The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and condensation respectively.

Evaporation

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.

Condensation

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.

Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.