Wind

In an ideal situation, one could draw the direction of winds blowing over an area simply by looking at the isobars on a weather map. But the earth is not an ideal situation. At least two important factors affect the direction in which winds actually blow: the Coriolis effect and friction. The Coriolis effect is a pseudoforce that appears to be operating on any moving object situated on a rotating body, such as a stream of air traveling on the surface of the rotating planet. The effect of the Coriolis force is to deflect winds from the straight-forward direction that we might expect them to take simply from an examination of isobars. In the Northern Hemisphere, the Coriolis effect tends to deflect winds to the right and in the Southern Hemisphere, it tends to drive winds to the left.

Imagine how the Coriolis effect will determine the movement of winds in the Northern Hemisphere. Suppose that air initially begins to move from west to east as a result of pressure gradient forces. At once, the Coriolis effect will begin to drive the stream of air to the right, that is, to the south. The actual path followed by the wind, then, is a compromise between the pressure gradient force and the Coriolis force. Since each of these forces can range widely in value, the precise movement of wind in any one case is also variable.

At some point, the two forces driving the wind are likely to come into balance. At that point, the wind begins to move in a straight line that is perpendicular to the direction of the two forces. Such a wind is known as a geostrophic wind.