Clouds And Rain (Condensation And Precipitation Of Water Vapour)

Air always contains some amount of water vapour at a given temperature. When the temperature of air falls below a certain temperature, the excess water vapour present in the air gets transformed into tiny droplets of water. This process is called condensation. The temperature at which condensation takes place is called the dew point. A surface called nucleus is required for the condensation to take place. Dust particles, tree leaves, grass, etc. are examples of nuclei.

During the day, a large amount of water vapour goes to air which then rises up, expands and gets cooled. When the temperature falls below the dew point, the excess of water present in the air gets condensed around the dust particles forming tiny droplets of water. When the temperature drops further, these droplets get condensed into tiny crystals of ice which being light floats in the sky. Millions of such droplets come together to form larger droplets called clouds. When these droplets become big enough and heavy, they cannot stay in the air. They come down on Earth as rain.

Fog is the condensation of water vapours at lower layers of atmosphere whereas smog is a combination of smoke and fog in industrial areas. Snow is ice crystals formed by the condensation of water vapour at very low temperature (below 0°C).

Patterns of rainfall of an area or region are determined by the prevailing wind patterns. In large parts of our country, rains are mostly brought by the south-west monsoons and to a smaller extent by north-east monsoons.

Weather reports often say ‘depressions’ have caused rains in some areas. What are ‘depressions’?