Wind

The picture described above applies to winds that blow in the upper atmosphere. At distances of more than a kilometer or so above the ground, pressure gradient and Coriolis forces are the only factors affecting the movement of winds. Thus, air movements eventually reach an equilibrium point between pressure gradient forces and the Coriolis force, and geostrophic winds blow parallel to the isobars on a weather map.

Such is not the case near ground level, however. An additional factor affecting air movements near the earth's surface is friction. As winds pass over the earth's surface, they encounter surface irregularities and slow down. The decrease in wind speed means that the Coriolis effect acting on the winds also decreases. Since the pressure gradient force remains constant, the wind direction is driven more strongly toward the lower air pressure. Instead of developing into geostrophic winds, as is the case in the upper atmosphere, the winds tend to curve inward towards the center of a low pressure area or to spiral outward away from the center of a high pressure area.

Friction effects vary significantly with the nature of the terrain over which the wind is blowing. On very hilly land, winds may be deflected by 30 degrees or more, while on flat lands, the effects may be nearly negligible.