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Cause of Wind
Name: Phil K.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 3/26/2004
Question:
How do we get wind?
Replies:
Phil K.,
The basic cause of all winds can be traced to contrasts in temperature. These differences
occur because air is not heated at all points with equal intensity. The differences also
occur on scales of varying magnitude. A coal-stove fire, for example, causes differences
of heating in a small cabin, At the seashore on a summer afternoon, differences in
temperature exist between the hot sand and cool water. On a planetary scale, the
equatorial belt is warmer than the Temperate Zones.
When air is heated, its molecules are agitated and their movement accelerated. They tend
to draw away from one another and the air expands. As the molecules expand, they occupy a
greater volume and the density of the heated air parcel is decreased. Like a huge
invisible bubble, the heated air starts to rise. Surrounding cooler air flows in to
replace the rising air. This movement of air, from cooler (higher pressure) to warmer
(lower pressure) areas is wind.
I hope that this helps.
Sincerely,
Bob Trach
Dear Phil-
Wind is the movement of air from a region of high pressure to a region of
lower pressure. These areas of high and low pressure arise from temperature
differences caused by the sun heating the earth, which in turn heats the
atmosphere.
Because sunlight is more intense at the equator than at the poles, the air
is heated more at the equator. The heated air rises and spreads towards the
poles in each direction. The cooler air at the surface near the poles flows
south to replace the warmed air at the equator. The rotation of the earth
diverts the winds in a curved path as it moves.
There are other influences that cause pressure and temperature differences,
but the final result is air movement in an attempt to equalize pressure
differences.
Wendell Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
Phil,
There a couple of physical mechanisms that contribute
to producing wind.
Horizontal temperature and atmospheric pressure
differences (called gradients) across the land surface
and in the air cause wind to occur.
Thinking very simplistically, the air at the equator is
warmer than at the poles, so warm air rises over
the equator and heads toward the cooler poles (warm
air tries to replace cooler air). As the air goes further
north it cools and sinks to the ground at the poles. This
results in cool air from the poles draining back towards the
equator. Now add a turning Earth; this deflects the air to
the right in the northern hemisphere and to the left in
the southern hemisphere.
Next, cool air is more dense than warm air, so the vertical
gradient of air pressure from the ground to the upper
atmosphere is greater (also meaning that the atmosphere
is not as deep) over the poles than over the equator. This also
means that air pressure (where it is cooler and because there
is less mass of air above the surface), tends to be lower
over the poles and higher over the equator (where the air is
warm and thus there is more mass of air above the surface).
Horizontal temperature differences tend to create horizontal
as well as vertical air pressure differences, as temperature
and pressure are directly related.
Horizontal pressure differences (which tend to align
with horizontal temperature differences because of the direct
relationship between temperature and air pressure) are most
responsible for causing the wind, and atmospheric circulations
of air (high and low pressure areas) caused by the turning
Earth, determine the wind direction.
The larger the horizontal pressure difference (gradient) is,
the greater the winds are. On cool Spring days
after a cold front has passed, it tends to be quite windy; if
you would look at a weather map that has isobars (lines of constant
pressure) on it, you would see that the isobars are close together,
reflecting the large horizontal change in air pressure over
short horizontal distances.
Something that contributes to higher wind speeds is convection,
the lifting of air parcels as they are warmed by the Earth's
surface (often reflected by individual cumulus or so-called "fair
weather" clouds, which are each produced over a thermal plume from
the ground). The vertical motion of the warm air rising in the thermal
plume is changed into enhanced horizontal wind by the general
motions of the atmosphere; I could try to explain this more fully,
but it would become too long a discourse for our purposes here.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
There are many mechanisms for generating wind from a "spring breeze" to a
"tornado". All of the mechanisms have a common origin. Air in regions of
higher density will flow into regions of lower density until the two
densities are equal. The difference in density can arise from a temperature
difference, a water vapor difference, or a pressure difference. In turn
these variables can arise form a variety of causes -- sunlight, the earth's
rotation, evaporation of water over a body of water, cold air (more dense)
passing over the crest of a mountain over a warm valley (less dense),...,
the list is long, but the central thread is a difference in density of two
bodies of air.
Vince Calder
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Update: June 2012
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