CONDITIONS FOR PRECIPITATION

CONDITIONS FOR PRECIPITATION There are three possible ways by which precipitation is produced.

(i) Convectional Precipitation is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point. Towering cumulonimbus clouds may form. Convection rain is often accompanied by lightning and thunder. In tropical latitudes this type of rain is usually torrential. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity, It is most common in equatorial regions and regions having a tropical monsoon climate.

(ii) Orographic means 'related to mountains'. The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises on the windward side, it is cooled at the adiabatic rate. If sufficiently cooled, precipitation results; when the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rainshadow, may exist on the leeward side.

Several of the important dry deserts of earth are of this type. Orographic rainfall is most common where no-shore winds rise up over hilly mountain regions lying parallel to the coast, e.g., British Columbia (Canada) and Scotland. In USA, prevailing westerlies bring moist air from the Pacific Ocean over the coast ranges of central and northern California and the great Sierra Nevada Range, Heavy rainfall is experienced on the windward side, while on the leeward side, air descends and becomes dry down the eastern face of sierras. This produces part pf America's great desert zone. Much of orographic rainfall is actually of convectional type, in that it takes the form of heavy convectional showers and thunderstorms. In monsoon areas, this type greatly augments the normal monsoon rainfall e.g., at Cheerapunji, on the windward slopes of the Khasi hills.

(iii) Cyclonic Precipitation (depression or frontal) occurs when large masses of air of different temperatures meet. The warm moist air of one air mass moves over the cold heavier air of another. Or, it is caused by air rising through horizontal convergence in an area of low pressure. Cyclonic rain is common throughout the doldrums where the trade winds meet. It is the precipitation along the frontal surfaces of a depression in mid and high latitudes. In tropical cyclones, the rainfall is often very heavy, but lasts only for a few hours. In temperate cyclones, it is much lighter but lasts for many hours, even days.

Thunderstorm It is an intense local storm accompanied by lightning and thunder that develops in large cumulon­imbus clouds, which result from rapid ascent of air under very unstable conditions. Such conditions occur either at the cold front of a depression or when the ground is intensely heated, and there must always be sufficient moisture in the air for cloud formation. The equatorial areas produce the highest frequency of thunderstorms a& the air is usually very humid and the hot sun produces the necessary up currents of air. Very heavy rainfall or hail occurs, accompanied by the discharge of electrical energy, produc­ing lightning and thunder.
Based on the cause of initial lift of air column, resulting in a storm, a classification can be made:
(i) Thermal or air mass thunderstorm set off by thermal convection caused by solar heating of the ground and lower layers of air; time or occurrence is typically in late afternoon when air temperatures near the ground are the highest;
(ii) Oro­graphic thunderstorms occur when the air is forced to rise
over a mountain range; the torrential monsoon rains of the Asiatic and East Indian mountain ranges are largely of this type; (iii) Frontal thunderstorm is caused when a layer of warm air is forced to rise over a layer of cold air.