Ever marvel why mountains are sometimes snowcapped or have a halo of clouds encircling their peaks, whereas their foothills and valleys are dry and clear? Orographic rain shadows—low-precipitation areas discovered on the leeward facet (the facet sheltered from wind) of mountains—are sometimes in charge. As rain-producing winds journey from west to east throughout mountain ranges, the mountains themselves block the climate’s passage, squeezing out moisture on one facet of the ridge and casting a “shadow” of dryness behind it on the opposite facet.
This rain shadow impact not solely explains why locations like Reno, Nevada, and Cody, Wyoming, have drier climates; it is also why some deserts, together with the Sahara Desert, which lies within the shadow of Africa’s Atlas Mountains, are drier than they in any other case could be.
The Formation of a Rain Shadow
Rain shadows kind when air strikes from west to east throughout mountain ranges, which act as limitations to the circulate of air. (Within the center latitudes—the areas between the tropics and the polar circles—all winds journey from west to east.) When winds blow in opposition to a mountain, they’ve nowhere to go besides be pressured to ascend its sloping terrain. As air hikes up the mountain slope, it expands and cools adiabatically. (As a normal rule, dry air usually cools by 5.5 levels F for each 1,000 ft it rises.)
What Is Adiabatic Heating/Cooling?
An adiabatic course of (from the Greek phrase “adiabatos,” that means “impassible”) is one the place heating or cooling happens with out warmth being actively added or eliminated. Enlargement and compression are adiabatic processes; air molecules occupy extra and fewer area, respectively, inflicting temperature fluctuations.
If a mountain’s elevation is excessive sufficient, the air cools to its dew level temperature, at which level it reaches saturation, or holds as a lot water vapor as it could. If the air is lifted past this level, its water vapor will start condensing, forming cloud droplets and finally precipitation. The now-moist air additionally continues cooling, however at a fee of three.3 levels F each 1,000 ft. When air is lifted on this fashion, that’s, over a topographic barrier, it is known as orographic carry.
If the air that reaches the mountaintop is cooler than the encompassing air already in place on the summit, it will need to sink down the leeward, or sheltered facet, of the mountain. Because it descends, it compresses and heats adiabatically. By now, there’s little moisture remaining within the air, so little or no precipitation falls on the east facet of the mountain’s crest.
By the point the air reaches the bottom of the mountain, it may be many levels hotter than it was initially. It will probably additionally transfer extra swiftly, too, since gravity pulls on the air mass because it travels 1000’s of ft downhill. Based on AccuWeather, a 40- to 50 mph wind alongside a mountain ridge can enhance to 100 mph by the point it reaches the mountain valleys. This phenomenon is named a chinook, or a foehn wind.
The taller the mountain vary, the extra pronounced its rain shadow impact shall be.
Areas The place Rain Shadows Happen
Rain shadows are discovered the place the world’s outstanding mountain ranges are.
For instance, the jap slopes of California and Nevada’s Sierra Nevada Mountains is dwelling to the most well liked place on Earth (134 levels F) and one of many driest locations in North America—the rain shadow desert generally known as Demise Valley, which sees 2 inches of rainfall on common yearly. Journey to the Sierra Nevada’s western slopes, nevertheless, and also you’ll discover an space so well-watered, it’s the one pure habitat of the large sequoia, probably the most large timber on Earth.
New Zealand’s Southern Alps create some of the outstanding rain shadow results on Earth. The over 12,000-foot tall mountains intercept the moisture-laden air streaming onshore from the Tasman Sea, squeezing greater than 390 inches of precipitation from them in a median yr. In the meantime, within the South Island’s Central Otago area, a distance lower than 70 miles from the Alps, annual rainfall totals as little as 15 inches aren’t unparalleled. This putting distinction could be simply seen on satellite tv for pc imagery: The shoreline west of the mountains seems a deep, verdant inexperienced shade, whereas the panorama east of the mountains is a dry and dusty tan.
Rain shadows may also be discovered within the neighborhood of the Rocky Mountains, the Appalachian Mountains, South America’s Andes Mountains, Asia’s Himalayas, and others. And among the world’s well-known deserts, together with the Gobi Desert in Mongolia, and Argentina’s Patagonia Desert, exist as a result of they’re on the leeward facet of mountains.