19.2. Orographic Rain

It is also known as the relief rain. When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed. 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 remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area. For example, Mahabaleshwar lying on the windwardside of Western Ghats receives annual rainfall of about 622 cm as against Pune on the leeward side only 70 km away from Mahabaleshwar receives only 66 cm annual rainfall.

The windward slope of a mountain, at the time of rainfall, has cumulus clouds while the leeward slope has stratus clouds. The orographic rainfall may occur in any season. It is longer duration. The orographic rainfall is supported by convectional and cyclonic processes of condensation. Most of the precipitation in the world is orographic in nature.

In India, Cherrapunji in Meghalaya plateau, the Western Ghats and the entire Himalayan region receive Orographic Rainfall.