The Wind in the Willows
When the atmospheric pressure varies between the horizontal points, the wind blows. As the English poet Christina Rossetti once wrote, "Who has seen the wind, neither I nor you." But that hasn't stopped us from measuring it. A wind vane is almost as old as the beginning of time. A simple pennant or flag will point in the direction from which the wind is coming. On airport runways, a cone-shaped bag opened at each end extends horizontally and points in the direction of the wind. This simple wind vane is called a windsock. Pilots can tell the wind direction as they land.
A pressure plate anemometer is the oldest type of anemometer. It measures wind speed by a single swinging metal plate rather than the cups of a modern anemometer.
An anemometer measures wind speed, while a weather vane or wind vane measures wind direction.
By the way and very importantly, the wind direction is never the direction the wind is going, but rather, the direction from which the wind is coming. A northeast wind is coming from the northeast. A south, or southerly, wind blows from the south.
What about wind speed? Again, back in the seventeenth century, English physicist Robert Hooke came up with a device consisting of a plate that moved proportionally to the wind speed. The plate swung out farther in a stronger wind. That became known as a pressure plate anemometer.
In more recent times, rotating cups mounted on a shaft have been used for measuring wind speeds (see the figure at the end). This cup anemometer records the wind speed as the cups spin. Those rotating cups might turn some gears or generate an electrical current. The greater current indicates higher wind speeds.
Some anemometers, called aerovanes, look like airplane propellers. The device swings into the wind, and then the propeller blades rotate with the increasing wind speed.
Something that easily beats an old rusty anemometer is the traditional Beaufort scale, which relates wind speeds to the motion of objects such as trees or water. Anemometers become inaccurate because of their constant exposure.
Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.