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THE 3-CELL THEORY

According to the 3-cell theory, Earth is divided into six circulation belts—three in the Northern Hemisphere and three in the Southern Hemisphere. The dividing lines are the equator, 30°N and S latitude, and 60°N and S latitude. The three cells of general circulation of the Northern Hemisphere are similar to those of the Southern Hemisphere. (Refer to fig. 3-1-6 during the following discussion.)

First, note the tropical cell of the Northern Hemisphere which lies between the equator and 30°N latitude. The air at the equator heats and rises due to convection. When it reaches the upper portions of the troposphere, it tends to flow to-ward the North Pole. By the time the air has reached 30°N latitude, the Coriolis effect has deflected it so much that it is moving eastward instead of northward. This results in a piling up of air (convergence) near 30°N latitude and a descending current of air (subsidence) toward the surface which forms a belt of high pressure. When the descending air reaches the surface where it flows outward (divergence), part of it flows poleward to become part of the mid-latitude cell; the other part flows toward the equator, where it is deflected by the Coriolis effect and forms the northeast trades.

Figure 3-1-5.—Coriolis effect on windflow.

The mid-latitude cell is located between 30° and 60°N latitude. The air which comprises this cell circulates poleward at the surface and equator-ward aloft with rising currents at 60° (polar front) and descending currents at 30° (high-pressure belt). However, in general, winds both at the surface and aloft blow from the west. This is easily explained for the surface wind by the Coriolis effect on the poleward-moving surface air. The west wind aloft is not as easily explained. Most authorities agree that this wind is fric-tionally driven by the west winds in the two adjacent cells.

The polar cell lies between 60°N latitude and the North Pole. The circulation in this cell begins with a flow of air at a high altitude toward the pole. This flow cools and descends at the North Pole and forms a high-pressure area in the polar regions. After reaching the surface of Earth, this air tends to flow equatorward and is deflected by the Coriolis effect so that it moves from the northeast. This air converges with the poleward flow from the mid-latitude cell and is deflected upward with a portion circulating poleward again and the remainder equatorward. The outflow of air aloft between the polar and mid-latitude cells causes a semipermanent low-pressure area at approximately 60°N latitude.

To complete the picture of the world’s general atmospheric circulation, we must associate these prevailing wind and pressure belts with some basic characteristics.

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