Magnitude and Direction of Forces

Friction

Friction opposes, or decelerates, the wind. Friction is largest in the atmospheric boundary layer, the air layer nearest the ground. Friction always slows down the wind, and therefore also reduces the Coriolis force. The magnitude of frictional force is determined by the roughness of the surface and the speed of the wind. The force of friction over still water or an ice pond is small. Trees in a forest strongly oppose the wind, so the frictional force of forests is large. While the wind may be blusterous over an ice pond, it will be more calm in the woods.

Meteorologists consider the effects of friction when dealing with the air within 20 to 200 meters of the surface. This layer is called the surface layer; it is here that frictional drag is important in determining wind direction and speed. In the surface layer friction acts to slow the wind, and thus always acts opposite to the wind direction. Its magnitude increases with increasing wind speed.