Deserts and Mountains

What's the term for a patch of land that has become a desert, because a mountain range has blocked precipitation?

And the answer: rain shadow.

Photo credit: Gordon Wiltsie via National Geographic

In a rain shadow effect, rain and snow can occur on one side of a mountain, but on the other side of the mountain, it's warm and dry. The Tibetan Plateau is an example of a rain shadow, where the enormous Himalayas block moisture from entering Tibet and China.

Rain shadows are dry areas on the backsides of mountains. As all tall structures do, the mountain creates a shadow, yet in this case it's a shadow of dryness. To create this effect, a climate needs three essential elements: the ocean, wind, and a mountain range to block the air.

Evaporation on the surface of the ocean creates moist air, while winds push the wet air inland until it reaches the base of the mountains. The air is forced to rise, and as it does, it expands and cools. Cooler air can't hold as much moisture, so clouds form and rain pours down. The result? A lush green landscape – on one side of the mountain, at least.

The now-dry air mass crosses the mountains, and begins to sink on the leeward side. As it falls, it compresses and warms, promoting evaporation. Dry air warms one degree Celsius per one hundred meter of elevation drop.

Some of the driest places in the world exist because of the rain shadow effect. A rain shadow creates arid land west of the Great Dividing range in Australia, east of the Sierra Nevada mountain range in California, and even southwest of the tropical Mauna Kea in Hawaii. Though, that's not to say the effect is wholly detrimental. Without the condensation of moisture on the mountainside, many landscapes would be devoid of lush forms of life that call these regions home.

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