7. WIND

Three Forces That Affect the Wind

1. Pressure Gradient Force (PGF) 2. Coriolis Force 3. Friction

Upper Air Wind

Above the friction layer (lowest few thousand feet), only PGF and Coriolis force affect the horizontal motion of air. Coriolis force deflects it to the right. With the forces in balance, wind will remain parallel to contours. This is called the geostrophic wind.

Coriolis Force

Affects all moving objects, deflects air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. At a right angle to wind direction and directly proportional to wind speed. Varies with latitude from zero at the Equator to a maximum at the poles. Influences wind direction, effects are more pronounced in middle and high latitudes.

Surface Wind

At the surface of the Earth, all three forces come into play. As frictional force slows the wind speed, Coriolis force decreases. However, friction does not affect PGF. PGF and Coriolis force are no longer in balance. The stronger PGF turns the wind at an angle across the isobars toward lower pressure until the three forces balance.

Friction Force

Friction between the wind and the terrain surface slows the wind. The rougher the terrain, the greater the frictional effect. The stronger the wind speed, the greater the friction. Normally decreases with height and becomes insignificant above the lowest few thousand feet. However, this may vary somewhat since both strong winds and rough terrain extend the friction layer to higher altitudes.

Naming of the Wind

Named according to the direction from which it is blowing. A west wind (270°) indicates the wind is blowing from the west to the east.

Surface Wind Flow

The angle of surface wind to isobars is about 10° over water, increasing to as high as 45° over ragged terrain. The end result is, in the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure, and counterclockwise and inward into low pressure. High 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.

Pressure Gradient Force (PGF)

The force due to differences in pressure within the atmosphere that causes air to move and wind to blow. It is directly proportional to the pressure gradient. Whenever a pressure difference develops over an area, the PGF begins moving the air directly across the contours/isobars. PGF is directed from higher height/pressure to lower height/pressure and is perpendicular to contours/isobars.

Magnitude of PGF

Wind speed is directly proportional to the PGF (contour/isobar gradient) and flows from high to low pressure. Closely spaced contours/isobars indicate strong winds, while widely spaced contours/isobars mean lighter wind.