Water vapour present in the air is known as humidity. It is expressed quantitatively in different ways. The actual amount of water vapour present in the atmosphere is known as 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 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 onto 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 zero degrees celsius, precipitation takes place in the form of fine flakes of snow and is called snowfall.