What is Orographic Rainfall?

By K Balaji|Updated : December 2nd, 2022

Orographic rainfall is a phenomenon that occurs when moist air moves over a range of mountains. It is the precipitation that happens when air that is filled with moisture is lifted and moves over a range of mountains. Another term for orographic rainfall is relief precipitation. 

Causes of Orographic Rainfall

When the mountains act as a barrier to the airflow, the air rises and the moist air is forced to move upslope. This causes the moist air to cool down and produce clouds or precipitation in the form of rain or snow. The precipitation and the clouds are more concentrated on the windward side of the mountain.

The form of precipitation can depend on whether the air is unstable or stable. Hence, if the air is unstable, there are more chances of thunderstorms. There are generally 2 causes of orographic rainfall:

  • The upward vertical propagation of the moist air upon an orographic slope is caused by the extreme heating of the surface of the mountain barrier.
  • The upward deflection of the large-scale horizontal flow of air is caused by orography.
  • When a mountain range blocks moist air from large water bodies and causes it to be lifted over the mountain range, heavy precipitation occurs over the mountains' upwind regions.
  • Mountains on the Lee side are known as the rain shadow because of their ridge and low rainfall.

Summary:

What is Orographic Rainfall? 

When moist air passes over a mountain range, a phenomenon known as orographic rainfall or relief precipitation occurs. The clouds act as a source of rain when the air rises and cools. Additionally, because of how the mountains block airflow, most precipitation occurs upwind of the mountain ridge.

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