Precipitation occurs when tiny droplets of water, ice or frozen water vapor join together into masses too big to be held above the earth. They then fall to ground as precipitation.

Precipitation in the form of drops of water is called rainfall, when the drop size is more than 5 mm.

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.

Conventional 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 takesplace 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 a saturated air mass encounters a mountain, it is compelled to ascend, causing it to expand (due to a decrease in pressure). As the air rises, its temperature decreases, leading to the condensation of moisture.
• 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, it undergoes condensation, and soon it reaches saturation.
• The surplus moisture falls down as orographic precipitation along the windward slopes.
• The chief characteristic of this sort of rain is that the windward slopes receive 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 bremain rainless and dry.
• The area on the leeward side, which receives less rainfall, is known as the rain-shadow area. The rain-shadow effect directly contributes to the formation of arid and semi-arid regions. 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.

Frontal Precipitation

• The meeting of two air masses with different temperatures produces turbulent conditions. Along the front convection occurs and causes 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.

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