Moisture in the Atmosphere Grade 10 Essays and Notes CAPS:

The atmosphere isn’t just made up of air but also contains water vapour.  This water vapour, which makes up  less than 0.001% of all the water on the Earth, is invisible. This tiny amount of water in the air is really important to our climate. When water vapour condenses onto tiny particles in the atmosphere, clouds form.

1. Water in the atmosphere in different forms, such as water vapour and liquid.

Moisture in the Atmosphere Grade 10 Essays Topics

Essay # 1. Nature and Characteristics of Atmosphere:

The present atmosphere of the Earth is not the original one, which was formed by the release of dissolved gases during cooling of molten rocks. The original atmosphere in all probabilities had carbon dioxide, nitrogen and water. Without the presence of free oxygen, the atmosphere was therefore a reducing atmosphere. The current atmosphere contains high amount of oxygen plus inert nitrogen (neutral), and is therefore oxidizing in nature.

The main source of oxygen in the atmosphere is green plants. Plants use sunlight to transform carbon dioxide and water into organic matter, and release oxygen in the atmosphere. Oxygen is required by higher forms of life. The oxygen in our atmosphere was almost all produced by plants (cynobacteria) or, more colloquially, blue-green algae.

Green plants and rain forests have converted an envelope of carbon dioxide and nitrogen into an oxygen rich atmosphere, making this planet suitable for us to live. Nitrogen is an ingredient in building proteins and nucleic acids (DNA) and essential element for living beings. Both the elements maintain same strength despite enormous amount consumed by increasing population each day due to perfect nature’s cycle of recovery.

Essay # 2. Composition of Atmosphere:

The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost negligible at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the earth’s surface.

i. Gases:

Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface.

It is largely responsible for the greenhouse effect. The volume of other gases is constant but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of the fossil fuels. This has also increased the temperature of the air.

Ozone is another important component of the atmosphere found between 10 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.

ii. Dust Particles:

Dust particles are generally concentrated layers of the atmosphere; yet, convectional air currents may transport them to great heights. The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.

iii. Water Vapour:

Water vapour is also a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions it may be less than one per cent of the air.

The atmosphere consists of different layers with vary­ing density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude. The column of atmosphere is divided into five different layers depending upon the temperature condi­tion. They are- troposphere, stratosphere, mesosphere, ionosphere and exosphere.

The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.

Thickness of the troposphere is greatest at the equa­tor because heat is transported to great heights by strong convectional currents. This layer contains dust particles and water vapour. All changes in climate and weather takes place in this layer. The temperature in litis layer decreases at the rate of 1°C for every 165 m of height. This is the most important layer for all biological activities.

The zone separating the tropsophere from strato­sphere is known as the tropopause. The air temperature at the tropopause is about minus 80°C over the equator and about minus 45°C over the poles. The temperature here is nearly constant, and hence, it is called the tropopause.

The stratosphere is found above the tropopause and extends up to a height of 50 km. One important feature of the stratosphere is that it contains the ozone layer. This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.

The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km. The upper limit of mesosphere is known as the mesopause.

The ionosphere is located between 80 and 400 km above the mesopause. It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer.

Temperature here starts increasing with height. The uppermost layer of the atmosphere above the ionosphere is known as the exosphere. This is the highest layer. Whatever contents are there, these are extremely rarefied in this layer, and it gradually merges with the outer space.

Essay # 4. Heating and Cooling of Atmosphere:

The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in the form of long wave. 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 each other, there is a flow of energy from the warm to the cool body. 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 dirunal (day and night) variation in daily weather is 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 the form of short waves and heats up its surface. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in the form of long wave. 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 greenhouse 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.

Heat Budget of the Planet Earth:

The earth as a whole does not accumulate or lose 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.

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.

Essay # 5. Water in the Atmosphere:

Water is present in the atmosphere in three forms namely—gaseous, liquid and solid. The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration. Thus, there is a continuous exchange of water between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation.

Water vapour present in the air is known as humidity. The actual amount of the water vapour present in the atmosphere is known as the absolute humidity. It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre. The absolute humidity differs from place to place on the surface of the earth.

The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity. The air containing moisture to its full capacity at a given temperature is said to be saturated.

It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage. The temperature at which saturation occurs in a given sample of air is known as dew point.

Evaporation and Condensation:

Evaporation is a process by which water is transformed from liquid to gaseous state. Heat is the main cause of evaporation. The temperature at which the water starts evaporating is referred to as the latent heat of vapourisation.

The transformation of water vapour into water is called condensation. Condensation is caused by the loss of heat. When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases. Then, the excess water vapour condenses into liquid form. If it directly condenses into solid form, it is known as sublimation.

In free air, condensation results from cooling around very small particles termed as hygro­scopic condensation nuclei. After condensation the water vapour or the moisture in the atmosphere takes one of the following forms—dew, frost, fog and clouds.

Forms of condensation can be classified on the basis of temperature and location. Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.

i. Dew:

When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky, calm air, high relative humidity and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.

i. Frost:

Frost forms on cold surfaces when condensation takes place below freezing point (0°C), i.e. the dew point is at or below the freezing point. The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.

iii. Fog and Mist:

When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. So, the fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero. Such a condition when fog is mixed with smoke, is described as smog.

The only difference between the mist and fog is that mist contains more moisture than the fog. In mist each nuclei contains a thicker layer of moisture. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.

Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by the dust, smoke, and the salt particles.

iv. Clouds:

Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes.

According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types:

(i) Cirrus;

(ii) Cumulus;

(iii) Stratus;

(iv) Nimbus.

a. Cirrus:

Cirrus clouds are formed at high altitudes (8,000-12,000 m). They are thin and detached clouds having a feathery appearance. They are always white in colour.

b. Cumulus:

Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000- 7,000 m. They exist in patches and can be seen scattered here and there. They have a flat base.

c. Stratus:

As their name implies, these are layered clouds covering large portions of the sky. These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.

d. Nimbus:

Nimbus clouds are black or dark gray. They are formed at middle levels or very near to the sur­face of the earth. These are extremely dense and opaque to the rays of the sun. Sometimes, the clouds are so low that they seem to touch the ground. Nimbus clouds are shapeless masses of thick vapour.

A combination of these four basic types can give rise to the following types of clouds: high clouds—cirrus, cirrostratus, cirrocumulus; middle clouds—altostratus and altocumulus; low clouds— stratocumulus and nimbostratus and clouds with extensive vertical development—cumulus and cumu­lonimbus.

Essay # 6. Atmospheric Pressure:

The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in units of mb and Pascals. The widely used unit is kilo Pascal written as hPa.

At sea level the average atmospheric pressure is 1,013.2 mb or 1,013.2 hPa. Due to gravity the air at the surface is denser and hence has higher pressure. Air pressure is measured with the help of a mercury barometer or the aneroid barometer. The pressure decreases with height.

Vertical Variation of Pressure:

In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation. It does not always decrease at the same rate.

Horizontal Distribution of Pressure:

Small differences in pressure are highly significant in terms of the wind direction and purposes of comparison.

World Distribution of Sea Level Pressure:

Near the equator the sea level pressure is low and the area is known as equatorial low. Along 30°N and 30°S are found the high-pressure areas known as the subtropical highs. Further polewards along 60°N and 60°S, the low-pressure belts are termed as the sub-polar lows.

Near the poles the pressure is high and it is known as the polar high velocity. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level.

Forces Affecting the Velocity and Direction of Wind:

The air is set in motion due to the differences in atmospheric pressure. The air in motion is called wind. The wind blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force.

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 in nature.

They oscillate with the apparent movement of the sun. In the northern hemisphere in winter they move southwards and in the summer northwards. 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 the wind.

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.

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 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.

General Circulation of the Atmospheric Pressure:

The pattern of planetary winds largely depends on:

(i) Latitudinal variation of atmospheric heating;

(ii) Emergence of pressure belts;

(iii) The migration of belts following apparent path of the sun;

(iv) The distribution of continents and oceans;

(v) The rotation of earth.

The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s climate. The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created.

The winds from the tropics converge at this low pressure zone. The converged air rises along with the convective cell. It reaches the top of the troposphere up to an altitude of 14 km and moves towards the poles. This causes accumulation of air at about 30°N and S.

Part of the accumulated air sinks to the ground and forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 30°N and S latitudes. Down below near the land surface the air flows towards the equator as the easterlies.

The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a cell in the tropics is called Hadley Cell. In the middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high.

At the surface these winds are called westerlies and the cell is known as the Ferrel Cell. At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell. These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.

The general circulation of the atmosphere also affects the oceans. The large-scale winds of the atmosphere initiate large and slow-moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean.

Air Masses:

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 air masses are classified according to the source regions.

There are five major source regions. These are:

(i) Warm tropical and subtropical oceans;

(ii) The subtropical hot deserts;

(iii) The relatively cold high latitude oceans;

(iv) The very cold snow covered continents in high latitudes; and

(v) Permanently ice covered continents in the Arctic and Antarctica.

Seasonal Wind:

The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heat, pressure and wind belts. The most pronounced effect of such a shift is noticed in the monsoon, especially over southeast Asia. The other local deviations from the general circulation system are as follows.

Local Winds:

Differences in the heating and cooling of earth surfaces and the cycles those develop daily or annually can create several common, local or regional winds.

Land and Sea Breezes:

The land and sea absorb and transfer heat differently. 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 sea to land as the sea breeze. In the night the reversal of condition takes place. The land loses heat faster and is cooler than the sea. The pressure gradient is from the land to the sea and hence results in land breeze.

Mountain and Valley Winds:

In mountain regions, during the day the slopes get heated up and air moves up the slope to fill the resulting gap the air from the valley blows up the valley. This wind is known as the valley breeze. During the night the slopes get cooled and the dense air descends into the valley as the mountain wind.

The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind. Another type of warm wind occurs on the leeward side of the mountain ranges. The moisture in these winds, while crossing the mountain ranges condense and precipitate. When it descends down the leeward side of the slope the dry air gets warmed up by adiabatic process.


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.

Extra Tropical Cyclones:

The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones. The passage of front causes abrupt changes in the weather conditions over the area in the middle and high latitudes. Extra tropical cyclones are formed along the polar front. The cold front moves faster than the warm front ultimately overtaking the warm front.

The warm air is completely lifted up and the front is occluded and the cyclone dissipates. The extra tropical cyclone affects a much larger area as compared to the tropical cyclone. The wind velocity in a tropical cyclone is much higher and it is more destructive. The extra tropical cyclones move from west to east but tropical cyclones move from east to west.

Tropical Cyclones:

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. 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. Tropical cyclones originate and intensify over warm tropical oceans.

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

(i) Large sea surface with temperature higher than 27°C;

(ii) Presence of the Coriolis force;

(iii) Small variations in the vertical wind speed;

(iv) A pre-existing weak low-pressure area or low- level-cyclonic circulation;

(v) Upper divergence above the sea level system.

A mature tropical cyclone is characterized by the strong spirally circulating wind around the centre, called the eye. The diameter of the circulating system can vary between 150 and 250 km. From the eye wall rain bands may radiate and trains of cumulus and cumulonimbus clouds may drift into the outer region.

The diameter of the storm over the Bay of Bengal, Arabian Sea and Indian Ocean is between 600-1200 km. The system moves slowly about 300-500 km per day. The cyclone creates storm surges and they inundate the coastal low lands.

Thunderstorms and Tornadoes:

They are of short duration, occurring over a small area but are violent. Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning.

When the clouds extend-to heights where sub­zero 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 characterized 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. Such a phenomenon is called a tornado.

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

Essay # 7. Temperature:

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.

Factors Controlling Temperature Distribution:

The temperature of air at any place is influenced by: 

(i) The latitude of the place;

(ii) The altitude of the place;

(iii) Distance from the sea, the airmass circulation;

(iv) The presence of warm and cold ocean currents;

(v) Distribution of temperature;

(vi) Inversion of temperature.

(i) The Latitude:

The temperature of a place depends on the insolation received.

(ii) 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 6.5°C per 1,000 m.

(iii) 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 comes under the moderating influence of the sea and land breezes which moderates the temperature.

(iv) 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.

(v) Distribution of Temperature:

The global distribution of temperature can be well understood by studying the temperature distribution in January and July. The Isotherms are lines joining places having equal temperature. In general, the effect of the latitude on temperature is well pronounced on the map, as the isotherms are generally parallel to the latitude.

The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere. In the northern hemisphere the land surface area is much larger than in the southern hemisphere. Hence, the effects of land mass and the ocean currents are well pronounced. In January, the isotherms deviate to the north over the ocean and to the south over the continent.

This can be seen on the North Atlantic Ocean. The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and the isotherms bend towards south in Europe. It is much pronounced in the Siberian plain.

The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere.

In July the isotherms generally run parallel to the latitude. The equatorial oceans record warmer temperature, more than 27°C. Over the land more than 30°C is noticed in the subtropical continental region of Asia, along the 30°N latitude. Along the 40°N runs the isotherm of 10°C and along the 40°S the temperature is 10°C.

(vi) Inversion of Temperature:

Normally, temperature decreases with increase in eleva­tion. It is called normal lapse rate. At times, the situa­tion is reversed and the normal lapse rate is inverted. It is called Inversion of temperature. Inversion is usually of short duration but nonetheless quite common. 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 the mornings are common occurrences especially during winter season. This inver­sion commonly lasts for few hours until the sun comes up and begins to warm the earth. The inversion takes place in the hills and the 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 damage.

Essay # 8. Precipitation:

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, when the temperature is lower than the 0°C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Besides rain and snow, other forms of precipitation are sleet and hail, though the latter are limited in occurrence and are sporadic in both time and space.

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. Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice reaching the surface of the earth are called hailstones.

Types of Rainfall:

i. Conventional Rain:

The air, on being heated, becomes light and rises up as 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.

ii. Orographic Rain:

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. 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.

World Distribution of Rainfall:

Different places of world receive varigated rainfall throughout the year. In general, as we proceed from equator to the poles, rainfall goes on decreasing steadily. The coastal areas of the world receive great amount 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 a great sources of water.

Between the latitudes 35 and 40 degree N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45 and 65 degree 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.

On the basis of the total amount of annual precipitation, major precipitation regimes of the world are identified as follows:

The equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone and the coastal areas of the monsoon Jand receive heavy rainfall of over 200 cm per annum. Interior continental areas receive moderate rainfall varying from 100-200 cm per annum.

The coastal’ areas of the continents receive moderate amount of rainfall. The central parts of the tropical land and the eastern and interior parts of the temperate lands receive rainfall varying between 50-100 cm per annum. Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive very low rainfall- less than 50 cm per annum.

Seasonal distribution of rainfall provides an important aspect to judge its effectiveness. In some regions rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of cool temperate regions.

Essay # 9. Solar Radiation:

The earth’s surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere. 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 nearest to the sun (147 million km). This position is called perihelion.

herefore, the annual insolation received by the earth on 3rd 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.

Variability of Insolation at the Surface of the Earth:

The factors that cause these variations in insola­tion are:

(i) The rotation of earth on its axis;

(ii) The angle of inclination of the sun’s rays;

(iii) The length of the day;

(iv) The transparency of the atmosphere;

(v) The configuration of land in terms of its aspect.

The last two, however, have less influence. The fact that the earth’s axis makes an angle of 66V2 with the plane of its orbit around the sun has a greater influence on the amount of insolation received at different latitudes.

The second factor that determines the amount of insolation received is the angle of inclination of the 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.

The Passage of Solar Radiation through the Atmosphere:

The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation.

Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky. The red colour of the rising and the setting sun.

Spatial Distribution of Insolation at the Earth’s Surface:

The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/min the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics. Generally, at the same latitude the insolation is more over the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.

Essay # 10. Global Warming:

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.

Greenhouse Gases (GHGs):

The primary GHGs of today’s concern are carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4), nitrous oxide (N2O) and ozone (O3). Some other gases such as nitric oxide (NO) and carbon monoxide (CO) easily react with GHGs and affect their concentration in the atmosphere.

The effectiveness of any given GHG molecule will depend on the magnitude of the increase in its concentration, its life time in the atmosphere and the wavelength of radiation that it absorbs. The chlorofluorocarbons (CFCs) are highly effective.

Ozone which absorbs ultraviolet radiation in the stratosphere is very effective in absorbing terrestrial radiation when it is present in the lower troposphere. Another important point to be noted is that the more time the GHG molecule remains in the atmosphere, the longer it will take for earth’s atmospheric system to recover from any change brought about by the latter. The largest concentration of GHGs in the atmosphere is carbon dioxide.

The emission of CO2 comes mainly from fossil fuel combustion (oil, gas and coal). Forests and oceans are the sinks for the carbon dioxide. Forests use CO2 in their growth. So, deforestation due to changes in land use also increases the concentration of CO2.

The time taken for atmospheric CO2 to adjust to changes in sources to sinks is 20-50 years. It is rising at about 0.5 per cent annually. Doubling of concentration of CO2 over pre-industrial level is used as an index for estimating the changes in climate, in climatic models.

Chlorofluorocarbons (CFCs) are products of human activity. Ozone occurs in the stratosphere where ultraviolet rays convert oxygen into ozone. Thus, ultraviolet rays do not reach the earth’s surface. The CFCs which drift into the stratosphere destroy the ozone. Large depletion of ozone occurs over Antarctica. The depletion of ozone concentration in the stratosphere is called the ozone hole. This allows the ultraviolet rays to pass through the troposphere.

International efforts have been initiated for reducing the emission of GHGs into the atmosphere. The most important one is the Kyoto Protocol proclaimed in 1997. This protocol went into effect in 2005, ratified by 141 nations.

Kyoto Protocol bounds the 35 industrialized countries to reduce their emissions by the year 2012 to 5 per cent less than the levels prevalent in the year 1990. The increasing trend in the concentration of GHGs in the atmosphere may, in the long run, warm up the earth. Once the global warming sets in, it will be difficult to reverse it.

The annual average near-surface air temperature of the world is approximately 14°C. An increasing trend in temperature was discernible in the 20th century. The greatest warming of the 20th century was during the two periods, 1901-1944 and 1977-1999.

Over each of these two periods, global temperatures rose by about 0.4°C. In between, there was a slight cooling, which was more marked in the Northern Hemisphere. The globally averaged annual mean temperature at the end of the 20th century was about 0.6°C above that recorded at the end of the 19th century.

Essay # 11. World Climate:

Three broad approaches have been adopted for classifying climate. They are empirical, genetic and applied. Empirical classification is based on observed data, particularly on temperature and precipitation. Genetic classification attempts to organize climates according to their causes. Applied classification is for specific purpose.

Koeppen’s Scheme 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 recognized five major climatic groups four of them are based on temperature and one on precipitation. 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.

1. Group A: Tropical Humid Climates:

Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn. The sun being overhead throughout the year and the presence of Inter Tropical Convergence Zone (INTCZ) make the climate hot and humid. Annual range of temperature is very low and annual rainfall is high.

The tropical group is divided into three types, namely (i) Af-Tropical wet climate; (ii) Am-Tropical monsoon climate; (iii) Aw-Tropical wet and dry climate:

(i) Tropical Wet Climate (Af):

Tropical wet climate is found near the equator. The major areas are the Amazon Basin in South America, western equatorial Africa and the islands of East Indies. Significant amount of rainfall occurs in every month of the year as thunder showers in the afternoon.

The temperature is uniformly high and the annual range of temperature is negligible. The maximum temperature on any day is around 30°C while the minimum temperature is around 20°C. Tropical evergreen forests with dense canopy cover and large biodiversity are found in this climate.

(ii) Tropical Monsoon Climate (Am):

Tropical monsoon climate (Am) is found over the Indian sub-continent, North Eastern part of South America and Northern Australia. Heavy rainfall occurs mostly in summer. Winter is dry.

(iii) Tropical Wet and Dry Climate (Aw):

Tropical wet and dry climate occurs north and south of Af type climatic regions. It borders with dry climate on the western part of the continent and Cf or Cw on the eastern part. Extensive Aw climate is found to the north and south of the Amazon forest in Brazil and adjoining parts of Bolivia and Paraguay in South America, Sudan and south of Central Africa.

The annual rainfall in this climate is considerably less than that in Af and Am climate types and is variable also. The wet season is shorter and the dry season is longer with the drought being more severe. Temperature is high throughout the year and diurnal ranges of temperature are the greatest in the dry season. Deciduous forest and tree-shredded grasslands occur in this climate.

2. Dry Climates-B:

Dry climates are characterized by very low rainfall that is not adequate for the growth of plants. These climates cover a very large area of the planet extending over large latitudes from 15°-60° north and south of the equator. At low latitudes, from 15°-30°, they occur in the area of subtropical high where subsidence and inversion of temperature do not produce rainfall.

On the western margin of the continents, adjoining the cold current, particularly over the west coast of South America, they extend more equatorwards and occur on the coast land. Dry climates are divided into steppe or semi-arid climate (BS) and desert climate (BW).

They are further subdivided as subtropical steppe (BSh) and subtropical desert (BWh) at latitudes from 15°-35° and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at latitudes between 35°-60°.

Subtropical Steppe (BSh) and Subtropical Desert (BWh) Climates:

Subtropical steppe (BSh) and subtropical desert (BWh) have common precipitation and temperature characteristics. Located in the transition zone between humid and dry climates, subtropical steppe receives slightly more rainfall than the desert, adequate enough for the growth of sparse grasslands. The rainfall in both the climates is highly variable. Fog is common in coastal deserts bordering cold currents. Maximum temperature in the summer is very high.

3. Warm Temperate (Mid-Latitude) Climates-C:

Warm temperate (mid-latitude) climates extend from 30°-50° of latitude mainly on the eastern and western margins of the continents. These climates generally have warm summers with mild winters.

They are grouped into four types:

(i) Humid Subtropical, i.e. dry in winter and hot in summer (Cwa);

(ii) Mediterranean (Cs);

(iii) Humid Subtropical, i.e. no dry season and mild winter (Cfa);

(iv) Marine west coast climate (Cfb).

(i) Humid Subtropical Climate (Cwa):

Humid subtropi­cal climate occurs poleward of Tropic of Cancer and Capricorn, mainly in North Indian plains and South China interior plains. The climate is similar to Aw climate except that the temperature in winter is warm.

(ii) Mediterranean Climate (Cs):

As the name suggests, Mediterranean climate occurs around Mediterranean sea, along the west coast of continents in subtropical latitudes between 30° 40° latitudes e.g.—-Central California, Central Chile, along the coast in south eastern and south western Australia. These areas come under the influence of subtropical high in summer and westerly wind in winter. Hence, the climate is characterized by hot, dry summer and mild, rainy winter.

(iii) Humid Subtropical (Cfa) Climate:

Humid subtropical climate lies on the eastern parts of the continent in subtropical latitudes. In this region the air masses are generally unstable and cause rainfall throughout the year. They occur in eastern United States of America, southern and eastern China, southern Japan, northeastern Argentina, coastal South Africa and eastern coast of Australia.

(iv) Marine West Coast Climate (Cfb):

Marine west coast climate is located poleward from the Mediterranean climate on the west coast of the continents. The main areas are- North western Europe, west coast of North America, north of California, southern Chile, southeastern Australia and New Zealand. Due to marine influence, the temperature is moderate and in winter, it is warmer than for its latitude.

4. Cold Snow Forest Climates-D:

Cold snow forest climates occur in the large continental area in the northern hemisphere between 40°-70° north latitudes in Europe, Asia and North America.

Cold snow forest climates are divided into two types:

(i) Df-cold climate with humid winter;

(ii) Dw-cold climate with dry winter.

The severity of winter is more pronounced in higher latitudes.

(i) Cold Climate with Humid Winters (Df):

Cold climate with humid winter occurs poleward of marine west coast climate and mid latitude steppe. The annual ranges of temperature are large. The weather changes are abrupt and short. Poleward, the winters are more severe.

(ii) Cold Climate with Dry Winters (Dw):

Cold climate with dry winter occurs mainly over Northeastern Asia. The development of pronounced winter anti-cyclone and its weakening in summer sets in monsoon, like reversal of wind in this region. Poleward summer temperatures are lower and winter temperatures are extremely low with many locations experiencing temperatures below freezing point for up to seven months in a year. Precipitation occurs in summer.

5. Polar Climates-E:

Polar climates exist poleward beyond 70° latitude. Polar climates consist of two types:

(i) Tundra (ET)

(ii) Ice Cap (EF)

(i) Tundra Climate (ET):

The tundra climate (ET) is so called after the types of vegetation, like low growing mosses, lichens and flowering plants. This is the region of permafrost where the sub-soil is permanently frozen. The short growing season and water logging support only low growing plants. During summer, the tundra regions have very long duration of daylight.

(ii) Ice Cap Climate (EF):

The ice cap climate (EF) occurs over interior Greenland and Antartica. Even in summer, the temperature is below freezing point. This area receives very little precipitation. The snow and ice get accumulated and the mounting pressure causes the deformation of the ice sheets and they break. They move as icebergs that float in the Arctic and Antarctic waters. Plateau Station, Antarctica, 79°S, portray this climate.

6. Highland Climates-H:

Highland climates are governed by topography In high mountains, large changes in mean temperature occur over short distances. Precipitation types and intensity also vary spatially across high lands. There is vertical zonation of layering of climatic types with elevation in the mountain environment.

Essay # 12. Climate Change:

Archaeological findings show that the Rajasthan desert experienced wet and cool climate around 8,000 B.C. The period 3,000-1,700 B.C. had higher rainfall. From about 2,000-1,700 B.C., this region was the centre of the Harappan civilization.

Dry conditions accentuated since then. In the geological past, the earth was warm some 500-300 million years ago, through the Cambrian, Ordovician and Silurian periods. During the Pleistocene epoch, glacial and inter-glacial periods occurred, the last major peak glacial period was about 18,000 years ago. The present inter-glacial period started 10,000 years ago.

Climate in the Recent Past:

The 1990s recorded the warmest temperature of the century and some of the worst floods around the world. The worst devastating drought in the Sahel region, south of the Sahara desert, from 1967-1977 is one such variability.

During the 1930s, severe drought occurred in south­western Great Plains of the United States, described as the dust bowl. Europe witnessed ‘Little Ice Age’ from 1550 to about 1850. From about 1885-1940 world temperature showed an upward trend. After 1940, the rate of increase in temperature slowed down.

Causes of Climate Change:

The astronomical causes are the changes in solar output associated with sunspot activities. Sunspots are dark and cooler patches on the sun which increase and decrease in a cyclic manner. According to some meteorologists, when the number of sunspots increases, cooler and wetter weather and greater storminess occur.

An another astronomical theory is Millankovitch oscillations, which infer cycles in the variations in the earth’s orbital characteristics around the sun, the wobbling of the earth and the changes in the earth’s axial tilt.

All these alter the amount of insolation received from the sun, which in turn, might have a bearing effect, on the climate. Volcanism is considered as another cause for climate change. The most important anthropogenic effect on the climate is the increasing trend in the concentration of greenhouse gases in the atmosphere which is likely to cause global warming.


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