The Windward Versus Leeward Side of a Mountain

Matterhorn clouds
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In meteorology, "leeward" and "windward" are technical terms that indicate the direction in which the wind is blowing in regard to a specific point of reference. These points of reference can be a number of things, including ships at sea, islands, buildings, and—as this article will explore—mountains.

In all circumstances in which the terms are used, the windward side of the reference point is the one that faces the prevailing wind. The leeward—or "lee"—side is the one sheltered from the wind by the reference point.

Windward and leeward aren't frivolous terms. When applied to mountains, they are important factors in weather and climate—one is responsible for enhancing precipitation in the vicinity of mountain ranges, while the other withholds it.

Windward Slopes Give Air (and Precipitation) a Boost

Mountain ranges act 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 (the windward side) 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 that falls on the windward slope and at the summit. Known as "orographic lifting," this event is one of three ways precipitation can form.

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. This is often the eastern side of the mountain range because prevailing winds in the mid-latitudes blow from the west, but that is not necessarily always the case.

In contrast to the moist windward side of a mountain, 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 been 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." This causes clouds to dissipate and further reduces the possibility of precipitation, an occurrence known as the "rain shadow effect." It is the reason why locations at the base of mountain lees tend to be some of the driest places on earth. The Mojave Desert and California's Death Valley are two such rain shadow deserts. 

Winds that blow down the lee side of mountains are called "downslope winds." They not only carry low relative humidity but also rush down at extremely strong speeds and can bring temperatures more than 50 degrees Fahrenheit warmer than the surrounding air. "Katabatic winds" like the Santa Ana Winds in Southern California are an example of such winds; these are infamous for the hot, dry weather they bring in autumn and for fanning regional wildfires. "Foehns" and "chinooks" are other examples of these warming downslope winds.