Mountains have a windward side and a leeward side. A “rain shadow” is an area of dry climate on and around their leeward side.

Rain shadows occur because of a pretty simple set of factors. First, prevailing winds bring moist air from oceans or lakes toward the mountain.

Next, the altitude of the mountains means that the air is forced up toward their peaks. As the moist air moves up, it gets colder and starts precipitating, leaving rain and/or snow on the windward side of the mountain.

Once the moist air reaches the leeward side of the mountain, it starts to sink and dry out. One dramatic example of a rain shadow is in India’s Ghat Mountains, where parts of the windward side get 10 times as much precipitation as the leeward side.

Senior writer Chris DeWeese edits Morning Brief, The Weather Channel’s newsletter.