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Foundations
Rain Shadows
Cross a single mountain range and the world can change completely: dripping green forest on one flank, cracked desert on the other. The mountain did not just divide the land, it divided the weather. This is a rain shadow, and it is one of the tidiest cause-and-effect stories in all of geography.
How it works
When moist air off the ocean runs into a mountain range, it has nowhere to go but up. As it rises it cools, and cool air cannot hold as much water, so the moisture condenses and falls as rain or snow on the windward side. By the time that air spills over the crest and sinks down the far side, it has been wrung dry, and it warms as it descends. The windward slope gets a soaking; the leeward slope, sitting in the mountain's "shadow," gets almost nothing.
Green on one side, brown on the other
The pattern shows up all over the planet. In the western United States, wet Pacific air drenches the windward slopes, then descends into the parched Great Basin and Death Valley behind the Sierra Nevada. In South America, the same Andes that feed lush eastern rainforests cast a rain shadow that helps create the dry Patagonian steppe. And on the wet side, foothills in the Himalaya include some of the rainiest places on Earth, where the rising air simply cannot hold its water.
Reading a rain shadow in a clue
A rain shadow gives a clue two flavors to play with. A place described as improbably dry "behind" or "east of" a big range is sitting in the shadow. A place drenched with rain against the windward side of mountains is on the wet flank. If a clue pairs a mountain range with a surprising climate, the rain shadow usually explains it, and points you to the correct side of the ridge.
Bottom Line
Mountains force moist air upward, so it rains on the windward side and the leeward side stays dry. That one mechanism produces coastal rainforests and interior deserts a short distance apart. When a clue puts a strange climate next to a mountain range, read the rain shadow and pick your side.
Frequently Asked Questions
What is a rain shadow?
A rain shadow is the dry area on the downwind side of a mountain range, created because the air has already dropped most of its moisture climbing the windward side.
Why does it rain more on one side of a mountain?
Moist air forced up a mountain cools and condenses, releasing rain on the windward slope. The air then descends the far side warm and dry, leaving the leeward slope arid.
What are examples of rain shadow deserts?
The Great Basin and Death Valley sit in the rain shadow of the Sierra Nevada, and the dry Patagonian steppe lies in the rain shadow of the Andes.
What is the windward side of a mountain?
The windward side is the slope facing the incoming wind, which receives heavy rain or snow. The opposite, sheltered slope is the leeward side, which sits in the rain shadow.