Condensation is the process of water vapour changing to the liquid state. If air is cooled below its dew point, some of the air’s water vapour becomes liquid.
Thus, any further cooling of the saturated air starts the process of condensation. Whenever the dew point temperature falls below the freezing point (0°C), water vapour may convert directly into ice by the process of sublimation.
However, Byers prefers the term ‘crystallization.’ He applies the term ‘sublimation’ to ice directly converting into water vapour without passing through the intermediary liquid state. Condensation may start with the addition of any further water vapour to the saturated air, or with the reduction of its temperature.
Condensation depends upon two factors: relative humidity of air, and degree of cooling. Therefore in arid lands a larger degree of cooling is necessary before the dew point is reached, while in humid climates a lesser degree of cooling will start the process of condensation.
In the process of evaporation heat is transformed into work which results in cooling the evaporating surface. Condensation, on the other hand, is the reverse process. Here an equal amount of energy is transformed into heat.
This released heat is called the latent heat of condensation which raises the temperature of air. It is to be borne in mind that this heat energy plays a vital role in different weather processes. The latent heat of condensation lowers the rate of cooling of air.
Under normal conditions, no sooner than the dew point is passed, condensation begins. But there are occasions when condensation starts only after air has cooled much below its dew point.
In such a condition, the air is said to be supersaturated. This happens only when air contains very small dust particles. Under abnormal conditions condensation may start even before the dew point is reached.