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is added or withdrawn due to evaporation and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 referr