Air contains water vapour. It varies from zero to four per cent by volume of the atmosphere and
plays an important role in the weather phenomena. 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 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 capacityat 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.
The temperature at which saturation occurs in a given sample of air is known as dew point.
WATER IN THE ATMOSPHERE EVAPORATION AND CONDENSATION
The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and
condensation respectively.
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
vaporization.
Hence, the greater the movement of air, the greater is the evaporation.
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. 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.
Forms of condensation can be classified on the basis of temperature and location.
CHAPTER -11 WATER IN THE ATMOSPHERE
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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 1. clear sky, 2. calm air, 3. high relative humidity, 4. 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 (0C), i.e. the dew
point is at or below the freezing point. The excess moisture is deposited in the form of minute ice
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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. 0
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.
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 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.
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.
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.
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;
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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 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. 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. 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.
Types of Rainfall
On the basis of origin, rainfall may be classified into three main types –
1. the convectional, 2. orographic or relief and 3. 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 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.
cyclonic Rainfall Orographic rainfall
CONVECTIONAL RAINFALL
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
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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
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.
1. In general, as we proceed from the equator towards the poles, rainfall goes on decreasing
steadily.
2. The coastal areas of the world receive greater amounts of rainfall than the interior of the
continents.
3. The rainfall is moreover the oceans than on the landmasses of the world because of being great
sources of water.
4. Between the latitudes 35 and 40 N and S of the equator,
5. the rain is heavier on the eastern coasts and goes on decreasing towards the west.
6. But, between 45 and 65 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.
7. 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 ANNUALPRECIPITATION, MAJOR PRECIPITATION
REGIMES OF THEWORLD ARE IDENTIFIED AS FOLLOWS.
1. 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.
2. Interior continental areas receive moderate rainfall varying from 100 - 200 cm per annum.
3. The coastal areas of the continents receive a moderate amount of rainfall.
4. 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.
5. 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.
6. Seasonal distribution of rainfall provides an important aspect to judge its effectiveness.
7. 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.