ATMOSPHERIC INSTABILITY AND STABILITY

Atmospheric Instability

Atmospheric Instability is a state of atmosphere in which air is set in convectional current rising upward from its pocket occurring when Environment Lapse Rate is greater than Adiabatic Lapse rate.

If a parcel of air is lifted and continues to rise after the lifting force disappears, the atmosphere is instability. In an instability layer, the lapse rate of a rising parcel is less than the lapse rate of the environment. Though the parcel cools as it rises, its temperature remains warmer than the surrounding air during its ascent through an instability layer. Because the parcel is warmer than the environment, the parcel has positive buoyancy and continues to rise on its own.

Atmospheric Stability

Atmospheric Stability is a state of atmosphere in which air is set in convectional current rising upward from its pocket occurring when Environment Lapse Rate is less than Adiabatic Lapse rate.

If some external force such as topographic lifting or convergence pushes the air upward, the temperature of the rising air relative to the environment suggests that the air would prefer to go back to its original position. In other words, though a parcel is being forced up, it has negative buoyancy meaning it wants to sink to its original position where it was in equilibrium with the environment. If pushed down, the air has positive buoyancy and wants to rise.

TEMPERATURE INVERSION

Normally, temperature decreases with increase in elevation, it is called normal lapse rate. At times, the situations are reversed and the normal lapse rate is inverted. It is called of temperature inversion.

Temperature inversion as atmospheric conditions where by air temperature increase with height in such away warm air overlies cold air.

Inversion is usually of short duration but quite common nonetheless. A long winter night with clear skies and still air is ideal situation for inversion. The heat of the day is radiated off during the night, and by early morning hours, the earth is cooler than the air above. Over polar areas, temperature inversion is normal throughout the year. Surface inversion promotes stability in the lower layers of the atmosphere. Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere. Dense fogs in mornings are common occurrences especially during winter season. This inversion commonly lasts for few hours until the sun comes up and beings to warm the earth. The inversion takes place in hills and mountains due to air drainage. Cold air at the hills and mountains, produced during night, flows under the influence of gravity. Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above. This is called air drainage. It protects plants from frost damages.

TYPES OF TEMPERATURE INVERSION

There are four kinds of temperature inversions, namely:

• Ground inversion,  

• Turbulence inversion,

• Subsidence inversion, and

• Frontal inversion.

Ground inversion

A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.
  

Turbulence inversion

A turbulence inversion often forms when quiescent air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer. The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.

Subsidence inversion

A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion. Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.

Frontal inversion

A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.

CAUSES OF TEMPERATURE INVERSION

Causes of temperature inversions are

• Radiation from the earth’s surface

-At the Night: Earth surface radiates heat away (cools fast)

-Air by ground cools quickly

–At the upper:  atmosphere cools slower, creating inversion

-It is Very common in winter.

• Subsidence (sinking air) associated with high pressure systems

• Frontal systems

-Cold air from cold water moves in and under warm air on land

• Valley causes of temperature inversions

At the Night: cold, dense air flows down slope

-Flows below warmer, less dense air

-Mountains trap air and pollution in valley and also helps bring cold, moist air

• Presence of ozone gases in the Atmosphere

• Presence of water vapour in the Atmosphere

• Terrestrial radiation during night.

QN

Classify temperature invention

ANSWER

Temperature invention is classified according to

1. Levels of its occurrence.
2. Causal factors and
3. Duration of occurrence.

EFFECTS OF TEMPERATURE INVERSION

• Air pollution increase

Warm air traps pollutants below, does not allow for vertical dispersion of pollutants

• Smog

High concentrations of sulfur dioxide and other gasses from combustion of fossil fuels by vehicles and industry Pollutants “trapped” by warm air above city, increasing concentration of pollutants and smog

• Freezing rain

Snow melts as it passes through warm air inversion, refreezes as it gets close to ground

• Prevents thunderstorms

Inversion doesn’t allow air to rise, condense, and create thunderclouds

• Radio and TV reception

From distant stations increases due to tropospheric ducting (waves bounce back to earth)

• Can keep coastlines and surrounding areas cooler in summer months

RECORDING AND READING OF TEMPERATURE

TEMPERATURE MEASUREMENT

Temperature is measured in degree Celsius (◦ C) or degree Fahrenheit (◦F).

INSTRUMENTS USED

Scientists use thermometers to measure temperature. Thermometers come in various different types including maximum and minimum temperature.

Maximum temperature

Maximum temperature measure highest temperature reached in a day.

Minimum temperature

Minimum temperature measure lowest temperature reached in a day.

Six’s thermometer

Also six’s thermometer used to measure minimum and maximum thermometer reached in a day.

Picture from https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSyDt5hC9pb4ft7QHWMXco-9Uf4LZPx0eJZgxp4tHfUlut7xkrL

THE FORMULA

Average (mean) Temperature= MAX TEMP + MIN TEMP / 2 (Max temp + min temp divided by 2)

Annual (Daily) diurnal range temperature = MAX TEMP –MIN TEMP

Montly temperature= DAILY MAX TEMP + DAILY MIN TEMP/ 2 ( Max temp + min temp divided by 2)

FOR Fahrenheit (◦F) to Celsius (◦C)

◦ C = 5/9 (◦F -32)

Where by ◦ C = degree Celsius and

◦F = degree Fahrenheit

FOR Celsius (◦C) to Fahrenheit (◦F)

◦F = 9/5 (◦ C +32)

Where by ◦ C = degree Celsius and

◦F = degree Fahrenheit

HUMIDITY

Humidity is the amount of water vapor in air. Or Humidity is the state of the atmosphere in relation to the amount of water vapour it contains. Humidity indicates the degree of o occur

QN

What are the conditions necessary for occurrence of humidity?

FACTORS INFLUENCING HUMIDITY

• Latitude.

• Distance from large water bodies seas and lakes.

• Temperature.

• Altitude.

• Natural vegetation

• Winds

SIGNIFICANCE OF HUMIDITY

• Humidity is important to make photosynthesis possible.

• Humidity is a key agent in both weather and climate, and it is an important atmospheric greenhouse gas.

• Humidity contributes both to achieving correct environmental conditions.

• A huge variety of manufacturing, storage and testing processes are humidity-critical.

• Humidity are used to prevent condensation, corrosion, mould, warping or other spoilage – highly relevant for foods, pharmaceuticals, chemicals, fuels, wood, paper, and many other products.

RECORDING AND READING OF HUMIDITY

Humidity is measured by a hygrometer which consists of wet and dry bulb thermometers.

The wet bulb thermometer is kept moist (wet) by wrapping it in Muslin which is then dipped in a container of distilled water. When the air is not saturated water evaporates from the muslin and cools the wet bulb.

The cooling effect causes the mercury to contract. The dry bulb is not affected in the same way as wet bulb because it does have a Muslin wrapping. It is affected by the surrounding air.

So when the air is not saturated the two thermometers show different readings, when the air is saturated the two thermometers show the same readings. Therefore when there is a big difference in reading between the two thermometers humidity is low and when there is small difference humidity is high.

EXPRESSIONS OF HUMIDITY

Humidity is expressed in

• Relative Humidity

• Specific humidity and

• Absolute humidity

Relative Humidity

Relative Humidity refers to actual amount of moisture in air relative to the maximum capacity amount that the air can hold.

Relative humidity varies with temperature

• Warm air has a higher capacity than cold air to hold moisture
• During day, relative humidity and temperature have inverse relationship

Specific humidity

Specific humidity refers to mass of water vapor per mass of air (g/kg)

• higher temperature, higher maximum specific humidity
• Warm air can “hold more water” than cold air

Absolute humidity

Absolute humidity refers to measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).

The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g of water vapor – 5g/m3.

PRECIPITATION

Precipitation is any type of water that forms in the Earth’s atmosphere and then drops onto the surface of the Earth. 

The process of continuous condensation in free air helps the condensed particles to grow in size. When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface. So after the condensation of water vapour, the release of moisture is known as precipitation. This may take place in liquid or solid form. The precipitation in the form of water is called rainfall.

CONDITION NECESSARY FOR PRECIPITATION TO OCCUR

• The Air must be saturated.

• The Air must contain small particles of matter such as dust around which the droplets form.

• The Air must be cooled below its dew point

HYDROLOGICAL CYCLE

Hydrological cycle is the cyclic movement of water containing basic continuous processes like evaporation, precipitation and runoff as Runoff – > Evaporation – > Precipitation – > Runoff. This is a continuous cycle which starts with evaporation from the water bodies such as oceans.

Components of Hydrological Cycles and its Definitions

Let us know about the components of hydrological cycle as follow:

1. Runoff: it is the water flowing over the land making its way towards rivers, lakes, oceans etc. as surface or subsurface flow.

It categorized into two runoff namely,

• Surface runoff and
• Sub surface run off

Surface runoff: it is the running water over the land and which ultimately discharge water to the sea.

Sub surface run off: The water getting infiltrated into pervious soil mass, making its way towards rivers and lakes can be termed as sub surface run off.

2. Precipitation: It is the fall of moisture from atmosphere to the earth’s surface in anyform. Example: rain, hail, snow, sleet, glaze, drizzle, snowflakes.

3. Evaporation: it is the conversion of natural liquids like water into gaseous form like air.

4. Condensation: It is the conversion of a vapor or gas to a liquid.

5. Transpiration: it is the evaporation taking place from any plant or greenery. Example, water droplet on a leaf getting evaporated into atmosphere

6. Evapo transpiration: it is the combination of evaporation and transpiration.

7.Infiltration: it is the process of filtration of water to the inner layers of soil based on its structure and nature. Pervious soils go through more infiltration than impervious. Infiltration in soils like sand, gravel and coarser material is more and for finer soil particles like clay and silt, infiltration is less.

Infiltration is inversely proportional to runoff. In a soil, if infiltration is less, then runoff is more, similarly more infiltration gives less runoff. Example: bitumen roads has more runoff than metallic red mud roads

Process of Hydrological Cycle

Process of hydrological cycle starts with oceans. Water in oceans, gets evaporated due to heat energy provided by solar radiation and forms water vapor. This water vapor moves upwards to higher altitudes forming clouds. Most of the clouds condense and precipitate in any form like rain, hail, snow, sleet. And a part of clouds is driven to land by winds. Precipitation, while falling to the ground, some part of it evaporates back to atmosphere.

Portion of water that reaches the ground, enters the earth’s surface infiltrating various strata of soil and enhancing the moisture content as well as water table. Vegetation sends a portion of water from earth’s surface back to atmosphere through the process of transpiration. Once water percolates and infiltrates the earth’s surface, runoff is formed over the land, flowing through the contours of land heading towards river and lakes and finally joins into oceans after many years. Some amount of water is retained as depression storage.

Further again the process of this hydrological cycle continues by blowing of cool air over ocean, carrying water molecules, forming into water vapor then clouds getting condensed and precipitates as rainfall. Similarly, then water gets percolated into soil, increasing water table then formation of runoff waters heading towards water bodies. Thus the cyclic process continues.

Picture from https://pmm.nasa.gov/education/water-cycle

CLASSIFICATION OF PRECIPITATION

Precipitation classified as follow

Snowfall

When the temperature is lower than the 0°C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals.  These crystals form flakes of snow.

Sleet and hail

Sleet is frozen raindrops and refrozen melted snow-water.

When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet. Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed. Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones. These are formed by the rainwater passing through the colder layers. Hailstones have several concentric layers of ice one over the other.

Glazed frost

It is a smooth, transparent and homogeneous ice coating occurring when freezing rain or drizzle hits a surface.

Hoar frost

It is a grayish-white crystalline deposit of frozen water vapour formed in clear still weather on vegetation.

Rainfall

The precipitation in the form of water is called rainfall.

TYPES OF RAINFALL

On the basis of origin, rainfall may be classified into three main types

• Convectional rainfall

• Orographic or relief rainfall and

• Cyclonic or frontal rainfall

Convectional rainfall

Convectional rainfall is formed when hot air is rises and cools and condenses forming rain. The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed. With thunder and lightning, heavy rainfall takes place but this does not last long. Such rain is common in the summer or in the hotter part of the day. It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.

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Orographic or relief rainfall

Orographic or Relief rainfall occurs when air has been blown over the sea and is then forced up over an area of high land.

When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed. The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. It is also known as the relief rain.

Cyclonic or frontal rainfall

Cyclonic or frontal rainfall occurs when warm air is forced to rise over cold air. The moisture in the warm air condenses as it cools which causes clouds and rain.

World Distribution of Rainfall

Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons.

In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily. The coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water. Between the latitudes 35 0 and 40 0 N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45 0 and 65 0 N and S of equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east. Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side. 

FACTORS FOR THE UNEVEN DISTRIBUTION OF PRECIPITATION

Temperature

The higher temperature lead the area experienced higher amount of precipitation than the areas has lowest temperature.

Relief and topography

The windward side of mountains and hills receive more rainfall than leeward side.

The vagaries of the monsoon

The unpredictability of the monsoon along with phenomena like monsoon troughs and depressions lead to uneven distribution of rainfall.

Amount of water to be evaporated

The higher amount of water to be evaporated may result into higher amount of precipitation of an area and vise versa.

Wind direction

The regions lying in the direction of flow of wind are rainier than those not lying on its direction.

Distance from the sea (Continentality)

As the distance from the sea increases, the moisture content of the wind decreases. So, the interior of landmass are much drier than those in coastal region.

IMPACTS OF PRECIPITATION

Positive Impact

• Encourage plant growth

• Temperature regulation

• Purification of the atmosphere

• Development of water bodies

• Soil development

Negative Impacts

• Precipitation cause soil erosion

• Precipitation cause mass wasting

• Outbreak of diseases

• Extreme precipitation cause occurrence of floods

• Precipitation cause water pollution

RECORDING AND MEASUREMENTS OF RAINFALL

Rainfall is measured by a rain gauge.

A rain gauge is an instrument used by meteorologists and hydrologists to gather and measure the amount of rainfall over a set period of time.

Amount of rainfall experienced in a year recorded by a means of graphs known as histogram or maps also known as isopleths map.

A line drawn on a map connecting points having equal rainfall at a certain time or for a stated period is called isohyets

FOGS

Fog is often described as a stratus cloud resting near the ground. Fog forms when the temperature and dew point of the air approach the same value (i.e., dew-point spread is less than 5°F) either through cooling of the air (producing advection, radiation, or hill fog) or by adding enough moisture to raise the dew point (producing steam or frontal fog). When composed of ice crystals, it is called ice fog.

TYPES OF FOGS

Advection fog

Advection fog forms due to moist air moving over a colder surface, and the resulting cooling of the near-surface air to below its dew-point temperature. Advection fog occurs over both water (e.g., steam fog) and land.

Radiation fog (ground or valley fog)

Radiation cooling produces this type of fog. Under stable nighttime conditions, long-wave radiation is emitted by the ground; this cools the ground, which causes a temperature inversion. In turn, moist air near the ground cools to its dew point. Depending upon ground moisture content, moisture may evaporate into the air, raising the dew point of this stable layer, accelerating radiation fog formation.

Hilly fog (Cheyenne fog)

This type occurs when sloping terrain lifts air, cooling it adiabatically to its dew point and saturation. Upslope fog may be viewed as either a stratus cloud or fog, depending on the point of reference of the observer. Upslope fog generally forms at the higher elevations and builds downward into valleys. This fog can maintain itself at higher wind speeds because of increased lift and adiabatic cooling. Upslope winds more than 10 to 12 knots usually result in stratus rather than fog. The east slope of the Rocky Mountains is a prime location for this type of fog.

Frontal fog

Associated with frontal zones and frontal passages, this type of fog can be divided into three types: warm-front pre-frontal fog; cold front post-frontal fog; and frontal-passage fog. Pre and post-frontal fog are caused by rain falling into cold stable air thus raising the dew point. Frontal passage fog can occur in a number of situations: when warm and cold air masses, each near saturation, are mixed by very light winds in the frontal zone; when relatively warm air is suddenly cooled over moist ground with the passage of a well marked precipitation cold front; and in low-latitude summer, where evaporation of frontal-passage rain water cools the surface and overlying air and adds sufficient moisture to form fog.

Ice fog

Ice fog is composed of ice crystals instead of water droplets and forms in extremely cold, arctic air (–29°C (–20°F) and colder). Ice fog of significant density is found near human habitation, in extremely cold air, and where burning of hydrocarbon fuels add large quantities of water vapor to the air. Steam vents, motor vehicle exhausts, and jet exhausts are major sources of water vapor that produce ice fog. A strong low level inversion contributes to ice fog formation by trapping and concentrating the moisture in a shallow layer.

CLOUDS

Clouds are visible mass of condensed water vapor floating in the atmosphere, typically high above the ground.

TYPES OF CLOUDS

Types of clouds categorized in term of height as follow,

• High clouds

• Medium clouds

• Lower clouds

High clouds

Types of clouds according to High clouds are

Cirrus

Cirrus are detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands. They may have a fibrous (hair-like) and/or silky sheen appearance. Cirrus clouds are always composed of ice crystals, and their transparent character depends upon the degree of separation of the crystals. As a rule when these clouds cross the sun’s disk they hardly diminish its brightness. Before sunrise and after sunset, cirrus is often colored bright yellow or red. These clouds are lit up long before other clouds and fade out much late.

Cirrostratus

Cirrostratus are transparent, whitish veil clouds with a fibrous (hair-like) or smooth appearance. A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky. A milky veil of fog (or thin Stratus) is distinguished from a veil of Cirrostratus of a similar appearance by the halo phenomena which the sun or the moon nearly always produces in a layer of cirrostratus.

Cirrocumulus

Cirrocumuluses are thin, white patch, sheet, or layered of clouds without shading. They are composed of very small elements in the form of more or less regularly arranged grains or ripples. In general Cirrocumulus represents a degraded state of cirrus and cirrostratus both of which may change into it and is an uncommon cloud. There will be a connection with cirrus or cirrostratus and will show some characteristics of ice crystal clouds.

Medium Clouds

Types of clouds according to medium clouds are

Altostratus

Altostratus are gray or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky. They are thin enough to regularly reveal the sun as if seen through ground glass. Altostratus clouds do not produce a halo phenomenon nor are the shadows of objects on the ground visible. Sometime virga is seen hanging from Altostratus, and at times may even reach the ground causing very light precipitation.

Altocumulus

Altocumulus are white and/or gray patch, sheet or layered clouds, generally composed of laminae (plates), rounded masses or rolls. They may be partly fibrous or diffuse. When the edge or a thin semitransparent patch of altocumulus passes in front of the sun or moon a corona appears. This colored ring has red on the outside and blue inside and occurs within a few degrees of the sun or moon.

Nimbostratus

Nimbostratus are continuous rain cloud. Resulting from thickening Altostratus, This is a dark gray cloud layer diffused by falling rain or snow. It is thick enough throughout to blot out the sun. The cloud base lowers into the low level of clouds as precipitation continues. Also, low, ragged clouds frequently occur beneath this cloud which sometimes merges with its base.

Lower Clouds

Types of clouds according to lower clouds are

Cumulus

Cumulus are detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes or towers with bulging upper parts often resembling a cauliflower. The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark and horizontal. Over land cumulus develops on days of clear skies, and is due diurnal convection; it appears in the morning, grows, and then more or less dissolves again toward evening.

Stratus

Stratus are generally gray cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible. Often when a layer of Stratus breaks up and dissipates blue sky is seen.

Cumulonimbus

Cumulonimbus are thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume. Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base. They produce precipitation, which sometimes is in the form of virga. Cumulonimbus clouds also produce hail and tornadoes.

Stratocumulus

Stratocumulus are  gray or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls. Except for virga they are non-fibrous and may or may not be merged. They also have regularly arranged small elements with an apparent width of more than five degrees (three fingers – at arm’s length).

ATMOSPHERIC PRESSURE

The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth. 

 Atmospheric pressure is the force with which atmosphere presses down on a unit area.

DISTRIBUTION OF ATMOSPHERIC PRESSURE

The distribution of atmospheric pressure is not uniform over the earth’s surface. It varies vertically as well as horizontally.

Factors causes un even distribution of atmospheric pressure

• Altitude

• Latitude

• Rotation of the earths

• Over head of the sun

• Temperature

RECORDING AND MEASUREMENTS OF ATMOSPHERIC PRESSURE

Atmospheric pressure is commonly measured with an instrument called barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises. 

There are two types of barometer, mercury barometer and aneroid barometer.

Atmospheric pressure recorded on a map which has lines joining all point called isobars.

WIND SYSTEMS

Wind is air in motion from high pressure to low pressure area. Wind made up of variety of gases, such as oxygen and carbon dioxide. 

The wind at the surface experiences friction. Rotation of the earth affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force. Thus, the horizontal winds near the earth surface respond to the combined effect of three forces – the pressure gradient force, the frictional force and the Coriolis force.

FACTORS AFFECTING WIND SYSTEMS (Wind direction, velocity or speed and strength)

• Pressure Gradient Force

• Frictional Force

• Coriolis Force

• Temperature

• Centripetal force

Pressure Gradient Force

The differences in atmospheric pressure produce a force. The rate of change of pressure with respect to distance is the pressure gradient. The pressure gradient is strong where the isobars are close to each other and is weak where the isobars are apart.

Frictional Force

It affects the speed of the wind. It is greatest at the surface and its influence generally extends upto an elevation of 1 – 3 km. Over the sea surface the friction is minimal.

Coriolis Force

The rotation of the earth about its axis affects the direction of the wind. This force is called the Coriolis force after the French physicist who described it in 1844. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high. The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator.

The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind. As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it. At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified. That is the reason why tropical cyclones are not formed near the equator.

Temperature

Air temperature varies between day and night and from season to season due to changes in the heating Earth’s atmosphere. Because of the sun’s warming effect, there are more winds during the day. Air masses also differ in temperature. A warm front precedes a warm air mass. Warm air is less dense than cold air, so warm air rides up and over the cold air, causing winds. Converselt, a cold front, the leading edge of a cold air mass, also creates wind.

Centripetal

Centripetal force

Centripetal force increases air speed and influences the direction of wind flowing around the center of the circulation. This acceleration creates a force at right angles to the flow of the wind and inward toward the center of the rotation, such as low and high pressure systems. The winds in a low pressure system, called cyclones, blow in a counterclockwise and inward direction in the Northern Hemisphere. Winds in high pressure systems, known as anticyclones, blow in a clockwise and outward direction in the Northern Hemisphere.

PRESSURE AND WIND

The velocity and direction of the wind are the net result of the wind generating forces. The winds in the upper atmosphere, 2 – 3 km above the surface, are free from frictional effect of the surface and are controlled mainly by the pressure gradient and the Coriolis force. When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.

The wind circulation around a low is called cyclonic circulation. Around a high it is called anti cyclonic circulation. The direction of winds around such systems changes according to their location in different hemispheres.

The wind circulation at the earth’s surface around low and high on many occasions is closely related to the wind circulation at higher level. Generally, over low pressure area the air will converge and rise. Over high pressure area the air will subside from above and diverge at the surface.

Apart from convergence, some eddies, convection currents, orographic uplift and uplift along fronts cause the rising of air, which is essential for the formation of clouds and precipitation.

CATEGORIES OF WIND SYSTEMS

Wind systems classified as follows,

• Local winds

• Monsoon winds

• Jet streams
• Planetary winds and

• Air masses

LOCAL WINDS

Local winds are small scale convective winds of local origin caused by temperature differences.

Differences in the heating and cooling of earth surfaces and the cycles those develop daily can create several local winds.

Ways that Local Winds Develop or originated

Some ways in which local winds develop are: 

1. Convection from daytime heating.
2. Unequal heating and cooling of the surface.
3. Gravity, including downdrafts.

TYPES OF LOCAL WINDS

A. Land breeze and Sea breeze

Land breeze

Land breeze is the movements of air from the land towards the sea that occur during night.

During the night the sea heats up faster and becomes warmer than the land. Therefore, over the sea the air rises giving rise to a low pressure area, whereas the land is relatively cool and the pressure over land is relatively high. Thus, pressure gradient from land to sea is created and the wind blows from the land to the sea as the land breeze.

Picture from https://i.pinimg.com/originals/23/d7/b5/23d7b5568e98236a0a31cf1954cfe6dc.gif

Sea breeze

Sea breeze is the movements of air from the sea towards the land that occur during the day time.

During the day the land heats up faster and becomes warmer than the sea. Therefore, over the land the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is relatively high. Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land as the sea breeze.

1. Katabatic winds and  Anabatic winds

Katabatic winds

Katabatic wind is the movements of air from the mountain downwards the valley during the nights.

When air over sloped terrain is cooled by conduction it becomes denser than near free air and drains to lower levels. The winds generated are known as katabatic winds. 

Katabatic winds depend on

• The degree of cooling along the slope (the colder the surface, the greater the potential for the generation of very dense air and hence greater wind speed)

• The roughness of the slope (the smoother the slope the greater the potential for uninterrupted and thus stronger flow)

• The steepness of the slope (gentle slopes are more favorable for katabatic development because steep slopes cause the wind to become turbulent, resulting in mixing with surrounding air and the consequential breakdown of continual downward movement of cold air).

Anabatic winds

Anabatic wind is the movements of air from the valley towards the mountain during the day.

 Air in contact with a slope warms by conduction and ascends (not necessarily following the slope). Such ascending winds are called anabatic winds. The upward flow will be strongest in the early afternoon and over sun facing slopes.

• Descending winds

Its dry and warm air moving downwards in the leeward sides of the mountain.

• Desert winds

Its dry and dusty winds which blow from the desert where air pressure is high.

• Convectional Winds

Are the strong air currents rising upward and mostly carry dust.

MONSOON WINDS

The type of wind system in which there is a complete or almost complete reversal of prevailing direction from season to season is known as the monsoon winds. The largest and best developed monsoonal area in the world is the South- East Asia including India. During the summer (April to September) the interior of the continents is intensely heated. This creates a low pressure into which winds are drawn from over the cooler surrounding oceans. In India South-west monsoon wind blows in summer. As the air from over the oceans is very moist, it results in heavy rainfall. During the winter (October to March) the continental interior becomes much cooler than the surrounding oceans; the wind direction is thus reversed, blowing from the continental high pressure to the low pressure over the oceans. This creates the North-East monsoon over India, which is generally a cool and dry wind.

JET STREAMS

 A jet stream is an area of strong winds ranging from 120-250 mph that can be thousands of miles long, a couple of hundred miles across and a few miles deep.  Jet streams usually sit at the boundary between the troposphere and the stratosphere at a level called the tropopause.  This means most jet streams are about 6-9 miles off the ground. 

PLANETARY WINDS

Planetary winds are the distribution of winds throughout the lower atmosphere. Confined within some latitudinal belts, these winds blow rather regularly throughout the year and are basically controlled by the latitudinal pressure belts.

The main planetary winds are

• The North-east and the South-east Trade winds

• The Temperate Westerlies and

• The Polar Easterlies, which blow from the polar high pressure area to the temperate low pre sure area.

Trade winds

Trade winds blow in a belt lying between 5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere. From the equator ward side of the Sub-tropical highs in the Northern hemisphere air flows towards the Equatorial low but it is deflected right according to Ferrle’s law and as a result instead of blowing as northerly wind, it bends westward to become North-east trade winds. In the Southern hemisphere winds originating from the Sub-tropical high pressure and blowing towards the Equatorial low pressure are similarly deflected westward to become the prevailing South-east trades. Trade winds are noted for their consistency, both in force and direction in many areas especially over open seas and derive their name from the nautical expression ‘to blow trade’ meaning ‘to blow along a regular track’. Zones of sub-tropical highs in latitudes about 30°-35°N and 30°-35°S are areas of descending air and are characterize by calms light variable winds, comparatively dry air and quiet, stable weather conditions. These zones of latitudes are called Horse latitude.

The Westerlies

The Westerly winds blow across latitudes 35°-60° of both hemispheres. The air streams flowing pole wards from the Sub-tropical high pressure areas deflects eastward in the Northern he sphere to form South-westerlies. Similar winds in the Southern hemisphere are known as North-westerlies. Unlike the trade winds, the westerlies are very variable in force and direction especially in the Northern hemisphere. In the Southern hemisphere, on the other hand, the Westerlies blow with great strength and regularity throughout the year over the almost uninterrupted expanse of ocean and have given the name Roaring forties to the region specially between latitudes 40°S and 50°S. Sometimes the name is applied to the winds themselves as they give a roaring sound on account of high speed.

The Polar easterlies

The Polar easterlies blow from the Polar high pressure area to the Temperature low pressure area. On their equator ward journey they are deflected westward to become North easterlies in the Northern hemisphere and South easterlies in the Southern hemisphere.

AIR MASSES

Air mass is defined as a large body of air having little horizontal variation in temperature and moisture.

When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an air mass.. The homogenous surfaces, over which air masses form, are called the source regions.

CLASSIFICATION OF AIR MASSES

The air masses are classified according to the source regions.

There are five major source regions. These are:

• Warm tropical and subtropical oceans;

• The subtropical hot deserts;

• The relatively cold high latitude oceans;

• The very cold snow covered continents in high latitudes;

•  Permanently ice covered continents in the Arctic and Antarctica.

TYPES OF AIR MASSES

Accordingly, following types of air- masses are recognised:

• Maritime tropical (mT);

• Continental tropical (cT);

• Maritime polar (mP);

• Continental polar (cp);

• Continental arctic (cA)

Tropical air masses are warm and polar air masses are cold.

Fronts

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis.

There are four types of fronts:

(a) Cold front;

(b) Warm front;

(c) Stationary front;

(d) Occluded front.

When the front remains stationary, it is called a stationary front. When the cold air moves towards the warm air mass, its contact zone is called the cold front, whereas if the warm air mass moves towards the cold air mass, the contact zone is a warm front. If an air mass is fully lifted above the land surface, it is called the occluded front. The fronts occur in middle latitudes and are characterized by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

EFFECTS OF AIR MASSES ON CLIMATE

• Air masses also form fronts, along which much of the world’s major weather action travels.

• It regulates temperature of a place

• It cause occurrence of precipitation i.e cyclonic rainfall.

CYCLONES AND ANTICYCLONES

A. Cyclones (Depression)

A cyclone is any low pressure system that spins counter-clockwise (in the Southern Hemisphere it spins clockwise). Cyclones range in size from tiny whirlwinds to thunderstorms to hurricanes to synoptic scale lows.

Low pressure cyclones that form outside of tropical regions are known as extra-tropical or mid latitudes cyclones. These are the weather systems that bring us snow storms in the winter and lines of severe thunderstorms in the summer. The cyclones are usually associated with frontal zones, separating the moist air ahead of the front from the dry air behind the front.

Air spirals counter-clockwise at the surface into a cyclone. The air is then forced upward by convergence. This rising air is what leads to the clouds and storms associated with the cyclone.

TYPES OF CYCLONES

There are two types of cyclones, namely,

• Temperate depression and

• Tropical cyclones

Temperate depression

Temperate depression occurs in region along the polar front where warm westerlies and cold polar winds converge. When a local low-pressure zone develops somewhere along the front, the front bulges. It forms within 30‎° and 50‎° latitude during winter months and grows into massive spiralling storms which can be 1000 miles wide.

Extra tropical cyclones form along the polar front. Initially, the front is stationary. In the northern hemisphere, warm air blows from the south and cold air from the north of the front. When the pressure drops along the front, the warm air moves northwards and the cold air move towards, south setting in motion an anticlockwise cyclonic circulation. The cyclonic circulation leads to a well developed extra tropical cyclone, with a warm front and a cold front.

Tropical cyclones

Tropical cyclones  are those which form in tropical areas. These start off as tropical depressions, then grow into tropical storms, and may eventually strengthen into hurricanes.

Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.

This is one of the most devastating natural calamities. They are known as

• Cyclones in the Indian Ocean,

• Hurricanes in the Atlantic,

• Typhoons in the Western Pacific and South China Sea, and

• Willy-willies in the Western Australia.

• Tornadoes in west Africa

Tropical cyclones originate and intensify over warm tropical oceans.

Conditions for the formation and intensification of tropical storms

The conditions favourable for the formation and intensification of tropical storms are:

• Large sea surface with temperature higher than 27° C;

• Presence of the Coriolis force;

• Small variations in the vertical wind speed;

• A pre-existing weak-low-pressure area or low-level-cyclonic circulation;

•  Upper divergence above the sea level system.

FORMATION OF DEPRESSION

Depression are formed in the temperate latitudes (i.e 60°N and 60°S) when humid tropical air meet cold polar air.

• Western winds meet polar winds.
• The zone where two different currents meets is called polar fronts

DEVELOPMENT OF A DEPRESSION

Stages in development of depression

There are three stages of development of depression, namely,

• Embryo depression
• Mature depression
• Decay depression

Embryo depression

The first stage is known as embryo stage

• In this stage Cold air moves in a generally westerly direction along the polar front.

• In this stage tropical air moves in a generally Easterly direction.

• The friction effects of the two air flows cause a wave to develop in the front.

Mature depression

In mature depression the cold front starts to catch up with the warm air.

• The wave bulges into the colder air and gets larger.

• Pressure falls at the top of the wave and due to the coriolis force wind blows around the low pressure point in clockwise direction in the southern hemisphere and anticlockwise in the northern hemisphere.

Decay depression

In decay depression the cold front catch up the warm air to form an occlusion or occluded front.

• No warm sector left of ground level.

• Less uplift -less condensation -less latent heat- less ppt – less cloud -pressure rises -wind decreases.

• Cold air replaces uplifted air and infills the depression.

WEATHER ASSOCIATED WITH DEPRESSION

The weather associated with the passage of a depression is as follow,

1. Passage of the warm front

Clear weather with a few high cirrus clouds. Winds will blow from southern west for a while. A cloud cover grows light show begin and grow heavier.

2. The warm sector

The warm sector is the area between the two fronts when it approaches and passes a place the rain stops, the weather clears temperature rise, the air is humid and the winds changes from southern east  to southern west.

3. Passage of the cold front

The weather changes rapidly as the cold front passes, the wind blows from the northern west, temperature falls, and there is heavy rain and thunderstorms with cumulonimbus clouds.

4. Passages of the depression

The sky clear and the temperature remain cool.

B. Anticyclones

Anticyclone is a region of atmospheric high pressure and associated with light winds. Both cyclones and anticyclones are manifested by circular oval or wedge shaped arrangement of isobar with lowest pressure at the centre in case of cyclones and highest pressure at the centre in case of anticyclones.

The law governing here known as Buys Ballot’s law, states that “in the northern hemisphere winds move in an anticlockwise direction around the centre of low pressure and clockwise around centers of high pressure; in the southern hemisphere the reverse is true.”

Both cyclones and anticyclones move as systems, cyclones moving much faster than the anticyclones. As a result the wind directions at a place shift with the passage of a cyclone or an anticyclone.

CLIMATIC HAZARDS

A climatic hazard is a climatic phenomenon that might have a negative effect on humans or the environment.

The following are the among of climatic hazard,

Thunderstorms and Lighting

Thunderstorms are caused by intense convection on moist hot days.

A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightening. When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorm. If there is insufficient moisture, a thunderstorm can generate dust- storms. A thunderstorm is characterised by intense updraft of rising warm air, which causes the clouds to grow bigger and rise to greater height. This causes precipitation. Later, downdraft brings down to earth the cool air and the rain. From severe thunderstorms sometimes spiralling wind descends like a trunk of an elephant with great force, with very low pressure at the centre, causing massive destruction on its way.

Tornadoes

Tornadoes occur in middle latitudes. The tornado over the sea is called water spouts.

These violent storms are the manifestation of the atmosphere’s adjustments to varying energy distribution. The potential and heat energies are converted into kinetic energy in these storms and the restless atmosphere again returns to its stable state.

Hurricanes

Hurricanes are large, swirling storms. They produce winds of 119 kilometers per hour (74 mph) or higher. Winds from a hurricane can damage buildings and trees.

Hurricanes form over warm ocean waters. Sometimes they strike land. When a hurricane reaches land, it pushes a wall of ocean water ashore. This wall of water is called a storm surge. Heavy rain and storm surge from a hurricane can cause flooding.

Strong winds e.g. typhoon in china

Strong winds are the most common means of destruction associated with hurricanes. Their sometimes continuous barrage can uproot trees, knock over buildings and homes, fling potentially deadly debris around, sink or ground boats, and flip cars.

Willy willies

Willy-willies are a severe tropical cyclone or a whirlwind that takes place over a desert.

They are visually defined by their entrainment and transport of surface debris but little is known of their sediment transport role in the landscape.

Ice storms

An ice storm is a type of winter storm characterized by freezing rain.

Drought

A drought is a period of below-average precipitation in a given region; resulting in prolonged shortages in the water supply, whether atmospheric, surface water or ground water. A drought can last for months or years.

CLIMATIC CHANGE

Climate change is a change in the usual weather found in a place. This could be a change in how much rain a place usually gets in a year. Or it could be a change in a place’s usual temperature for a month or season.

CAUSES OF CLIMATIC CHANGE

• Variation of solar energy

• Variation in atmospheric Carbon dioxide

• Volcanic eruption

•  Changes in oceanic circulation

• Composition of the atmosphere Gases in the atmosphere can be increased.

• Excessive rainfall

• Ultra violet radiation increases.

EVIDENCES OF PAST CLIMATE CHANGE

Rocks

The characteristics of different rocks depend on the environment in which the sediments were deposited.

Some sands and gravels are dropped by glaciers as they melt and they become a distinctive rock called till.

Where till is found there must have been glaciers and therefore it must have been cold.

Rocks that form in a hot desert environment are often coloured red with iron deposits.

Also at high temperatures, sea water can evaporate quickly leaving behind a layer of salt on the ground which becomes preserved in the rocks and is another indicator of a hot climate.

Fossils

Different species of plants and animals need different conditions to survive. Some plants and animals can be very sensitive to climate and do not adapt easily to change.

For example coral reefs live in tropical waters. They need a particular temperature, a specific depth of water and the right amount of light. If the depth of the water changes just a fraction, they cannot survive.

Therefore where fossil corals are found it is possible to estimate fairly precisely the environment they lived in by assuming that they needed the same conditions as those that thrive today.

Pollen and spores

Plants produce pollens and spores that are particularly useful in helping to determine climate. They are tiny with a resistant outer case and are produced in millions.

This means that they can be covered in mud quickly and are more easily preserved as fossils than large animals.

Each plant has different shaped pollen or spores so when the fossil is put under a microscope it is possible to identify the type of plant it came from.

Different plants are adapted to different climates therefore looking at all the types of pollen present in a layer of rock can be a good indication of the climate at the time when they were living.

Shrinking ice sheets

The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA’s Gravity Recovery and Climate Experiment show Greenland lost 150 to 250 cubic kilometers (36 to 60 cubic miles) of ice per year between 2002 and 2006, while Antarctica lost about 152 cubic kilometers (36 cubic miles) of ice between 2002 and 2005.

Sea level rise

Global sea level rose about 8 inches in the last century. The rate in the last two decades, however, is nearly double that of the last century.

GLOBAL WARMING PHENOMENA

 Global warming refers to the situation in which the atmosphere traps and retains heat energy from the sun in the lower level leading to the rise in temperature.

Due to the presence of greenhouse gases, the atmosphere is behaving like a greenhouse. The atmosphere also transmits the incoming solar radiation but absorbs the vast majority of long wave radiation emitted upwards by the earth’s surface. The gases that absorb long wave radiation are called greenhouse gases. The processes that warm the atmosphere are often collectively referred to as the greenhouse effect.

EFFECTS OF GLOBAL WARMING

• The rise in temperature

• The melting of ices

• Occurrence of strong storms

• Disappearance of some animals and plant species 

• Occurrence of precipitation 

• Decline of production due to drought and desertification process

• Occurrence of disease example cancer

MITIGATING MEASURES AGAINST GLOBAL WARMING

• Discouraging the uses of burning of material that release harmful greenhouse gases such as CO 2, CFC’s.

• Use alternative sources of energy

• Formation of an international policies and cooperation among different nations in the fight against air pollution.

CLIMATE CLASSIFICATION

Climate classifications are orderly arrangements of data dealing with climatic controls and elements. The purpose of such schemes is to identify climate types and subtypes. Maps and graphs display the climates (for example, wet tropical climates). The classifications typically identify climate regions and subregions that cover broad areas that are subcontinental in size.

KOEPPEN’ S CHEME OF CLASSIFICATION OF CLIMATE

The most widely used classification of climate is the empirical climate classification scheme developed by V. Koeppen.  Koeppen identified a close relationship between the distribution of vegetation and climate. He selected certain values of temperature and precipitation and related them to the distribution of vegetation and used these values for classifying the climates. It is an empirical classification based on mean annual and mean monthly temperature and precipitation data. He introduced the use of capital and small letters to designate climatic groups and types. Although developed in 1918 and modified over a period of time, Koeppen’s scheme is still popular and in use.

Koeppen recognised five major climatic groups; four of them are based on temperature and one on precipitation. Table 1.0 lists theclimatic groups and their characteristics according to Koeppen. The capital letters: A,C, D and E delineate humid climates and B dry climates.

The climatic groups are subdivided into types, designated by small letters, based on seasonality of precipitation and temperature characteristics. The seasons of dryness are indicated by the small letters: f, m, w and s, where f corresponds to no dry season,

m – Monsoon climate, w- winter dry season and s – summer dry season. The small letters a, b, c and d refer to the degree of severity of temperature. The B- Dry Climates are subdivided using the capital letters S for steppe or semi-arid and W for deserts. The climatic types are listed in Table 12.2. The distribution of climatic groups and types is shown in Table 1.1.

Table 1.0: Climatic Groups According to Koeppen

Group	Characteristics
A – Tropical     B – Dry Climates   C – Warm Temperate     D – Cold Snow Forest Climates     E – Cold Climates   H – High Land	Average temperature of the coldest month is 18° C or higher   Potential evaporation exceeds precipitation   The average temperature of the coldest month of the (Mid-latitude) climates years is higher than minus 3°C but below 18°C   The average temperature of the coldest month is minus 3° C or below   Average temperature for all months is below 10° C   Cold due to elevation

Table 1.I : Climatic Types According to Koeppen

Group	Type	Letter Code	Characteristics
A-Tropical Humid Climate	Tropical wet  Tropical monsoon   Tropical wet and dry	Af Am   Aw	No dry season Monsoonal, short dry season Winter dry season
B-Dry Climate	Subtropical steppe   Subtropical desert   Mid-latitude steppe    Mid-latitude desert	BSh   BWh   BSk   BWk	Low-latitude semi arid or dry Low-latitude arid or dry   Mid-latitude semi arid or dry Mid-latitude arid or dry
C-Warm temperate (Mid-latitude) Climates	Humid subtropical   Mediterranean    Marine west coast	Cfa   Cs   Cfb	No dry season, warm summer   Dry hot summer   No dry season, warm and cool summer
D-Cold Snow-forest Climates	Humid continental   Subarctic	Df   Dw	No dry season,severe winter   Winter dry and very severe
E-Cold Climates	Tundra   Polar ice cap	ET    EF	No true summer   Perennial ice
H-Highland	Highland	H	Highland with snow cover

Merits of Koppen’s Scheme

• It is simple.

• It is quantitative because it has used numerical values to define boundaries of climatic groups.

• It makes it easy to ascribe a given place to a particular climate sub-group on the basis of temperature and precipitation.

Demerits of Koppen’s Scheme

• It is inconsistent because he used mean temperature for his A, C, D and E zones; whereas his zone B is based on a precipitation – evaporation ratio.
  
  
• It is not comprehensive enough because it has not taken case of the climates of mountainous regions and regions affected by fog.
  
  
• The boundaries of Koppen’s climate types are too strictly empirical.
  
  
• Koppen’s criteria for identifying dry climates are too simple to be of much use.

TYPES OF CLIMATE

Five types of climate are

• Hot climates

• Warm climates

• Cool climates

• Cold climates and

• Arctic climates

Hot climates

Hot climates are the type of climates found within the tropics, mainly between 23½° north and 23½° south of the equator.

Hot climates include the following climate sub types

• Equatorial climate

•  Tropical continental climate

• Tropical monsoon climate

• Tropical marine climate

• Tropical desert climate

Warm climates

Warm climates border the hot tropical deserts. They occur between 300 and 400 north and south of the equator.

There are four broad types of warm climates:

• Warm temperature western margin.

• Warm temperature continental.

• Warm temperature eastern margin.

• Warm temperature desert. 

Cool climates

Cool climates are experienced in regions between 350 norths and 600 south of the equator. They are characterized by definite seasonal variations in temperature.

There are four types of cool climates:

• Cool temperate continental (British type).

• Cool temperate continental (Siberian type).

•  Cool temperate eastern margin (Laurentian type).

• Temperate desert. 

Cold climates

Cold climates are mainly experienced in regions between latitudes 600 N and 680 N.

There are three types of cold climates:

•  Cold temperate western margin.

• Cold temperate eastern margin.

• Cold temperate continental.

Arctic climates

 These types of climates are experienced in regions beyond the Arctic Circle (661/20 N) and around Arctic Ocean. They are also known as polar deserts.

The main features of these climates are

• Low amounts of precipitation (rain),
• mild summers and
• Very cold winters.

Arctic climates comprises of Tundra and Polar climates Tundra climate Location This region occurs in the northern coast of North America, southern coast Greenland and the Arctic coast of Europe and Asia.

There are two types of Arctic climates are

• Tundra climate and

• Polar climate

VEGETATION

Vegetation is the collective name for plants in a particular area. It can be natural (man have not planted them) or artificial (planted by man).

Factors which influence vegetation

There are five major factors which influence the nature and growth of vegetation. These are: 

• Climatic factors, 

• soils, 

• Geomorphic factors

• Human activities and 

• Biotic factor.

Where the factors are favorable, plants grow well, where they are not, plant growth is slowed down.

Climatic factors

These include temperature, rainfall, insolation and winds.

• The most critical of these factors are temperature and rainfall and a proper balance must be maintained between the two for plants to grow well. An example, in cold lands,plants growth is impossible below 6°C no matter how heavy the rainfall is. That is why, in the temperate lands, plants grow well only in the summer when temperatures rise beyond this critical level, but shed their leaves and remain dormant in the winter when temperature drop below 6°C. In the cold tundra region, there is no plant life most part of the year.

In hot dry lands where the temperature is adequate all the year round for plant growth, rainfall becomes the critical factor. In these lands, where rainfall is light and below 25cm per annum, even this little amount is lost by evaporation, so deserts prevail. Where it is moderate and seasonal, grasslands occur while heavy rainfall all year round gives rise to forest.

Temperature and rainfall affect not only the density of the plant cover, but also the types and abundance of plant species. Certain trees like Iroko, Obeche and Mahogany are tropical species because they tolerate high temperatures and heavy rainfall. Pines, oak and poplar, which can thrive in lower temperatures, are found in the temperate region. Also, the colder the climate, the fewer the plant species that can survive. Hotter, wetter climates have infinitely more plant species.

• Winds affect vegetation too.

Persistent strong winds in one direction may permanently disfigure trees and bend them in the direction may permanently disfigure the trees and bend them in the direction of the winds. In the African savannas, strong winds from diverse directions force the plants to adopt and umbrella shape and thus show a thin edge to the wind.

• Sunlight also affects vegetation.

As we know, plants need sunlight to grow well. That is why the leaves and branches always grow towards the source of light. We say that they are positively heliotrophic.

It is for these reasons that, in the tropical rainforests, some trees pierce through dense canopy of the leaves of the leaves of other trees and grow really tall in search of sunlight. It is for this reason too, that farmer cut down some trees and lob off branches of others on their farms before they plant their crops. This makes the crops get enough sunlight to grow well.

Soils

Soil factors, otherwise called edaphic factors, are also relevant. The actual thing about rainfall that matters to plants is not the amount that has fallen, but the quantity that remains in the soil to become available to plant roots. The nature of relief and soils is therefore important to vegetation in this regard.

A soil that retains water supports denser vegetation than one that does not . Also, different types of soils determine the types of plants that grow on them. Thus, oil palms grow well on the acidic soils of eastern Nigeria, while a mangrove thrives on the waterlogged soild of the Niger Delta.

Geomorphic factors

Geomorphic factors, affect plant growth in at least three ways.

• First, where every other thing is equal, steep slopes, which have rapid run-off, have less dense vegetation than more time to sleep into the soil.

• However, since we know that highlands induce orographic precipitation, the windward side of highlands, where more rain falls, supports denser vegetation than the drier leeward side. That is why, for instance, the Adamawa Highlands in Nigeria have richer vegetation than the leeward side of Cameroun Mountains.

• Thirdly, very high mountains have an arrangement of vegetation all their own. The warmer slopes with deeper soils are normally forested. The middle slopes have shrubs and grass, while temperate ferns predominate on the cold hilltops where the soils are thin. Some mountain peaks, even in the equatorial region, like the Kilimanjaro Mountain (always) and the Cameroun Mountain (sometimes), are clad with snow.

Human activities

Man can destroy as well as grow plants. He cuts down trees to give way to roads, houses, factories and other cultural features. He burns bushes for farming, and harvests timber from the forests.

However, man also alters the plant cover of a place by planting more trees either as ornamental, crop plantations, farms or forest reserves.

Biotic factors

The biotic factors include the influence of living organisms, both plants and animals upon the vegetation.

Any activity of the living organism which may cause marked effects upon vegetation in any way is referred to as biotic effect.

The biotic effect may be both direct and indirect. It may be beneficial to the plants in some respects but detrimental in other respects.

PLANT COMMUNITY AND ASSOCIATION

PLANT COMMUNITY

Plant community is a group of plants sharing a common environment that interact with each other.

Certain plant communities often occur together on the landscape due to shared environmental requirements. This provides a way to organize biological information, creating mappable units for land management and conservation planning.

PLANT ASSOCIATION

Plant association is a grouping of  plant species, or a plant community, that recurs across the landscape. Plant association are used as indicator of environmental conditions such as temperature, moisture,light, etc

PLANT SUCCESSION

Plant succession is a directional non-seasonal cumulative change in the types of  plant species that occupy a given area through time.

It involves the processes of colonization, establishment, and extinction which act on the participating plant species. Most successions contain a number of stages that can be recognized by the collection of species that dominate at that point in the succession. Succession begins when an area is made partially or completely devoid of vegetation because of a disturbance .

Some common mechanisms of disturbance are

• Fires,
• wind storms,
• volcanic eruptions,
•  logging,
• climate change,
• severe flooding,
• disease, and
•  Pest infestation.

Succession stops when species composition changes no longer occur with time, and this community is said to be a climax community.

The concept of a climax community assumes that the plants colonizing and establishing themselves in a given region can achieve stable equilibrium. 

TYPES OF PLANT SUCCESSION

Types of Plant succession are

Primary succession

Primary succession is the establishment of plants on land that has not been previously vegetated.

Begins with colonization and establishment of pioneer species.

Secondary succession

Secondary succession is the invasion of a habitat by plants on land that was previously vegetated.

Removal of past vegetation may be caused by natural or human disturbances such as fire, logging, cultivation, or hurricanes.

Allogenic succession

Allogenic succession is caused by a change in environmental conditions which in turn influences the composition of the plant community.

Autogenic succession

Autogenic succession is a succession where both the plant community and environment change, and this change are caused by the activities of the plants over time.

Progressive succession

Progressive succession is a succession where the community becomes complex and contains more species and biomass over time.

Retrogressive succession

Retrogressive succession is a succession where the community becomes simplistic and contains fewer species and less biomass over time. Some retrogressive successions are allogenic in nature.

For example, the introduction of grazing animals results in degenerated rangeland.

FACTORS INFLUENCING PLANT SUCCESSION

Light

Light requirements differ for many plants. During succession, light can influence which plants can most readily colonize an area following a disturbance.

Soil

The type of soil can dictate which plants are able to become established. Clay soils can hold more nutrients, and sand soils do not hold as much water.

Water

Water is essential to all forms of life; however, the required amount varies among species. Water quality and quantity can play a strong role in promoting or inhibiting species colonization.

Nutrients

Nutrients such as nitrogen and phosphorus are often limited in succession areas. Species with lower nutrient requirements have a greater chance of successfully maintaining a population.

Time

Ecological succession is a slow process, relying on soil formation and vegetation communities. The amount of time required to reach a stable ecosystem is related to the size and intensity of the most recent disturbance

ECOLOGY AND ECOSYSTEM

Ecology is the study of the relationships between plants, animals, people, and their environment.

Ecosystem is a system, or a group of interconnected elements, formed by the interaction of a community of organisms with their environment.

TYPES OF ECOSYSTEM

Two types of ecosystem are

• Terrestrial Ecosystem and
• Aquatic Ecosystem

Terrestrial Ecosystem

A terrestrial ecosystem is an ecosystem that exists on land, rather than on water. Such ecosystem is a community of organisms existing and living together on the land.

Aquatic Ecosystem

An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. 

FACTORS AFFECTING ECOSYSTEM

• Diseases

• Climate

• Human activities such as cultivation, mining, overgrazing etc

• Natural disaster such as earth quakes, land slide etc

• Death.

NATURAL VEGETATION

Natural vegetation is vegetation that has not been grown by humans. It doesn’t need help from humans and gets whatever it needs from its natural environment.

TYPES OF NATURAL VEGETATION

Some types of natural vegetation are

• Forests

• Grass lands and

• Desert and semi desert vegetation

Forests

A forest is a large area dominated by trees. 

Forests cover 1/3 of the earth’s surface. Forests exist in dry, wet, bitterly cold, and swelteringly hot climates. These different forests all have special characteristics that allow them to thrive in their particular climate.

TYPES OF FORESTS

Types of forest are

• Tropical evergreen forest

• Tropical monsoon forest

• Temperate evergreen forest

• Mediterranean vegetation

• Coniferous forest

Tropical evergreen forest

The tropical evergreen forests usually occur in areas receiving more than 200 cm of rainfall and having a temperature of 15 to 30 degrees Celsius. They occupy about seven per cent of the earth’s land surface and habours more than half of the world’s plants and animals. They are found mostly near the equator. 
These forests are dense and multi-layered. They harbour many types of plants and animals. The trees are evergreen as there is no period of drought. They are mostly tall and hardwood type. Leaves are broad and give out excess water through evapo-transpiration. 

Tropical monsoon forest

Tropical monsoon forests are founded in the tropics. However, they are only found in places that experience the tropical monsoon climate. The tropical monsoon climate is characterized by the high temperature and rainfall over than 1500 per year. Tropical monsoon forests are therefore located in South Asia, southern china, northern Australia and southern Asia. In the World map, tropical are mainly found between 100 and 250N and S of the equator.

Temperate evergreen forest

Temperate evergreen forests are found predominantly in areas with warm summers and cool winters, and vary enormously in their kinds of plant life. In some, needle leaf trees dominate, while others are home primarily to broadleaf evergreen trees or a mix of both tree types.

Temperate evergreen forests are common in the coastal areas of regions that have mild winters and heavy rainfall, or inland in drier climates or montane areas. Many species of trees inhabit these forests including pine, cedar, fir, and redwood.

Temperate rain forests only occur in 7 regions around the world – the Pacific Northwest, the Validivian forests of southwestern South America, the rain forests of New Zealand and Tasmania, the Northeastern Atlantic (small, isolated pockets in Ireland, Scotland, and Iceland), southwestern Japan, and those of the eastern Black Sea.

Mediterranean vegetation

Mediterranean vegetation is any scrubby, dense vegetation composed of broad leaved evergreen shrubs, bushes, and small trees usually less than 2.5 m (about 8 feet) tall and growing in regions lying between 30° and 40° north and south latitudes. These regions have a climate similar to that of the Mediterranean area, which is characterized by hot, dry summers and mild, wet winters. Around the Mediterranean Sea this vegetation is called macchie, maquis, or garigue; it is known as chaparral in southwestern North America, as Cape flora in southern Africa, and as mallee in southwestern Australia. 

Coniferous forest

Coniferous forests consist mostly of conifers, trees that grow needles instead of leaves, and cones instead of flowers. Conifers tend to be evergreen, that is, they bear needles all year long. These adaptations help conifers survive in areas that are very cold or dry. Some of the more common conifers are spruces, pines, and firs.

Precipitation in coniferous forests varies from 300 to 900 mm annually, with some temperate coniferous forests receiving up to 2,000 mm. The amount of precipitation depends on the forest location. In the northern boreal forests, the winters are long, cold and dry, while the short summers are moderately warm and moist. In the lower latitudes, precipitation is more evenly distributed throughout the year.

GRASSLAND

Grassland is an area of land that mostly contains grasses.

TYPES OF GRASSLAND

Types of Grassland are

• Tropical grassland

• Temperate grassland

Tropical grassland

Tropical grasslands are located near the equator, between the Tropic of Cancer and the Tropic of Capricorn. They cover much of Africa as well as large areas of Australia, South America, and India.

Tropical grasslands are dominated by grasses, often 3 to 6 feet tall at maturity. They may have some drought-resistant, fire-resistant or browse-resistant trees, or they may have an open shrub layer. They develop in regions where the climax community should be forest, but drought and fire prevent the establishment of many trees.

Temperate grassland

Temperate grasslands are located north of the Tropic of Cancer (23.5 degrees North) and south of the Tropic of Capricorn (23.5 degrees South). The major temperate grasslands include the veldts of Africa, the pampas of South America, the steppes of Eurasia, and the plains of North America.

Grasses are the dominant vegetation. Trees and large shrubs are largely absent. Seasonal drought, occasional fires and grazing by large mammals all prevent woody shrubs and trees from becoming established. A few trees such as cottonwoods, oaks and willows grow in river valleys, and a few hundred species of flowers grow among the grasses. The various species of grasses include purple needlegrass, blue grama, buffalo grass, and galleta. Flowers include asters, blazing stars, coneflowers, goldenrods, sunflowers, clovers, psoraleas, and wild indigos.

DESERT AND SEMI DESERT VEGETATION

The natural vegetation of poor coverage occurring in areas with no or little rainfall in a year.

TYPES OF DESERT AND SEMI DESERT VEGETATION

 Types of desert and semi desert vegetation are

• Tropical desert vegetation

• Semi desert and scrub

• Tundra

Tropical desert vegetation

The tropical desert is an environment of extremes: it is the driest and hottest place on earth. Rainfall is sporadic and in some years no measurable precipitation falls at all. The terribly dry conditions of the deserts are due to the year-round influence of subtropical high pressure and continentality. In tropical areas the heat enhances evaporation and the dryness conditions of the areas with little precipitation. Rain also occurs in a few events and quickly the moisture is absorbed by the soil or evaporated. These climatic conditions do not allow the geochemical processes of weathering to happen and most of the rock transformations are due to physical processes of contraction and expansion with the break of rocks in fragments. In this case, most of the desert surface is occupied by fragmented rocks, outcrop rocks or sand. Lack of vegetation, bare rock or sand, mountains and canyons, mesas or buttes, dunes, basins and playas, washes and arroyos, alluvial fans and generally angular topography are all deserts characteristics.

Semi desert and scrub

A transitional formation type situated between true desert and more thickly vegetated areas (e.g. between thorn forest and desert or between savannah and desert). The vegetation is sparser than that of the thorn forest and succulents are more common, as a consequence of the drier climate. Most of the plants are shallow-rooted, and so able to exploit before it evaporates any precipitation that percolates into the surface layer of the soil.

Tundra

Tundra is a major zone of treeless level or rolling ground found in cold regions, mostly north of the Arctic Circle (Arctic tundra) or above the timberline on high mountains (alpine tundra). Tundra is known for large stretches of bare ground and rock and for patchy mantles of low vegetation such as mosses, lichens, herbs, and small shrubs. This surface supports a meagre but unique variety of animals. The Finns called their treeless northern reaches the tunturi.

CLIMATOLOGY

Climatology is the science of studying the average atmospheric conditions of a region in long-term perspective.

The primary goal of Climatology is to study the unique characteristics of atmosphere in controlling the global climate, origin, types of climates, causes and processes influencing the climatic variations, elements of weather and the impact of climate on humans or vice-versa.

FACTORS INFLUENCING CLIMATE

There are lots of factors that influence our climate as follows,

Altitude
Normally, climatic conditions become colder as altitude increases. “Life zones” on a high mountain reflect the changes, plants at the base are the same as those in surrounding countryside, but no trees at all can grow above the timberline. Snow crowns the highest elevations.

Prevailing global wind patterns

There are 3 major wind patterns found in the Northern Hemisphere and also 3 in the Southern Hemisphere. These are average conditions and do not essentially reveal conditions on a particular day. As seasons change, the wind patterns shift north or south. So does the intertropical convergence zone, which moves back and forth across the Equator. Sailors called this zone the doldrums because its winds are normally weak.

Latitude and angles of the sun’s rays

As the Earth circles the sun, the tilt of its axis causes changes in the angle of which sun’s rays contact the earth and hence changes the daylight hours at different latitudes. Polar Regions experience the greatest variation, with long periods of limited or no sunlight in winter and up to 24 hours of daylight in the summer.

Surface of the Earth

Just look at any globe or a world map showing land cover, and you will see another important factor which has a influence on climate: the surface of the Earth. The amount of sunlight that is absorbed or reflected by the surface determines how much atmospheric heating occurs. Darker areas, such as heavily vegetated regions, tend to be good absorbers; lighter areas, such as snow and ice-covered regions, tend to be good reflectors. The ocean absorbs and loses heat more slowly than land. Its waters gradually release heat into the atmosphere, which then distributes heat around the globe.

Distance from the sea

Distance from the sea has an influence in both temperature and rainfall of a region. During summer onshore winds have cooling effect on the land which is warm. -During winter the sea is warmer than the land.

Ocean Currents

Current flowing along the Coasts tends to modify the climate of the Coastal regions. Where onshore wind blow over a cold ocean current are cooled from the below and the moisture they are carrying is condensed and dropped over the sea as rain.

THE NATURAL REGIONS OF THE WORLD

The natural regions of the world are divided on the basis of climate. The climate includes both temperature and rainfall in a given region.

Major natural regions of the world include the following

1. EQUATORIAL REGION

The region is found between 0° and 5° north and south of the equator but in some regions it may extend up to 10° north or south of the equator. Examples of areas found within this region include the Amazon basin, Congo basin, the southern Ivory Coast, south Ghana, western coastal Nigeria, and eastern coastal Malagasy Republic.

The climate experienced on equatorial region is called Equatorial climate.

The vegetations found in equatorial region are called tropical rain forest or tropical evergreen forest.

CHARACTERISTICS OF EQUATORIAL REGION

The following are characteristics of equatorial region

• There are no marked seasons of the year

• High temperature about 27°C throughout the year

• The annual temperature range is about 3°C.

• The daily mean temperatures are about 26°C all the year round.

• The daily temperature range is rarely more than 8°C because of the thick cloud cover.

• Rainfall is heavy and is usually convection rain.

• Rainfalls usually occur in the afternoons and they are accompanied by lightning and thunder.

• The annual rainfall is about 2000mm

• High humidity and intensive cloud cover throughout the year.

• Crops grown are cassava, groundnuts, maize, millet, beans, bananas.

• Human and economic activities are Plantation agriculture, fishing, cultivation, peasantry sedentary agriculture, lumbering.

• Common animals found are Monkey, gorillas, crocodiles, and hippopotamus.

• Common vegetations found are mahogany, ebony, rosewood, iron wood, green heart, balsa, palm and tree ferns. 
• • 2. TROPICAL REGION (savannah region)

The region is found between 6° – 20° north and south of the equator.

Area located include: East and Central Africa, Brazilian plateau, Venezuela, Africa and N. Australia.

The climates experienced on tropical region are called tropical continental climate and tropical maritime climate.

The vegetations found in tropical region are called tropical grass

CHARACTERISTICS OF TROPICAL REGION

• High temperature, in hot summers (32°C) and cooler winters (20°C).

• The annual temperature range is about 11° C.

• The highest temperatures occur just before the rainy season begins.

• Heavy rains, mainly convection, occur in the summer.

• Total annual rainfall is around 765mm, though this increases in the areas lying close to the equatorial climate region.

• This region is characterized by tall grass and trees which are more numerous near the equatorial forest region.

• Region is suitable for herbivores animals such as giraffes, elephants, buffaloes, rhino, zebras, antelopes, wildebeests and many other animals.

• There are also carnivorous animals such as lions, leopards, hyenas, etc.

• The region also supports a variety of species of birds, reptiles and insects.

• People living in this region mainly engage in livestock keeping, cultivation and tourism. Also lumbering is practiced. 

• • 3. TROPICAL DESERT

The tropical desert climate occurs on the western margins of landmasses between latitude 20° to 30° north and south of the equator.

The climate is experienced in all the major tropical deserts of the world.

 The hot deserts occupy about one third of the earth’s surface.

The principal tropical deserts occur on the continents as follows:

• Africa: Sahara, Kalahari and Namib Deserts

•  Asia: the desert of Jordan, Syria, Iran, Iraq, Saudi Arabia and Israel, and the desert of India.

• North America: Mohave, Colorado and Mexican Deserts.

• South America: Atacama Desert

• Australia: Great Australian Desert

The vegetations found in tropical region are called tropical desert vegetation

CHARACTERISTICS OF TROPICAL DESERT

• High mean monthly temperature, 29°C in the hot season.

• During the cool season, temperatures can be as low as 10°C.

• Daytime temperatures can rise to 47°C or more. 

• Night temperature can drop to as low as 5°C. 

• Annual temperature range is very high. It can reach 26°C.

• Desert conditions hardly support meaningful human activities because of the very high temperatures experienced during the day.

• However, animal, the camel, has adapted to desert conditions and can survive for many days without water. 

• There are depressions of varying sizes where underground water reaches the surface. These depressions are called oases (singular, oasis).

• 4.THE  MONSOON REGION

 The areas which mainly experience monsoon type of climate are South East Asia, Northern, Southern China, and the Indian subcontinent. This type of climate is most marked in India.

The vegetations found in monsoon region are called tropical monsoon forest.

CHARACTERISTICS OFMONSOON REGION  

• Seasonal reversal of winds (monsoon winds); onshore during one season and offshore during another season.

• Onshore wind brings heavy rain to coastal regions while offshore winds bring little or no rain, except where they cross a wide stretch of the sea.

• Temperatures range from 32°C in the hot season to about 25°C in the cool season, giving an annual range of about 7°C.

• Annual rainfall varies greatly, depending on relief and the angle at which onshore winds meet the highlands (aspect).

• There are three marked seasons: cool, dry season; hot, dry season; and hot, wet season. 

• Climate described as having a hot, wet season and cool, dry season.

• The main human and economic activities carried out in areas experiencing this type of climate include rice growing and livestock husbandry.

• Crops grown are rice, wheat, millet, maize, and sorghum. Sugarcane, cotton and juice are important lowland crops grown in India, Pakistan and Bangladesh. The other crops grown are tea (Sri-lanka, Bangladesh and India) and rubber in Malaysia. 

• Animals kept in this climatic region include pigs, cattle, buffalos, sheep, goats, and poultry.

• 5. MEDITERANEAN REGION

This region occurs between 30°N and 45°N and 30°S and 40°S on the western sides of the continents.

 Places experiencing the Mediterranean climate are on the coastal lands around the Mediterranean Sea (the Maghreb, Spain, Italy, Greece, Egypt and Israel), the western sides of north and South America (central California and central Chile), South Australia (Perth and Adelaide) and South Africa (Cape Province).

The vegetations found in Mediterranean region are called Mediterranean forest.

CHARACTERISTICS OFMEDITERANEAN REGION

• Temperatures range from 21°C in the summer to 10°C (or below) in the winter.

• Mean annual temperature is about 15°C.

• Annual total rainfall varies from 500 to 900 mm.

• Hot, dry summers and cold, wet winters. 

• The Mediterranean climate can generally be described as having hot, dry summers and middy, rainy winters.

• Crops grown include fruits and cereals. It is in this region that much of the world’s citrus fruits are grown. Citrus fruits include oranges, lemons, grapes and limes. Other fruits grown here are peaches, apricots, plums, cherries, olives, almonds and pears. The cereals include maize, wheat, rice and barley.

•  Agriculture has given rise to specialized industries such as wine-making, flour milling, fruit canning and food processing industries. 

• 6. MOUNTAIN REGION

This region occurs in the main mountain areas of the world. The areas that experience such climates include the East Africa Mountains, the Ethiopian highlands, the mountains and plateaus of central Asia, the Alps of Europe, the Andes of South America and the Rockes of North America.

CHARACTERISTICS OFMOUNTAIN REGION

• Pressure and temperature generally decrease with increase in altitude.

• Precipitation increases with altitude.

• In areas around mountains within the tropic, temperatures may range from high at the foot of a mountain to very cold at the peak, e.g. Mount Kilimanjaro.

• Climatic conditions are suitable for given human activities.

• Economic activities are animal keeping, forestry, tourism.

• 7. THE WARM TEMPERATE EASTERN COAST REGION

Warm temperate eastern margin (China type) occurs in the eastern sides of the continents between latitudes 23° and 35° north and south of the equator. The countries having this type of climate are central China, south eastern USA, and southern Brazil, eastern part of Argentina, South Africa, southern Brazil, and eastern part of Argentina, South Africa, southern Japan, and south eastern Australia.

CHARACTERISTICS OF THE WARM TEMPERATE EASTERN COAST REGION

• Temperatures are about 26° C in summer and 13° C in the winter.

• The total annual rainfall varies is about 1000 mm.

• The rain is convectional and torrential type and it mostly falls in the summer. 

•  Temperatures and rainfall in this type of climate make it possible to grow crops and keep animals. 

• The crops grown include rice, maize, cotton, sugarcane and tobacco.

• Economic activities include Lumbering, animal keeping, agriculture. 

• 8. THE WARM TEMPERATE INTERIOR REGION

It occurs in the interior of the continents, between 20° and 35° north and south of the equator. The best examples of the region are Murray-Darling lowlands of Australia; The high Veldt of South Africa; and the central Paraguay and central Argentina (both in South America); central lowlands of North America (Oklahoma and Texas and in northern Mexico); central European lowlands, and the plains of Manchuria.

The vegetations found in warm temperate interior region are called temperate grassland.

CHARACTERISTICS OF THE WARM TEMPERATE INTERIOR REGION

• Temperatures range from 26°C in the summer to 10°C in the winter.

• The annual rainfall varies from 380 to 700 mm, depending on the distance from the sea.

• Rainfall is convectional type and falls mainly in spring and early summer.

• The main economic activities carried out in this region are cattle rearing, Tourism and crop growing.

• 9. COLD TEMPERATE CONTINENTAL REGION

This region found on coastal areas of Scandinavia and Alaska.

The vegetations found Cold temperate continental region are called coniferous forest.

CHARACTERISTICS OF THE COLD TEMPERATE CONTINENTAL REGION

• Short, cold summers with temperatures of about 12°C.

• Long winters with temperatures ranging from -2C to 4°C.

•  Annual rainfall is about 750 mm.

• Rain falls in most months except the winter when show falls. 

• The main economic activities practiced in this region include agriculture, mining and manufacturing. 

• A dairy cattle farming is mainly practiced in the Scandinavian countries such as Norway Denmark and Sweden.  
• 10. TUNDRA AND POLAR REGIONS

Tundra regions found in the northern coast of North Americas, southern coast Greenland and the Arctic coast of Europe and Asia.

Polar Regions found in the interiors of Iceland, Greenland and Antarctica.

CHARACTERISTICS OF THE TUNDRA AND POLAR REGIONS

• Temperatures are permanently below 0°C.

•  Precipitation is in the form of blizzards (now storms).

• The winters consist of continuous night, and summers of continuous day.

• Most these regions are uninhabited and hence limited human activities take place here.

• The natural occupations are hunting, fishing and herding of reindeer.

THE END

HAVE A NICE STUDY

THE EARTH

The Earth is the third planet in distance from the sun and the fifth largest; The Earth is the only planet that supports life. It is distance from the Sun is approximately 149,503,000 kilometers.

CHARACTERISTICS OF THE EARTH

The Earth has the following characteristics

1. It is spherical in shape or flattened sphere (oblate spheroid)
2. It is diameter from the North Pole through the South Pole is 12,713 kilometers. 
3. It is diameter from the east-west is 12,757 kilometers (at the equator).
4. The circumference of the Earth is approximately 40,000 kilometers. 
5. The Earth rotates from west to east.

ORIGIN OF THE UNIVERSE

The details of the origin of Universe are unknown, but the basic theories have been established to show the origin of Universe. It believed that the Earth occurred as a result of origin of the universe.

Some theories established to describe the origin of Universe are 

1. Big Bang theory
2. Nebula Hypothesis
3. Creation theory (Biblical theory).

1. Big Bang theory

This theory postulated by Edwin Hubble in 1920s, Big Bang theory explained that about 15 billion years ago a tremendous explosion started the expansion of the universe. This explosion is known as the Big Bang. At the point of this event all of the matter and energy of space was contained at one point. Matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse. If the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born.

Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals (“very small planets”), some of which reached diameters of several hundred kilometers. In successive stages these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including Earth, were near the sun, and the gaseous planets were in more distant orbits.

2. Nebula Hypothesis

This theory postulated by Immanuel Kant, It suggests that the Solar System formed from nebulous material as follow

A huge mass of swirling cold gas and dust (Nebula) in an area in the Milky Way Galaxy, this cloud of gas and dust began to condense or pull together under the force of its own gravity.

As a result nebula conserved angular momentum of the material drawn the centre, it spanning anticlockwise, due to this the material around the centre of the condensing nebula flattened out into disc like shape.

The centre of the nebula continued to contract due to gravity. Eventually, pressure and temperatures in this mass became high enough that nuclear fusion started and central mass become a star and the Sun.

3. Creation theory(Biblical theory)

Creation theory is the religious belief that the universe and life originated from God commands as written in the Bible (Genesis 1:1-31) as follow

1 In the beginning, when God created the universe,[a] 2 the earth was formless and desolate. The raging ocean that covered everything was engulfed in total darkness, and the Spirit of God[b] was moving over the water. 3 Then God commanded, “Let there be light”—and light appeared. 4 God was pleased with what he saw. Then he separated the light from the darkness, 5 and he named the light “Day” and the darkness “Night.” Evening passed and morning came—that was the first day.

6-7 Then God commanded, “Let there be a dome to divide the water and to keep it in two separate places”—and it was done. So God made a dome, and it separated the water under it from the water above it. 8 He named the dome “Sky.” Evening passed and morning came—that was the second day.

9 Then God commanded, “Let the water below the sky come together in one place, so that the land will appear”—and it was done. 10 He named the land “Earth,” and the water which had come together he named “Sea.” And God was pleased with what he saw. 11 Then he commanded, “Let the earth produce all kinds of plants, those that bear grain and those that bear fruit”—and it was done. 12 So the earth produced all kinds of plants, and God was pleased with what he saw. 13 Evening passed and morning came—that was the third day.

14 Then God commanded, “Let lights appear in the sky to separate day from night and to show the time when days, years, and religious festivals[c] begin; 15 they will shine in the sky to give light to the earth”—and it was done. 16 So God made the two larger lights, the sun to rule over the day and the moon to rule over the night; he also made the stars. 17 He placed the lights in the sky to shine on the earth, 18 to rule over the day and the night, and to separate light from darkness. And God was pleased with what he saw. 19 Evening passed and morning came—that was the fourth day.

20 Then God commanded, “Let the water be filled with many kinds of living beings, and let the air be filled with birds.” 21 So God created the great sea monsters, all kinds of creatures that live in the water, and all kinds of birds. And God was pleased with what he saw. 22 He blessed them all and told the creatures that live in the water to reproduce and to fill the sea, and he told the birds to increase in number. 23 Evening passed and morning came—that was the fifth day.

24 Then God commanded, “Let the earth produce all kinds of animal life: domestic and wild, large and small”—and it was done. 25 So God made them all, and he was pleased with what he saw.

26 Then God said, “And now we will make human beings; they will be like us and resemble us. They will have power over the fish, the birds, and all animals, domestic and wild,[d] large and small.” 27 So God created human beings, making them to be like himself. He created them male and female, 28 blessed them, and said, “Have many children, so that your descendants will live all over the earth and bring it under their control. I am putting you in charge of the fish, the birds, and all the wild animals. 29 I have provided all kinds of grain and all kinds of fruit for you to eat; 30 but for all the wild animals and for all the birds I have provided grass and leafy plants for food”—and it was done. 31 God looked at everything he had made, and he was very pleased. Evening passed and morning came—that was the sixth day.

THE SHAPE OF THE EARTH

The shape of the earth is a flattened sphere (oblate spheroid) or spherical in shape. There are many ways to prove that the earth is spherical.

The following are some of them

A. CIRCUMNAVIGATION OF THE EARTH

The first voyage around the world by Ferdinand Magellan and his crew, from 1519 to 1522, proved beyond doubt that the earth is spherical. No traveler going round the world by land or sea has ever encountered an abrupt edge, over which he would fall. Modern air routes and ocean navigation are based on the assumption that the earth is round.

B. SHIP’S VISIBILITY

When a ship appears over the distant horizon, the top of the mast is seen first before the hull. In the same way, when it leaves habour, its disappearance over the curved surface is equally gradual. If the earth were flat, the entire ship would be seen or obscured all at once.

C. SUNRISE AND SUNSET

The sun rises and sets at different times in different places. As the earth rotates from west to east, places in the east see the sun earlier than those in the west. If the earth were flat, the whole world would have sunrise and sunset at the same time.

D. THE LUNAR ECLIPSE

The shadow cast by the earth on the moon during a lunar eclipse is always circular. It takes the outline of an arc of a circle. Only a sphere can cast such a circular shadow.

E.AERIAL PHOTOGRAPHS

Pictures taken from high altitudes by rockets and satellites show clearly the curved edge of the earth. This is perhaps the most convincing and the most up-to-date proof of the earth’s sphericity.

F. PLANETARY BODIES ARE SPHERICAL

All observations from telescopes reveal that the planetary bodies, the sun, moon, satellites and stars have circular outlines from whichever angle you see them. They are strictly spheres. Earth, by analogy, cannot be the only exception.

G. DRIVING POLES ON LEVEL GROUND ON A CURVED EARTH

Engineers when driving poles of equal length at regular intervals on the ground have found they do not give a perfect horizontal level. The centre pole normally projects slightly above the poles at either end because of the curvature of the earth. Surveyors and field engineers therefore have to make certain corrections for this inevitable curvature, i.e. 12.6 cm to 1 km.

H. THE CIRCULAR HORIZON

The distant horizon viewed from the deck of a ship at sea, or from a cliff on land is always and everywhere circular in shape. This circular horizon widens with increasing altitude and could only be seen on a spherical body.

STRUCTURE OF THE EARTH

Structure of the Earth is the layer, solid or mineral part of the Earth. The structure of the earth consists of

1. External structure (Outer zone) and
2. Internal structure (Inner zone).

1. EXTERNAL STRUCTURE OF THE EARTH (Outer zone)

External structure of the earth consists of layers such as Atmosphere.

ATMOSPHERE

Atmosphere is the thin layer of gases held on the earth by gravitation attraction.

– It composed by abiotic (non-living matter) and biotic (living organism).

– Non-living matter found in the atmosphere includes mixture of gases, water vapor and dust particles. Atmosphere consists of different gases such as carbon dioxide, oxygen, hydrogen, nitrogen and other gases.

– The living organism includes the smallest or microscopic organisms like bacteria.

CHARACTERISTICS OF ATMOSPHERE

Characteristics of atmosphere categorized into two groups as follow 

A. According to its composition.

B. According to its vertical structure from the ground level into interplanetary space.

COMPOSITION OF ATMOSPHERE

Atmosphere categorized into biotic and abiotic matters. Biotic matters include the life in atmosphere above 30m from the ground and abiotic matters include materials with no life found in air which is gases, water and solid matters.

ATMOSPHERIC GASES

The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (0.009%) and carbon dioxide (0.03%) and other gases include neon, helium, Krypton, xenon.

WATER VAPOUR IN THE ATMOSPHERE

The presence of water vapor in atmosphere plays a large role in determining the weather. Clouds and precipitation occur as a result of the phase change that occurs when water vapor condenses into liquid water.

Sources of water vapour in the atmosphere

The sources of water vapour in the atmosphere include the following

• Evaporation of water bodies.
• Evaporation of hot springs.
• Evaporation of water from the soil.
• Plants transpiration.
• Evapo- transpiration by plants.
• Volcanic eruption.

SOLID PARTICLES IN THE ATMOSPHERE 

The air seems to be completely clear; it is full of atmospheric particles which are invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, soot, sea salt particles etc.

STRUCTURE OF ATMOSPHERE

Structure of Atmosphere is viewed on its vertical layers from the Earth’s surface into the interplanetary space, in its vertical dimension, atmosphere has varied conditions in temperature and chemical composition and these have enabled the meteorologists to divide it into a number or layers.

Atmosphere in its vertical dimension is broadly divided

A.according to Contrasting temperature conditions in it with altitude from ground level into further space.

B. Contrasting chemical composition with altitude.

A. Structure of Atmosphere according to Contrasting temperature conditions

In this group, Atmosphere categorized in terms of altitude

The main atmospheric layers according to contrasting temperature conditions in it with altitude from ground level are

I. Troposphere.

II. Stratosphere.

III. Mesosphere.

IV. Thermosphere.

I. Troposphere

The troposphere is the lowest layer of Earth’s atmosphere. Most of the mass (about 75-80%) of the atmosphere is in the troposphere. Most types of clouds are found in the troposphere, and almost all weather occurs within this layer.

The bottom of the troposphere is at Earth’s surface. The troposphere extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level. The height of the top of the troposphere varies with latitude (it is lowest over the poles and highest at the equator) and by season (it is lower in winter and higher in summer). It can be as high as 20 km (12 miles or 65,000 feet) near the equator, and as low as 7 km (4 miles or 23,000 feet) over the poles in winter.

Air is warmest at the bottom of the troposphere near ground level. Air gets colder as one rises through the troposphere. That’s why the peaks of tall mountains can be snow-covered even in the summertime.

II. Stratosphere

The stratosphere is a layer of Earth’s atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.

III. Mesosphere

The mesosphere is a layer of Earth’s atmosphere. The mesosphere is directly above the stratosphere and below the thermosphere. It extends from about 50 to 85 km (31 to 53 miles) above our planet.

Temperature decreases with height throughout the mesosphere. The coldest temperatures in Earth’s atmosphere, about -90° C (-130° F), are found near the top of this layer.

The boundary between the mesosphere and the thermosphere above it is called the mesopause. At the bottom of the mesosphere is the stratopause, the boundary between the mesosphere and the stratosphere below.

The mesosphere is difficult to study, so less is known about this layer of the atmosphere than other layers. Weather balloons and other aircraft cannot fly high enough to reach the mesosphere. Satellites orbit above the mesosphere and cannot directly measure traits of this layer. Scientists use instruments on sounding rockets to sample the mesosphere directly, but such flights are brief and infrequent. Since it is difficult to take measurements of the mesosphere directly using instruments, much about the mesosphere is still mysterious.

IV. Thermosphere

The thermosphere is a layer of Earth’s atmosphere. The thermosphere is directly above the mesosphere and below the exosphere. It extends from about 90 km (56 miles) to between 500 and 1,000 km (311 to 621 miles) above our planet.

Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C (360° F) hotter in the daytime than at night, and roughly 500° C (900° F) hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher.

The boundary between the thermosphere and the exosphere above it is called the thermopause. At the bottom of the thermosphere is the mesopause, the boundary between the thermosphere and the mesosphere below.

Although the thermosphere is considered part of Earth’s atmosphere, the air density is so low in this layer that most of the thermosphere is what we normally think of as outer space. In fact, the most common definition says that space begins at an altitude of 100 km (62 miles), slightly above the mesopause at the bottom of the thermosphere. The space shuttle and the International Space Station both orbit Earth within the thermosphere!

Below the thermosphere, gases made of different types of atoms and molecules are thoroughly mixed together by turbulence in the atmosphere. Air in the lower atmosphere is mainly composed of the familiar blend of about 80% nitrogen molecules (N2) and about 20% oxygen molecules (O2). In the thermosphere and above, gas particles collide so infrequently that the gases become somewhat separated based on the types of chemical elements they contain. Energetic ultraviolet and X-ray photons from the Sun also break apart molecules in the thermosphere. In the upper thermosphere, atomic oxygen (O), atomic nitrogen (N), and helium (He) are the main components of air.

B. Structure of Atmosphere according to contrasting chemical composition with altitude

The main atmospheric layers according to contrasting chemical composition with altitude are.

I. Homosphere

II. Heterosphere

I. Homosphere

The homosphere is the lower of the two and the location in which turbulent mixing dominates the molecular diffusion of gases. In this region, which occurs below 100 km (about 60 miles) or so, the composition of the atmosphere tends to be independent of height.

II. Heterosphere

Above 100 km, in the zone called the heterosphere, various atmospheric gases are separated by molecular mass, with the lighter gases being concentrated in the highest layers. Above 1,000 km (about 600 miles), helium and hydrogen are the dominant species. Diatomic nitrogen (N2), a relatively heavy gas, drops off rapidly with height and exists in only trace amounts at 500 km (300 miles) and above. This decrease in the concentration of heavier gases with height is largest during periods of low Sun activity, when temperatures within the heterosphere are relatively low. The transition zone, located at a height of around 100 km between the homosphere and heterosphere, is called the turbopause.

IMPACTS OF ATMOSPHERE ON LIFE

Positive Impacts

1. Some atmospheric layers particularly troposphere consists of much of useful gases to living organisms.
2. Atmosphere is associated with the weather making processes such clouds formation which result into precipitation.
3. Atmosphere act as protective shield to Earth.
4. Atmosphere allows air communication. It makes transmission of radio, telephone and television.

Negative Impacts

1. Carbon dioxide gas present in the Atmosphere causes rate of temperature increase as it absorbs long wave radiation from the earth surface contributing to the problem of green house effect.
2. Atmosphere has some constituents which cause the air borne diseases to people.
3. Pollutant gases present in the Atmosphere contribute to a problem of acidic rain occurrence.
4. The depletion of ozone layer, results into the following problems

      -Melting of Ice due to rise in temperature.

      -Rise in sea level because of Ice melting on the landscape.

      -Skin cancer diseases to people.

      -Death and disappearance of some plant and animal species because of the adverse atmospheric changes.

DIAGRAM OF VERTICAL STRUCTURE OF THE ATMOSPHERE

2. STRUCTURE OF THE EARTH (Inner zone)

The internal structure of the earth consists of three zones, these are

I. Core

II. Mantle

III. Crust.

I. CORE

The earth is an almost spherical body of approximately 6370 km in equatorial radius. The center is occupied by a CORE, a spherical zone about 3470 km in radius. It is believed that the outer core has the properties of liquid (molten). However, the inner most part of the core with a radius of 1225km may be solid.

Iron, with small proportion of nickel, and radioactive elements is considered as the substance in the core. Temperature in the Earth‘s core may lie between 2200 ℃ and 2750 ℃.

II. MANTLE

Outside the core lies the mantle, a layer about 2895km thick, composed of mineral matter in solid state. The mantle is probable composed largely of mineral olivine. Its upper parts, at very high pressure, and under great pressure, have acquired “plastic’ form allows them to flow and to convect very slowly. Transitional layer which separates mantle and core known as Gutenberg discontinuity.

III. CRUST

 The outermost and thinnest of the Earth zone is the crust, a layer some 8 to 40 km thick (under certain mountains). Crystal rocks vary not only in thickness and density, but also in composition. It includes oceanic crust and continental crust.

Oceanic crust (mostly basaltic, basic rock) is that rock beneath the ocean, basins. It is also called sima because they are rich in iron, silicon and magnesium. It is the lower layer of the crust.

Continental crust (mostly granite rock) is the upper layer of the earth crust. It is also called sial because they are acidic rock and rich in silica and aluminum.

The base of the crust is marked by a rather clearly defined break called the Mohorovicic Discontinuity or the Moho . This sharp boundary lies 32-48 km beneath the earth surface. In other words, MOHO is the boundary between the crust and mantle.

Asthenosphere and Lithosphere

 Asthenosphere

– It is part of the upper mantle, about 85 to 700 km below sea level.

– It is a soft layer beneath the lithosphere.

– It is semi-plastic state material (fluid and mobile) which is very hot (1400 ℃)

– The lithospheric crust moves over the soft asthenosphere.

– The thermal generated conventional currents cause the lithosphere above to drift.

Lithosphere

– It is rigid outermost portion of the earth above the asthenosphere. In include sima (continental crust) and sial (oceanic crust).

– The convectional currents cause this layer to split into 7 major plates and 6 minor plates. Each plate has an average 100 km thick.

– The continental plate (sima) is thicker than oceanic one plate (sial).

CHARACTERISTICS OF EARTH’S CRUST

1. The crust has the most complicated and variation in its composition. The most abundant element in crust is Oxygen by weight and by volume.
2. Oceanic crust is denser than that of continental crust.
3. The earth’s crust is less than 1% by mass and volume of entire earth.
4. The earth’s crust is sub divided into the large parts technically called plates and these plates are supposed to be floating over asthenosphere (upper part of semi-solid Mantle)
5. The Earth’s crust is the outer most layer of the earth and exposed to atmosphere and that’s why it’s the coldest layer.
6. The normal earth crust’s geothermal gradient is 2-3 degrees centigrade per 100 m and pressure gradient is 0.3 KB/km

DIAGRAM OF INTERNAL STRUCTURE OF THE EARTH

Roles of gravity in stability and dynamic state of the planet earth.

Planet Earth is held in its orbit around the Sun by the force of gravity. Without gravity, Earth would fly off into outer space and you would fly off the Earth. All objects have gravity. Gravity is a pulling force that an object exerts on other objects. Massive objects exert a greater gravitational pull than less massive objects.

QN

What are the roles of gravity in stability and dynamic state of the planet earth?

ANSWER

1. Gravity is the primary force that holds the Earth together, which certainly contributes to the overall stability of the planet.
2. The differential pressure provided by gravitation allows hot, relatively low density materials to rise; and cold, relatively high density materials to sink. The rising and sinking coupled with the rotation of the planet produces the coriolis force which produces swirling currents in the material (such as magma and molten iron) that drive the tectonic plates and produce other dynamic effects.

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