Rain Shadows - dry areas on downwind sides of mountains - are prominent on the Big Island of Hawaii and east of the Cascade Range in the Pacific Northwest. Many rain shadows exist around the world - including north of Asia’s Himalayas, west of Austalia's Great Dividing Range, and east of California's Sierra Nevada Range.

Three essential items are needed to develop the rain shadow effect - an ocean nearby, winds blowing steadily onshore, and a mountain range to block the traveling air mass. Evaporation on the surface of the ocean creates moist air. Prevailing winds push the wet air inland until it hits the base of the mountains. The air is forced to rise. As the air lifts, it expands and cools. Cooler air can’t hold as much moisture, so clouds form and it rains a bunch, resulting in a lush, green landscape. The now dry air mass crosses the mountains and begins to sink on the leeward side of the range. It compresses and warms, promoting evaporation. The result? Dry areas on downwind sides of mountains. A rain shadow!

Tom Foster (http://HUGEfloods.com) and Nick Zentner (Central Washington University) have been hiking together in Washington for years. This ’Rain Shadow’ episode is part of a “2 Minute Geology” video series.