If there is any obstruction like a mountain or a high land in the direction of the humid winds coming from over sea, these winds dash against the windward side of the mountain and rise.
The rising air becomes cool and clouds are formed from the vapour, which cool and give rain on windward side.
This is called Orographic or relief Rain.
After showering some rain these winds become less humid.
They cross the mountain and descend on the leeward side of the mountain.
Air pressure increases of the descending winds and they become warmer.
As a result, these winds give less rainfall on the other side of the mountains.
This area on the leeward side is called Rain shadow area.
Konkan and Malabar coast on the western side of Western Ghats are windward regions so there is more rainfall, while the Deccan Plateau on the eastern side of Western Ghats is a rain shadow area so there is less
rainfall.
On the windward side of Western Ghats, Mumbai gets about $200\ cm$ rainfall annually, while there is hardly $80\ cm$ of rain in Pune which is on the Leeward side of the Western Ghats.
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