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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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