Aviation Weather

full text of the classic FAA guide

FRICTION

Friction between the wind and the terrain surface slows the wind. The rougher the terrain, the greater is the frictional effect. Also, the stronger the wind speed, the greater is the friction. One may not think of friction as a force, but it is a very real and effective force always acting opposite to wind direction.

As frictional force slows the windspeed, Coriolis force decreases. However, friction does not affect pressure gradient force. Pressure gradient and Coriolis forces are no longer in balance. The stronger pressure gradient force turns the wind at an angle across the isobars toward lower pressure until the three forces balance as shown in figure 27. Frictional and Coriolis forces combine to just balance pressure gradient force. Figure 28 shows how surface wind spirals outward from high pressure into low pressure crossing isobars at an angle.

FIGURE 27. Surface friction slows the wind and reduces Coriolis force but does not affect pressure gradient force; winds near the surface are deflected across the isobars toward lower pressure.

FIGURE 28. Circulation around pressure systems at the surface. Wind spirals outward from high pressure and inward to low pressure, crossing isobars at an angle.

The angle of surface wind to isobars is about 10° over water increasing with roughness of terrain. In mountainous regions, one often has difficulty relating surface wind to pressure gradient because of immense friction and also because of local terrain effects on pressure.

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