Prevailing Winds are those that blow constantly in certain parts of the world. They are produced by hot air moving north and south from the Equator and by cold air moving away from the poles. The prevailing winds are the Polar Easterlies, found in the extreme north and south; the Westerlies, blowing between 30° and 60° north and south of the Equator; and the Trade winds, which blow north-east and south-east, either side of the Equator.
Since the atmosphere is fixed to the earth by gravity and rotates with the earth, there would be no circulation if some force did not upset the atmosphere’s equilibrium. The heating of the earth’s surface by the sun is the force responsible for creating the circulation that does exist.
Because of the curvature of the earth, the most direct rays of the sun strike the earth in the vicinity of the equator resulting in the greatest concentration of heat, the largest possible amount of radiation, and the maximum heating of the atmosphere in this area of the earth. At the same time, the sun’s rays strike the earth at the poles at a very oblique angle, resulting in a much lower concentration of heat and much less radiation so that there is, in fact, very little heating of the atmosphere over the poles and consequently very cold temperatures.
Cold air, being more dense, sinks and hot air, being less dense, rises. Consequently, the rising warm air at the equator becomes even less dense as it rises and its pressure decreases. An area of low pressure, therefore, exists over the equator.
Warm air rises until it reaches a certain height at which it starts to spill over into surrounding areas. At the poles, the cold dense air sinks. Air from the upper levels of the atmosphere flows in on top of it increasing the weight and creating an area of high pressure at the poles.
The air that rises at the equator does not flow directly to the poles. Due to the rotation of the earth, there is a buildup of air at about 30° north latitude. (The same phenomenon occurs in the Southern Hemisphere). Some of the air sinks, causing a belt of high-pressure at this latitude.
The sinking air reaches the surface and flows north and south. The air that flows south completes one cell of the earth’s circulation pattern. The air that flows north becomes part of another cell of circulation between 30° and 60° north latitude. At the same time, the sinking air at the North Pole flows south and collides with the air moving north from the 30° high pressure area. The colliding air is forced upward and an area of low pressure is created near 60° north. The third cell circulation pattern is created between the North Pole and 60° north.
Because of the rotation of the earth and the coriolis force, air is deflected to the right in the Northern Hemisphere. As a result, the movement of air in the polar cell circulation produces the polar easterlies. In the circulation cell that exists between 60° and 30° north, the movement of air produces the prevailing westerlies. In the tropic circulation cell, the northeast trade winds are produced. These are the so-called permanent wind systems of the each.
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