Definition
A region of relatively dry air and reduced precipitation on the leeward (downwind) side of a mountain range, caused when moist air is forced upward on the windward side, cools, and drops most of its moisture as rain or snow before descending the other side warmer and drier.
Plain English
The dry area found behind a mountain range, where the air arrives after losing most of its moisture climbing up the other side.
Context Anchor
Seen in weather planning for flights near mountains, especially when comparing conditions on opposite sides of a range.
Derivation
The term comes from the idea that the mountain casts a 'shadow' — not from sunlight, but from rainfall. Just as an object blocks light and creates a shaded area behind it, the mountain blocks moisture and creates a dry area behind it.
Why Pilots Care
Pilots may find clearer skies and better visibility on the dry side but must still watch for strong downslope winds and mechanical turbulence near the terrain.
Analogy
Like standing in the shade of a tall building while the sunny side stays bright, except the 'shade' here is created by rain being stopped.
Grounding Statement
Picture moist Pacific air hitting the Sierra Nevada: it dumps rain and snow on the western slopes, then spills down the eastern side dry and warm — which is why Nevada sits in the Sierra's rain shadow.
Intuition Check
A rain shadow is not a dark shadow made by a cloud. It is a dry weather area caused by mountains removing moisture from moving air.
Example Sentence 1
The valley sits in the rain shadow of the coastal range, so we usually find clear skies and good visibility on the eastern side.
Example Sentence 2
The METAR showed improving conditions once the aircraft entered the rain shadow behind the coastal range.