Types of Precipitation
On the basis of origin, rainfall may be classified into three main types – the convectional, orographic or relief and the cyclonic or frontal.
Convectional Rainfall
- The, air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat and consequently, condensation takes place and cumulous clouds are formed. This process releases latent heat of condensation which further heats the air and forces the air to go further up.
- Convectional precipitation is heavy but of short duration, highly localised and is associated with minimum amount of cloudiness. It occurs mainly during summer and is common over equatorial doldrums in the Congo basin, the Amazon basin and the islands of south-east Asia.
Orographic Rainfall
- When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands (because of fall in pressure); the temperature falls, and the moisture is condensed.
- This type of precipitation occurs when warm, humid air strikes an orographic barrier (a mountain range) head on. Because of the initial momentum, the air is forced to rise. As the moisture laden air gains height, condensation sets in, and soon saturation is reached. The surplus moisture falls down as orographic precipitation along the windward slopes.
- The chief characteristic of this sort of rain is that the windward slopesreceive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area (Some arid and semi-arid regions are a direct consequence of rain-shadow effect. Example: Patagonian desert in Argentina, Eastern slopes of Western Ghats). It is also known as the relief rain.
- Example: Mahabaleshwar, situated on the Western Ghats, receives more than 600 cm of rainfall, whereas Pune, lying in the rain shadow area, has only about 70 cm.
- The Wind Descending on the Leeward Side is heated adiabatically and is called Katabatic Wind.
Frontal Precipitation
- This type of rainfall occurs along the zone of contact between a warm and cool air mass.
- When two large air masses of different temperature meet, the warmer and hence lighter air is lifted above the cooler air.
- Warm air then rises, cools and condenses to form rain.
- The boundary that separates cold air and warm air is called a front.
