Under certain favourable condition when a warm air mass and cold air mass meet,the wormer air mass is lifted over the colder one with the formation of a front.The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitate.
     Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth’s surface. Rain, drizzle, hail and snow are examples of precipitation. In India, rain is the most common form of precipitation.
     Evapotranspiration is the process which returns water to the atmosphere and thus completes the hydrologic cycle. Evapotranspiration consists of two parts, Evaporation and Transpiration. Evaporation is the loss of water molecules from soil masses and water bodies. Transpiration is the loss of water from plants in the form of vapour. We proceed on to discuss precipitation, and its most important component in India context, the rainfall.
Causes of precipitation
    For the formation of clouds and subsequent precipitation, it is for necessary that the moist air masses to cool in order to condense. This is generally accomplished by adiabatic cooling of moist air through a process of being lifted to higher altitudes. The precipitation types can be categorized as.
Frontal precipitation This is the precipitation that is caused by the expansion of air on ascent along or near a frontal surface.
Convective precipitation Precipitation caused by the upward movement of air which is warmer than its surroundings. This precipitation is generally showery nature with rapid changes of intensities.
Orographic precipitation      Precipitation caused by the air masses which strike the mountain barriers and rise up, causing condensation and precipitation. The greatest amount of precipitation will fall on the windward side of the barrier and little amount of precipitation will fall on leave ward side.
     For the Indian climate, the south-west monsoon is the principal rainy season when over 75% of the annual rainfall is received over a major portion of the country. Excepting the south-eastern part of the Indian peninsula and Jammu and Kashmir, for the rest of the country the south-west monsoon is the principal source of rain.
     From the point of view of water resources engineering, it is essential to quantify rainfall over space and time and extract necessary analytical information.

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