17. Precipitation

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 on to 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 the 00C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals. These crystals form flakes of snow. Usually the amount of snowfall is included in the rainfall figures. Besides rain and snow, other forms of precipitation are sleet and hail, though the latter are limited in occurrence and are sporadic in both time and space.

Sleet: Snow is not frozen rain. The term sleet is used for the frozen raindrops and the re-frozen melted snow water in the cold layer of the air near the Earth’s surface. Sleet also refers to a mixture of snow and rain.

Hailstones: Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones.

Hailstone mostly in the cumulo-nimbus clouds. Small droplets of water are formed in the lower part of the clouds due to condensation. Many of these small droplets join together to form large ones. The strong rising convection current carries these raindrops to the higher levels, which causes freezing and gives rise to small ice pellets. The strength of the vertical current is highly variable. Thus the ice pellets are not taken up continuously. They fall for some distance, slightly melt at the lower levels and are carried up again. This happens several times until the weight of the ice pellets becomes so heavy that they cannot be carried up by the current. Ultimately these ice pellets fall as hailstones on the Earth. Hailstones have several concentric

layers of ice one over the other. The size of the halistones depends upon the amount of ice it collects during its ascent and descent in the atmosphere by the convection current.

Hailstones occur widely in the world, except in the polar regions, the hot deserts and the equatorial region. The occurrence of hailstones is common during the spring and the early summer in the sub tropical and the temperate regions.