Relief rain occurs when warm, moist air ascends along the slope of a mountain. The air then condenses and rainfall occurs on the windward side. The leeward side of the mountain receives very little rainfall. This is called the rain shadow region.
Relief rain is also termed orographic rain. It's generally seen in coastal regions where a chain of mountains runs parallel to the coast. The oncoming sea breeze bares moisture from the sea and carries the moisture towards the hills.
Many parts of the West Indies and the United Kingdom receive relief rain. The western coast of India, along the Western Ghats, also receives orographic rain.