What Causes Wind?
Jeffrey T., Key West, FL
Answered by Justin Caudell
Jeffrey, one of nature’s most devastating phenomena is wind and I am glad that you want to find out just
how it is we have wind.
Wind is caused by air flowing from high pressure to low pressure. Since the Earth is rotating, however,
the air does not flow directly from high to low pressure, but it is deflected to the right in the Northern
Hemisphere or to the left in the Southern Hemisphere, so thus the wind flows around the high and low
pressure areas. This effect of the wind feeling the Earth turn underneath it is important for very large and
long lived pressure systems. For small, short-lived systems such as in the cold outflow of a thunderstorm, the
wind will flow directly from high pressure to low pressure.
The closer the high and low pressure areas are together, the stronger the pressure gradient, and the
stronger the winds. On weather maps, lines of constant pressure are drawn, which are called isobars. These
isobars are usually labeled with their pressure value in millibars and usually are marked as high pressure is
red and low pressure blue. The closer these lines are together, the stronger the wind. The curvature of the
isobars is also important to the wind speed. Given the same pressure gradient (isobar spacing), if they are
curved anti-cyclonically, the wind will be stronger. If the isobars are curved cyclonically, the wind will be
weaker.
Near the surface of the Earth, friction from the ground slows the wind down. During the day, when
convective mixing is stirring up the lower atmosphere, this effect is minimized. At night, however, when
convective mixing has stopped, the surface wind can slow considerably, or even stop altogether.
Wind can also be thought of one way that the atmosphere moves excess heat around. All wind is, directly
or indirectly, helping to transport heat either away from the surface of the Earth, where sunlight causes an
excess of energy buildup, or from warm regions (usually the tropics) to cooler regions (usually the higher
latitudes). Extratropical cyclones accomplish much of this heat transport outside of the tropics, while in the
tropics the trade winds, monsoons, and hurricanes transport much of the heat.
The highest wind speed ever recorded on the surface of the Earth was 231 mph on April 12 1934, atop
Mt. Washington, New Hampshire.