Chapter-11- 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. It is expressed quantitatively in

different ways.

 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 ability of the air to hold water vapour depends entirely on its temperature

 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 for evaporation. The temperature at which the water starts evaporating is

referred to as the latent heat of vapourisation.

 Increase in temperature increases water absorption and retention capacity of the given parcel

of air.

 Movement of air replaces the saturated layer with the unsaturated layer. Hence, the greater

the movement of air, the greater is the evaporation.

 Condensation- The transformation of water vapour into water.

 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.

 Particles of dust, smoke and salt from the ocean are particularly good nuclei because they

absorb water.

 Condensation also takes place when the moist air comes in contact with some colder object and

it may also take place when the temperature is close to the dew point.

 Condensation, therefore, depends upon the amount of cooling and the relative humidity of

the air. Condensation is influenced by the volume of air, temperature, pressure and

humidity.

Condensation takes place:

(i) when the temperature of the air is reduced to dew point with its volume remaining constant;

(ii) when both the volume and the temperature are reduced;

(iii) when moisture is added to the air through evaporation. However, the most favourable

condition for condensation is the decrease in air temperature.

 After condensation the water vapour or the moisture in the atmosphere takes one of the

following forms — dew, frost, fog and clouds. Condensation takes place when the dew point is

lower than the freezing point as well as higher than the freezing point.

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.

Frost

 Frost forms on cold surfaces when condensation takes place below freezing point (00C),

i.e. the dew point is at or below the freezing point.

 The excess moisture is deposited in the form of minute ice crystals instead of water droplets.

 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.

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.

 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.

 In urban and industrial centres smoke provides plenty of nuclei which help the

formation of fog and mist.

 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 nuceli 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.

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; 4- nimbus.

Cirrus

 Cirrus clouds are formed at high altitudes (8,000 - 12,000m).

 They are thin and detatched clouds having a feathery appearance. They are always

white in colour.

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.

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.

Nimbus

 Nimbus clouds are black or dark gray. They form 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

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 00C, 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. Besides rain and snow, other forms of precipitation are 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.

 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.

Types of Rainfall

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

orographic or relief and the cyclonic or frontal.

Convectional 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 lightening, 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.

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.

 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 Rain

already covered in last chapter

World Distribution of Rainfall

 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 350 and 400 N and S of the equator, the rain is heavier on the eastern

coasts and goes on decreasing towards the west.

 But, between 450 and 650 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 land 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. T

 he 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.