Atmospheric Circulation

Any student of meteorology understands that conceptual models have only limited applicability to the real world. A number of factors in the real world differ from the ideal conditions used to construct a model. These factors insure that actual weather conditions will be far more complex than the general conditions described above.

For example, both the Hadley and Ferrell models assumed that the earth has a homogeneous composition and that the sun always shines directly over the equator. Neither condition, of course, is actually true. For example, most parts of the planet are covered with water, and land masses are distributed unequally among this watery background. The flow of air in any one cell, then, may be undisturbed for long stretches in one region (as across an ocean), but highly disrupted in another region (as across a mountainous area).

Useful tools for meteorologists interested in studying air movements are charts of air pressure at various locations on the earth's surface. These charts are of value because, whatever models may predict, we known that in the real world air movements tend to occur from regions of higher pressure to those of lower pressure.

Such charts indicate that certain parts of the planet tend to be characterized by unusually high or low pressure centers at various times of the year. In general, about eight semipermanent high and low pressure cells have been identified. The term semipermanent is used for such cells because they seem to reappear every year on a regular basis.

For example, a semipermanent high pressure area occurs over the Bermuda Islands and persists throughout the year. A semipermanent low pressure—the Icelandic low—is usually found somewhat to the north of the Bermuda high, although it tends to shift from east to west and back again during various parts of the year. During the winter in the northern hemisphere, a semipermanent high exists over Siberia, although by summer it has disappeared and been replaced by a semipermanent low over India. The existence of these semipermanent highs and lows accounts for fairly predictable air movements over relatively large areas of the earth's surface.