Mountains exist all across the world and serve a wide range of purposes from defining important landscapes to providing a plethora of opportunities for recreational activity. Additionally, they help maintain a suitable habitat for certain plant and animal life, offer beautiful views of nature, and, interestingly, can even create their own weather. When air moving across the ground reaches the base of a mountain, it is forced to move upward to the atmosphere. This lifting of air originating at the surface can eventually result in the formation of clouds and potentially even precipitation.
There are four primary atmospheric mechanisms that allow the formation of clouds to occur, each of which deals with the upward motion of air. A very common mechanism is frontal lifting. This describes the displacement of air masses (essentially large areas of the atmosphere where air shares similar qualities) over one another. Convergence refers to winds flowing in opposing directions at the surface and the subsequent upward motion of air. A third possibility, air may move vertically in the atmosphere as a result of buoyancy, as less dense relatively warm air rises above denser cold air. Lastly, a mountain or other geographic feature can provide a barrier that forces horizontally moving air upward. This process is known as orographic lift. Just as with the other three mechanisms, orographic lift can result in the formation of clouds and precipitation.
When air rises in the atmosphere as a result of any of these four primary atmospheric mechanisms, it cools. Since cold air cannot hold the large amount of water vapor warm air can, condensation of vapor into liquid droplets (and thus cloud formation) becomes much more likely to occur. Due to the higher elevation of mountain ranges, much of this cloud formation may take place near or just above the mountain top and result in the appearance of fog (essentially a cloud that has formed very close to the ground). Fog frequently settles into areas of lower elevation as relatively warm and dense air at the base of the mountain rises upward, making room for the cooler and denser fog to sink to the surface. When this physical process occurs within the context of a mountain range, the fog is referred to as valley fog. However, it is important to note that cloud formation as a result of orographic lift is not limited to the height of the mountain and may extend hundreds to even thousands of feet above the summit.
Another interesting feature of mountain weather is called a rain shadow. Rain shadows are created on the back (leeward) side of a mountain where air descends downslope. As air is pushed downward and approaches the surface, it warms and makes cloud formation unlikely to nearly impossible. The combination of downward moving air and warmer temperatures create dry atmospheric conditions unfavorable for precipitation. One well known example of this phenomenon exists on the eastern side of the Sierra Nevada mountain range, which runs directly north and south along the border of Nevada and California. Due to the extremely low elevation of the leeward side of the Sierra Nevada and the perpendicular westerly wind flow, these mountains have helped create one of the strongest rain shadows on Earth. As a result, the area on the eastern side of these mountains known as Death Valley is considered one of the hottest and driest places in all of North America.
While primarily known as a geographic feature, mountains have important meteorological impacts and influence not only short term weather, but also long term climate. From clouds that last hours to the yearly effects of low precipitation, mountains are an important part of our environment, atmosphere and ultimately, the Earth we live on.
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©2019 Weather Forecaster Dennis Weaver