Grades 3–5 · Earth & Space
Why can one side of a mountain be wetter than the other?
Moist air carried by wind may meet a mountain range. To cross the mountains, some of that air rises. At higher elevations the surrounding pressure is lower, so the rising air expands and cools.
In the diagramRising and descending air create different conditionsrising air coolsdescending air warmswetter wind-facing sidedrier rain shadowOne wind pattern; actual precipitation varies.
Cooling can allow water vapor to condense into cloud droplets. If enough droplets or ice particles grow, rain or snow falls on the wind-facing side. The air has then lost some of its carried water.
Air descending on the other side encounters higher pressure and warms as it is compressed. Warmer descending air is less likely to form clouds from its remaining moisture. That side can therefore receive much less precipitation.
This relatively dry area is called a rain shadow. The mountain does not simply catch every raindrop like an umbrella. Its effect on air movement, temperature and moisture helps create different conditions on its two sides.
About this explanation
Starting knowledge: Know that air contains water vapor and that cooling can help droplets form.
Sources
- NOAA — Teek and Tom Educator’s Guide: Lesson 2