Kind Of A Drag |
 Dave Thurlow, Host |
I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook. Every wind that blows near the surface of the earth is slowed down by friction as air meets ground, making our world much calmer than it could be. The effect is most dramatic within a few inches of the ground; that's why it's recommended to lie as low as possible if you're caught outdoors in a tornado. But even a little higher, friction can make a difference.
Accurate anemometers are required to be ten meters, or about thirty-three feet, above ground to minimize the impact of surface friction. You have to go up about 300 meters before the effects of surface friction on the wind are gone completely. At this height, the winds can be more than twice as strong as they are below. Sometimes you can see this difference with clouds racing overhead, while the winds at ground level are relatively weak. Friction's effect on wind is greatest where there are trees and hills, so places like Pennsylvania tend to be less windy than the open plains of the Dakotas.
Ironically, friction can make some hurricane winds actually stronger. When a hurricane moves onshore, its winds drop off because the land surface is rougher than the ocean. However, scientists have found that the turbulent up and down winds produced by land friction can cause higher winds aloft to come down to the surface in short damaging gusts. These can be even stronger than the hurricane's gusts over open water, a fact that's really a drag.
Funding for The Weather Notebook is provided by The National Science Foundation. Our show is underwritten by Subaru, maker of the all Weather Legacy. Subaru -- the beauty of All-Wheel Drive.