Environment Planet Earth What Is a Rain Shadow? By Tiffany Means Writer University of North Carolina at Asheville Johns Hopkins University Tiffany Means is a meteorologist who has worked for CNN, the National Oceanic and Atmospheric Administration, and more. Since 2017, she has worked as a freelance science writer covering natural disasters, the climate crisis, and the environment. our editorial process Twitter Twitter LinkedIn LinkedIn Tiffany Means Updated May 31, 2021 An orographic, or mountain-triggered, rain shower passes over New Zealand's Southern Alps near Queenstown. Nigel Killeen / Getty Images Share Twitter Pinterest Email Planet Earth Weather Outdoors Conservation 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—low-precipitation areas found on the leeward side (the side sheltered from wind) of mountains—are often to blame. As rain-producing winds travel from west to east across mountain ranges, the mountains themselves block the weather's passage, squeezing out moisture on one side of the ridge and casting a “shadow” of dryness behind it on the other side. This rain shadow effect not only explains why places like Reno, Nevada, and Cody, Wyoming, have drier climates; it's also why some deserts, including the Sahara Desert, which lies in the shadow of Africa’s Atlas Mountains, are drier than they otherwise would be. The Formation of a Rain Shadow VectorMine / Getty Images Rain shadows form when air moves from west to east across mountain ranges, which act as barriers to the flow of air. (In the middle latitudes—the regions between the tropics and the polar circles—all winds travel from west to east.) 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. (As a general rule, dry air typically cools by 5.5 degrees F for every 1,000 feet it rises.) What Is Adiabatic Heating/Cooling? An adiabatic process is one where heating or cooling occurs without heat being actively added or removed. For example, when air expands (or compresses) its molecules occupy more (less) space and move more slowly (energetically) within that space, thereby causing a decrease (increase) in temperature. If a mountain's elevation is high enough, the air cools to its dew point temperature, at which point it reaches saturation, or 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 F every 1,000 feet. When air is lifted in this style, that is, over a topographic barrier, it's called orographic lift. If the air that reaches the mountaintop is cooler than the surrounding air already in place at the summit, it'll want to sink down the leeward, or sheltered side, of the mountain. As it descends, it compresses and heats adiabatically. By now, there's little 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, too, since 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. Regions Where Rain Shadows Occur A rain shadow, as viewed from the eastern (leeward) side of a mountain. Truhon Leong / Getty Images Rain shadows are found where the world's prominent mountain ranges are. For example, the eastern slopes of California and Nevada’s Sierra Nevada Mountains is home to the hottest place on Earth (134 degrees F) and one of the driest places in North America—the rain shadow desert known as Death Valley, which sees 2 inches of rainfall on average 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 as low as 15 inches aren’t unheard of. This striking difference can be easily viewed on satellite imagery: The shoreline west of the mountains appears a deep, verdant green color, while the landscape east of the mountains is a dry and dusty tan. The rain shadow on New Zealand's South Island is so prominent, it's visible from space. Antii Lipponen / Flickr / CC By 2.0 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. View Article Sources "Temperatures - Dry Bulb/Web Bulb/Dew Point." Weather.Gov. Sosnowski, Alex. "What Are Chinook Winds?" Accuweather. "Weather - Death Valley National Park." National Park Service, 2020. "Giant Sequoia (Sequoiadendron Giganteum)." University Of California Agriculture And Natural Resources. Scarsbrook, Mike, and Charles Pearson. "Rain, Snow And Ice." Te Ara - The Encyclopedia Of New Zealand, 2008.