Atmospheric Circulation

Coriolis Force

The Coriolis force is the effect of earth rotation on the direction of the wind. The Coriolis force arises for two reasons, first our directional system of latitude and longitude has been fixed to a rotating earth. Thus, our frame of reference for monitoring the direction of a free-moving object above the earth is constantly changing. The second reason is the amount of turning about a vertical axis varies from a maximum at the poles and minimum at the equator. Demonstrate this by standing a pencil on end at the north pole and turn the globe. The pencil completes one full rotation. But standing the pencil on end at the equator and rotating the earth yields no rotation about a vertical axis. Figure PC.5 shows the deflection that a missile experiences when shot from the North Pole towards the Equator. Because the Earth, and the target, has rotated underneath the free-moving missile, it appears that the missile has veered off course (changed direction). Such is the case for winds blowing above the surface. The deflection works the same way for an east-west wind, the path with will curve to the right as it moves across the surface. Go to Coriolis Force animation

Figure PC.5 The influence of the Coriolis force on wind.
Courtesy NASA JPL (Source)

Though the air is deflected to the right of its path in the Northern hemisphere, in the Southern Hemisphere wind is deflected to the left of its path. Why the difference? It all has to do with perspective. Pick up a globe and spin it in a west to east direction. Now look down on it from above the North Pole. It appears to by spinning in counterclockwise direction. Now keep it spinning from west to east, lift globe over your head and look at it from above the South Pole. It appears to be going in a clockwise direction. (Kind of weird, huh?)

Friction

The last influence on wind is the friction that features, especially hills and mountains, create as air moves across the surface. The friction imposed on the wind mechanically slows the wind and diverts its direction. The friction layer is the layer of air that is influenced by friction caused by the surface. The friction layer varies in height across the Earth depending on the surface configuration. Generally speaking wind speed increases with height above the surface. Mount Washington is the windiest place in the United States as explained in the video "Angry Skies" from National Geographic.

Geostrophic Winds

As air moves in response to a pressure gradient, the air initially moves directly from higher pressure toward lower pressure. The Coriolis effect then bends the wind away from its straight-line path. Friction further alters the path of the wind hence, air blows across isobars at an angle (we'll see this later).  Above the "friction layer", only the pressure gradient and Coriolis effect operate on wind. At particular latitudes, the opposing pressure gradient and Coriolis forces can balance one another high in the troposphere above the friction layer. When this occurs, winds tend to blow parallel to isobars. Winds that blow roughly parallel to isobars are called "geostrophic winds". The fast-moving jet streams are type of geostrophic wind.

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