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Name: Lori F.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 7/29/2003


Question:
I understand that the Coriolis effect explains the movement of wind belts and ocean currents. However, I am curious about two factors: 1) Why does the wind belt deviate at 60 degrees north (polar easterlies) to 30 degrees north, (the prevailing westerlies) and consequently the trades at 30 degrees to the equator. 2) Why does the direction of each wind belt in the southern hemisphere create a mirror image of the northern wind belts? In other words, why are there the shifts at certain latitudes?


Replies:
Lori,

The Coriolis effect causes turning of air masses, counterclockwise (to the left) for rising air (low pressure) and clockwise (to the right) for descending air (high pressure) in the northern hemisphere, and the opposite directions in the southern hemisphere.

Radiative heating causes rising air at the equator, which draws air from higher latitudes towards the equator. The Coriolis force is very small at the equator and increases with latitude. The rising air at the equator sets up a cell, called the Hadley cell, after the man who theorized it, with air lofted above the equator cooling as it rises and moves towards the pole in a somewhat westerly direction as the Earth moves beneath it. The air cools enough to descend at about 30 degrees (north or south). Underneath this cell, closer to the Earth, the Coriolis force turns the descending air to the right, explaining the persistence of high pressure in the tropics between the equator and 30 degrees. This directs air to the west, although weakly because of a weak north-south temperature gradient, thus creating the easterly trade winds of the tropics (northeast in the northern hemisphere and southeast in the southern hemisphere).

Before we go further, you asked "Why does the direction of each wind belt in the southern hemisphere create a mirror image of the northern wind belts?" If you look at the motion of the Earth from above the north pole and from above the south pole, you will see that the apparent rotation is in opposite directions (clockwise at the north pole, counterclockwise at the south pole). This explains why the southern atmospheric motions mirror the northern motions.

At the poles (above 30 degrees), radiation from the Sun is weak and the surface temperature is cold. Air moving to these latitudes from the south and air that is in the atmosphere above radiatively loses energy and cools rapidly. Since the general air motion is downward near the poles, and the Coriolis effect is very strong there, persistent weak high pressure is produced over the pole and thus easterly winds. This is sometimes called the polar cell.

From 30 to 60 degrees latitude, cooled air is descending from aloft from the Hadley cell to the south and draining along the Earth's surface from the polar cell to the north. Moderate radiative heating, plus the Coriolis effect, produce large synoptic scale eddies (alternate low and high pressure systems) that, partially due to conservation of momentum, must move in a general easterly direction (these westerly winds balance the tropical and polar easterlies). This is a weak cell in itself, called the Ferrel cell, again, named after the man who theorized it. It is fueled by the strong temperature gradient between the sub-tropics and the polar region. Furthermore, the high pressure areas of the subtropics naturally produce westerly winds at their northern boundaries, adding to the production ofwesterly winds in this region.

Along the boundaries between these three regions a discontinuity or shear occurs, producing the polar front jet to the north and the subtropical jet to the south. Both of these jets tend to exhibit westerly winds. In summer, the sub-tropical jet tends to be very weak or disappear entirely and the polar front jet moves to lower latitudes, producing severe weather, especially in the central United States. See the answers to jet stream questions on this web page for more details about the jet streams.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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