What is a Rain Shadow?

In the northernmost region of the large island of Hawai’i, there is a tall mountain that separates the east from the west: Kohala Mountain. It has long been a trademark of the region for its incredible effect on the island. It’s responsible for creating the obvious rift in geography between the leeward and windward sides of the mountain that one can see in the image above. What about the mountain though causes one side to be lush with vegetation, while the other is basically a desert?

The answer has to do with orographic uplift, which is the upward movement of air along given mountains. Hawai’i lies within the area covered by the easterly trade winds, causing the eastern side of Kohala Mountain to be the first mountainous region to intercept winds coming over the ocean in northern Hawai’i. That moist air from the Pacific Ocean rises up over the mountains though orographic uplift and then condenses into clouds at the top of the mountain. Consequently, the windward side of Kohala receives plenty of precipitation and moisture, which creates its lush vegetation. However, once the air condenses and precipitates over the east, there is not much moisture left for the west side of the mountain. Instead, dry air falls back down the west side of the mountain and warms up, creating a desert. This is an example of a rain shadow, and they are present all over the world.

One very notable example aside from Hawai’i is the mid-west of the United States. In the continental United States, winds primarily come from the west over the Pacific Ocean. When that air reaches the Rocky Mountains, it creates a rain shadow over the leeward side of the mountains, which is part of the reason why it tends to be so dry in the mid-west. Additionally, the area west of the mountains gets a lot of rain and is why Seattle is known for being so rainy. Other instances of the rain shadow effect creating dry areas include the Atacama Desert in Chile due to the Andes Mountains, and the Gobi Desert in Mongolia due to the Himalayan Mountains.

Since rain shadows heavily impact people and wildlife on both sides of mountain chains, it’s an important phenomenon to know in order to understand the geography and climate of different areas around the world. Although it is only one piece of the puzzle in deciphering why different parts of the planet have different climates, the rain shadow is still an essential element in determining the behavior of the weather throughout the world.

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© 2018 Weather Forecaster Cole Bristow