LEEWARD VERSUS WINDWARD SIDE OF A MOUNTAIN - Mountain ranges acts as barriers to the flow of air across the surface of the earth. When warm air travels from a low valley to the foothills of a mountain range, it rises along the slope of the mountain as it encounters higher terrain. As the air is lifted up the mountain slope, it cools as it rises. This cooling often results in the formation of clouds, and precipitation, which falls on the windward slope and at the summit. As this already dry air descends down the lee, it warms and expands which causes clouds to dissipate and further reduces the possibility of precipitation. It is the reason why locations at the base of a mountain lee tend to be some of the driest places on Earth.

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Leeward Versus Windward

The Leeward vs. Windward Side of a Mountain

by Tiffany Means

In meteorology, leeward and windward are technical names for the directional sides of a mountain.

The windward side is that side which faces the prevailing wind (upwind), whereas the leeward, or "lee" side, is the side sheltered from the wind by the mountain's very elevation (downwind).

Windward and leeward aren't just arbitrary terms, they are important weather and climate factors.

One is responsible for enhancing precipitation in the vicinity of mountain ranges, and the other, for withholding it.

Windward Mountain Slopes Give Air (and Precipitation) a Boost

Mountain ranges acts as barriers to the flow of air across the surface of the earth.

When a parcel of warm air travels from a low valley region to the foothills of a mountain range, it is forced to rise along the slope of the mountain as it encounters higher terrain.

As the air is lifted up the mountain slope, it cools as it rises (a process known as adiabatic cooling).

This cooling often results in the formation of clouds, and eventually, precipitation which falls on the windward slope and at the summit. Known as orographic lifting, this event is one of three ways precipitation can form (the other two are frontal wedging and convection). 

The Northwestern United States and the Front Range Foothills of Northern Colorado are two examples of regions that regularly see precipitation induced by orographic lift.

Leeward Mountain Slopes Encourage Warm, Dry Climates

Opposite from the windward side is the lee side -- the side sheltered from the prevailing wind.

(Because prevailing winds in the mid-latitudes blow from the west, the lee side can generally be thought of as the eastern side of the mountain range. This is true most times -- but not always.)

In contrast to the windward side of a mountain which is moist, the leeward side typically has a dry, warm climate.

This is because by the time air rises up the windward side and reaches the summit, it has already stripped of the majority of its moisture.

As this already dry air descends down the lee, it warms and expands (a process known as adiabatic warming), which causes clouds to dissipate and further reduces the possibility of precipitation.

This occurrence is known as the rain shadow effect. It is the reason why locations at the base of a mountain lee tend to be some of the driest places on Earth.

The Mojave Desert and California's Death Valley are two such rain shadow deserts. 

Downslope winds (winds that blow down the lee side of mountains) not only carry low relative humidity, they also rush down at extremely strong speeds and can bring temperatures as much as 50+ degrees Fahrenheit warmer than the surrounding air. 

Katabatic winds, foehns, and chinooks are all examples of such winds.

The Santa Ana Winds in Southern California are a well-known katabatic wind infamous for the hot, dry weather they bring in autumn and for fanning regional wildfires. 

Tiffany Means

·   Studied atmospheric sciences and meteorology at the University of North Carolina

·   Former administrative assistant for the National Oceanic and Atmospheric Administration

·   Member of the American Meteorological Society

Experience

Tiffany Means is a former writer for ThoughtCo who contributed articles about weather for five years. She has interned with the domestic and international weather departments at CNN, written monthly climate reports for NOAA’s National Centers for Environmental Prediction, and participated in a number of science outreach events, including the Science Olympiad Competition. Means has personally experienced such weather greats as the Blizzard of 1993 and the floods of Hurricane Francis (2004) and Ivan (2004).

Education

·   Bachelor's degree in atmospheric sciences and meteorology from the University of North Carolina at Asheville

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https://www.thoughtco.com/leeward-and-windward-sides-of-mountain-3444015