NCERT Textbook: Water in the Atmosphere - UPSC PDF Download

 Page 1


WATER IN THE ATMOSPHERE
Y
ou have already learnt that the 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 capacity
at a given temperature is known as the relative
humidity. With the change of air temperature,
the capacity to retain moisture increases or
decreases and the relative humidity is also
affected. It is greater over the oceans and least
over the continents.
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
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is added or withdrawn due to evaporation and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 vapourisation.
Increase in temperature increases water
absorption and retention capacity of the given
parcel of air. Similarly, if the moisture content
is low, air has a potentiality of absorbing and
retaining moisture. Movement of air replaces
the saturated layer with the unsaturated layer.
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
CHAPTER
© NCERT
not to be republished
Page 2


WATER IN THE ATMOSPHERE
Y
ou have already learnt that the 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 capacity
at a given temperature is known as the relative
humidity. With the change of air temperature,
the capacity to retain moisture increases or
decreases and the relative humidity is also
affected. It is greater over the oceans and least
over the continents.
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
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 vapourisation.
Increase in temperature increases water
absorption and retention capacity of the given
parcel of air. Similarly, if the moisture content
is low, air has a potentiality of absorbing and
retaining moisture. Movement of air replaces
the saturated layer with the unsaturated layer.
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
CHAPTER
© NCERT
not to be republished
WATER IN THE ATMOSPHERE 99
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; (iv) 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.
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
(0
0
C), 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. 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 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;
(iv) 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
© NCERT
not to be republished
Page 3


WATER IN THE ATMOSPHERE
Y
ou have already learnt that the 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 capacity
at a given temperature is known as the relative
humidity. With the change of air temperature,
the capacity to retain moisture increases or
decreases and the relative humidity is also
affected. It is greater over the oceans and least
over the continents.
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
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 vapourisation.
Increase in temperature increases water
absorption and retention capacity of the given
parcel of air. Similarly, if the moisture content
is low, air has a potentiality of absorbing and
retaining moisture. Movement of air replaces
the saturated layer with the unsaturated layer.
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
CHAPTER
© NCERT
not to be republished
WATER IN THE ATMOSPHERE 99
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; (iv) 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.
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
(0
0
C), 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. 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 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;
(iv) 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
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 100
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.
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
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 – 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
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.
Identify these cloud types which are
shown in Figure 11.1 and 11.2.
Figure 11.1
Figure 11.2
© NCERT
not to be republished
Page 4


WATER IN THE ATMOSPHERE
Y
ou have already learnt that the 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 capacity
at a given temperature is known as the relative
humidity. With the change of air temperature,
the capacity to retain moisture increases or
decreases and the relative humidity is also
affected. It is greater over the oceans and least
over the continents.
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
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 vapourisation.
Increase in temperature increases water
absorption and retention capacity of the given
parcel of air. Similarly, if the moisture content
is low, air has a potentiality of absorbing and
retaining moisture. Movement of air replaces
the saturated layer with the unsaturated layer.
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
CHAPTER
© NCERT
not to be republished
WATER IN THE ATMOSPHERE 99
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; (iv) 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.
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
(0
0
C), 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. 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 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;
(iv) 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
© NCERT
not to be republished
FUNDAMENTALS OF PHYSICAL GEOGRAPHY 100
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.
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
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 – 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
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.
Identify these cloud types which are
shown in Figure 11.1 and 11.2.
Figure 11.1
Figure 11.2
© NCERT
not to be republished
WATER IN THE ATMOSPHERE 101
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
You have already read about extra tropical
cyclones and cyclonic rain in Chapter 10.
Please consult Chapter 10 to understand
cyclonic rainfall.
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.
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. 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.
EXERCISES
1. Multiple choice questions.
(i) Which one of the following is the most important constituent of the
atmosphere for human beings?
(a) Water vapour (c) Dust particle
(b) Nitrogen (d) Oxygen
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Page 5


WATER IN THE ATMOSPHERE
Y
ou have already learnt that the 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 capacity
at a given temperature is known as the relative
humidity. With the change of air temperature,
the capacity to retain moisture increases or
decreases and the relative humidity is also
affected. It is greater over the oceans and least
over the continents.
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
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 vapourisation.
Increase in temperature increases water
absorption and retention capacity of the given
parcel of air. Similarly, if the moisture content
is low, air has a potentiality of absorbing and
retaining moisture. Movement of air replaces
the saturated layer with the unsaturated layer.
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
CHAPTER
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WATER IN THE ATMOSPHERE 99
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; (iv) 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.
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
(0
0
C), 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. 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 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;
(iv) 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
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 100
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.
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
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 – 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
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.
Identify these cloud types which are
shown in Figure 11.1 and 11.2.
Figure 11.1
Figure 11.2
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WATER IN THE ATMOSPHERE 101
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
You have already read about extra tropical
cyclones and cyclonic rain in Chapter 10.
Please consult Chapter 10 to understand
cyclonic rainfall.
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.
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. 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.
EXERCISES
1. Multiple choice questions.
(i) Which one of the following is the most important constituent of the
atmosphere for human beings?
(a) Water vapour (c) Dust particle
(b) Nitrogen (d) Oxygen
© NCERT
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FUNDAMENTALS OF PHYSICAL GEOGRAPHY 102
 (ii) Which one of the following process is responsible for transforming liquid
into vapour?
(a) Condensation (c) Evaporation
(b) Transpiration (d) Precipitation
(iii) The air that contains moisture to its full capacity :
(a) Relative humidity (c) Absolute humidity
(b) Specific humidity (d) Saturated air
(iv) Which one of the following is the highest cloud in the sky?
(a) Cirrus (c) Nimbus
(b) Stratus (d) Cumulus
2. Answer the following questions in about 30 words.
(i) Name the three types of precipitation.
(ii) Explain relative humidity.
(iii) Why does the amount of water vapour decreases rapidly with altitude?
(iv) How are clouds formed? Classify  them.
3. Answer the following questions in about 150 words.
(i) Discuss the salient features of the world distribution of precipitation.
(ii) What are forms of condensation?  Describe the process of dew and frost
formation.
Project Work
Browse through the newspaper from 1st June to 31st December and note the news
about extreme rainfall in different parts of the country.
© NCERT
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