Precipitation and its Forms
The falling of water droplets and ice crystals and their other forms on ground is called precipitation. It includes drizzle, rain, cloud burst, snowfall, sleet and hail.
When vapour rise up in atmosphere, condensation takes place and the tiny water droplets and ice crystals form the clouds. It is, however, not necessary that these clouds may cause precipitation. These tiny droplets ascend further and under certain favourable conditions join together. When they become bigger and heavy and cannot remain suspended in atmosphere, the condensed particles fall on the ground in the form of precipitation.
Forms of Precipitation
Drizzle: The light rain falling in very tiny drops is called drizzle. These droplets are tiny particles of 0.5 mm diameter or smaller. They are so tiny that even light wind may blow them away. It is frequent over large areas of the World's oceans particularly in the colder regions of the subtropics.
Rainfall: Rainfall is in the form of drops of water. These raindrops may vary in diameter from 0.5 to 7 mm. Sudden and violent rainfall is termed as a shower. It lasts only for a short while. The shower drops of rainfall are large and heavy. They may even be of 8 mm diameter.
Snowfall: When the temperature of surrounding air during condensation falls below the freezing point, the condensed particles may fall as snowflakes or as minute particles of cold ice. Snowfall occurs from the clouds formed with the mixture of ice crystals and very cold water droplets. Major snow-prone areas include the polar regions, the upper half of the Northern Hemisphere and mountainous regions of Southern Hemisphere.
Sleet: Sleet includes both rain drops and snow flakes. It is of two types: 1) Sleet due to frozen rain drops. 2) Sleet due to refrozen melted snow water.
Formation of Sleet: The condition under which the temperature of a layer of air near the ground falls below the freezing point and the layer of air above it has the temperature above the freezing point is most favorable for the formation of sleet. This precipitation falls from upper warm layers and upon crossing the cold layer below it reaches the ground in partially solidified form as pellets of ice together with cold rain drops. Areas where sleet is common are UK, USA, Ireland and most Commonwealth countries.
Hail: The precipitation in the form of hard solidified pellets of ice is known as hail. These pellets may be rounded and small sized like those of peas.They generally have a diameter of 5 mm or more. The small ice granules may also grow in size and their structure may resemble that of an onion and may grow to the size of a tennis ball. In this case, the bigger granules have frozen layered structure. The bigger granules cause great harm to standing crops. Taken together as a whole, agricultural crops suffer losses worth crores of rupees all over the world due to hailstorms.
Formation of Hail: The precipitation in the form of hail is generally caused by thunderstorms. During thunderstorms, cumulus clouds may be found up to 12 km height. Under such conditions, the rising air current takes water droplets to great heights. These then get frozen. The frozen pellets with their weight try to fall on the ground but strong air current bounces them back. They then again get frozen around other smaller particles present in the atmosphere. It may happen several times until the pellets are large enough or the currents may have weakened that the air is unable to hold them. This is known as the hailstorm.
Areas of HailStorm: In two zones of 30° to 60° North and South, hail storms are very common.