General Circulation

Warm air rises and cool air sinks, or subsides. If the Earth had no continents and did not turn, then the circulation of the atmosphere would appear much like this.

[pic] http://maritime.haifa.ac.il/departm/lessons/ocean/lect14.htm The air at the Equator would be heated by the warm Earth and so rise, flow toward the poles where it is cooled by the cold Earth, subside at the North Pole and South Pole, and then flow back to the Equator along the surface of the Earth.

But the Earth does have continents and it does spin on its axis, so the “General Circulation” of the atmosphere actually looks more like the images on the next page. These are sometimes referred to as the three-cell general circulation model. Air rises at the Equator and near 50( to 60(, and it sinks near 30( and 90( (the poles). The winds do not blow directly north and south due to the coriolis effect. The coriolis “force” deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Winds are named for the direction from which they blowing. As indicated, the general circulation of the atmosphere is from the East at latitudes poleward of about 50( North Latitude and 60( South Latitude. The general circulation of the atmosphere is also from the East at latitudes equatorward of about 30( north or south of the Equator. The general circulation is from the west at middle latitudes, and these are the “Prevailing Westerlies” that we experience in most of the continental United States. This is why weather systems generally move from west to east across North America.

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http://maritime.haifa.ac.il/departm/lessons/ocean/wwr196.gif

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http://www.earth.rochester.edu/fehnlab/ees215/fig15_1.jpg

Several distinctive features result from this three-cell general circulation model.

Convergence means “coming together.” The convergence of the atmosphere at the Equator is called the InterTropical Convergence Zone (ITCZ). This is a region of generally low pressure, also referred to as the “Doldrums.”

The cells immediately north and south of the equator, in which the winds are usually from the East, are called the Hadley Cell. The winds in this region are called the tropical easterlies. In the Northern Hemisphere, these are called the “Northeast Trade Winds,” and in the Southern Hemisphere, they are called the “Southeast Trade Winds.” A band of generally high pressure near 30( North and South of the Equator is called the Subtropical High.

The band of high pressure near 30( latitude is also called the Horse Latitudes. The name Horse Latitudes is thought to have originated in the seventeenth century, when sailing ships were carrying cargos of horses to the West Indies. With high pressure and good weather, the ships could be stranded for days or weeks without winds to propel them toward their destination. Horses died for lack of food and water waiting for winds to move the ships.

The middle latitude prevailing westerly cells are called the Ferrel Cell. It is named after William Ferrel, and American scientist who published his “Essay on the Winds and Currents of the Ocean” in 1856. In this essay and later papers, he showed by rigorous mathematics that winds are affected by the rotation of the Earth.

The cells near the North Pole and South Pole, in which the winds are mostly from the east, are called the Polar Cell. The winds are called the polar easterlies. A band of generally low pressure between the Ferrel Cell and the Polar Cell in the Northern Hemisphere and the Southern Hemisphere is called the Subpolar Low.

References:

Byers, Horace R., General Meteorology, Third Edition. McGraw-Hill Book Company, Inc., New York, 1959.

Cole, Franklyn W., Introduction to Meteorology, Third Edition. John Wiley and Sons, New York, 1980.

Forrester, Frank H., 1001 Questions Answered About the Weather. Dover Publications, Inc., New York, 1981.