What Is a Rain Shadow?

Ever wonder why mountains are often snowcapped or have a halo of clouds encircling their peaks, while their foothills and valleys are dry and clear? Orographic rain shadows are low-precipitation areas found on the side of a mountain that's sheltered from the wind, also called the leeward side. As rain-producing winds travel from west to east across mountain ranges, the mountains themselves block the weather's passage, collecting moisture on one side of the ridge and casting a “shadow” of dryness behind it on the other side.  

The rain shadow effect explains why places like Reno, Nevada, and Cody, Wyoming, have drier climates. Rain shadows are also the reason why some deserts are drier than they would be otherwise. Here, we look at how and where rain shadows occur.

How Rain Shadows Form

Rain shadows form when air moves from west to east across mountain ranges, which act as barriers to the flow of air.

When winds blow against a mountain, they have nowhere to go except be forced to ascend its sloping terrain. As air hikes up the mountain slope, it expands and cools adiabatically. Dry air typically cools by 5.5 degrees Fahrenheit for every 1,000 feet it rises.

If a mountain's elevation is high enough, the air cools to its dew point temperature, at which point it reaches saturation and holds as much water vapor as it can. If the air is lifted beyond this point, its water vapor will begin condensing, forming cloud droplets and eventually precipitation. The now-moist air also continues cooling but at a rate of 3.3 degrees every 1,000 feet. When air is lifted over a topographic barrier, it's called orographic lift.

If the air that reaches the mountaintop is cooler than the surrounding air already at the summit, it will sink down the leeward side of the mountain. As it descends, it compresses and heats adiabatically. By now, there isn't much moisture remaining in the air, so very little precipitation falls on the east side of the mountain's crest. 

By the time the air reaches the base of the mountain, it can be many degrees warmer than it was originally. It can also move more swiftly because gravity pulls on the air mass as it travels thousands of feet downhill. According to AccuWeather, a 40- to 50-mph wind along a mountain ridge can increase to 100 mph by the time it reaches the mountain valleys. This phenomenon is known as a chinook or a foehn wind.

The taller the mountain range, the more pronounced its rain shadow effect will be.

Where Do Rain Shadows Occur?

Rain shadows are found on the world's prominent mountain ranges. 

For example, the eastern slopes of California and Nevada’s Sierra Nevada Mountains is home to the hottest place on Earth (134 degrees) and one of the driest places in North America—the rain shadow desert known as Death Valley, which sees an average of two inches of rainfall every year. Travel to the Sierra Nevada's western slopes, however, and you’ll find an area so well-watered, it’s the only natural habitat of the giant sequoia, the most massive trees on Earth.

New Zealand’s Southern Alps create one of the most remarkable rain shadow effects on Earth. The over 12,000-foot tall mountains intercept the moisture-laden air streaming onshore from the Tasman Sea, squeezing more than 390 inches of precipitation from them in an average year. Meanwhile, in the South Island’s Central Otago region, a distance less than 70 miles from the Alps, annual rainfall totals can be as low as 15 inches.

This striking difference can be easily viewed on satellite imagery: The shoreline west of the mountains appears as a deep, verdant green color, while the landscape east of the mountains is a dry and dusty tan.  

Rain shadows can also be found in the vicinity of the Rocky Mountains, the Appalachian Mountains, South America’s Andes Mountains, Asia’s Himalayas, and others. And some of the world's famous deserts, including the Gobi Desert in Mongolia and Argentina's Patagonia Desert, exist because they're on the leeward side of mountains.