Ask A Scientist Weather Archive

name         Lori F.
status       educator
age          40s

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? 
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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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