PRECIPITATION

PRECIPITATION
Formation of water particles or ice within the cloud that fall towards earth's surface is precipitation. It occurs when condensation takes place rapidly within the cloud. Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface, appearing as streamers below cloud base-a phenomenon known as virgo.

(i) RAIN Form of precipitation consisting of the drops of water formed by the coalescence of minute condensation droplets within the clouds. In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm, although smaller drops can be called rain if they are widely scattered.
(ii) DRIZZLE Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together. Normally drizzle is produced by stratus and stratocumulus clouds.

(iii) SLEET Generally it implies. a form of precipitation consisting of either partly-melted snowflakes or rain and snow falling together. In USA, it implies a form of precipi­tation consisting of frozen raindrops that have subse­quently eartially melted.

(iv) SNOW A form of precipitation consisting of crystals of ice. It is produced when condensation takes place at a temperature below freezing point, so that the minute
crystals (spicules) of ice form directly from the water vapour. These may fall as they are but more commonly they combine together to form snowflakes, which display an infinite variety of patterns.

(v) HAIL Precipitation in the form of ice-pellets (hail stones) that develop in and fall from cumulonimbus clouds, either at a cold front or where intense heating of surface causes rapidly-ascending convection currents. The hail stone develops in the updraught of air as water vapour freezes onto the surface of a nucleus embryo of ice in the cloud. When it has grown sufficiently, its weight overcomes the force of the updraught and it falls. Hail consists of rounded lumps of ice, having an internal structure of concentric layers much like an onion.