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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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