Prevailing Westerlies
 

Hello, I am in a Meteorlogy class and I am having trouble
understanding the prevailing westerlies. If the Coriolis effect is
moving the trade winds from east to west in the northern hemisphere,
why do the prevailing westerlies move from the west to the east.
Shouldn't the Coriolis effect deflect these winds from the east to the
west like the trade winds? Please make this more clear for me.
Thanks in advance!

Prevailing Westerlies
 

"David" wrote in message
m...
| Hello, I am in a Meteorlogy class and I am having trouble
| understanding the prevailing westerlies. If the Coriolis effect is
| moving the trade winds from east to west in the northern hemisphere,
| why do the prevailing westerlies move from the west to the east.
| Shouldn't the Coriolis effect deflect these winds from the east to the
| west like the trade winds? Please make this more clear for me.
| Thanks in advance!

The prevailing westerlies are only partly down to the "Coriolis effect".
Just as significant is the meridional circulation due to air being heated at
the equator and cooled at the poles.

A gentleman called Hadley proposed that this would create a large
circulation (called, unsurprisingly, the "Hadley Cell") with air rising at
the equator and sinking at the poles. However, this would produce
*easterlies* over the whole earth's surface, which is not what is observed.

What actually is observed is a reverse circulation in middle latitudes
called the "Ferrel Cell" - a zone of westerlies between the polar easterlies
and the trade winds (sub-tropical easterlies) in the "Hadley Cell" itself.
To wave the hands about, air rising at the equator heads north or south and
then is deflected eastwards at high levels and accumulates at the
high-pressure belt around 30 degrees North or South. The air is forced to
descend and warm, some heading poleward to drive the reverse Ferrel
circulation. North of the zone of lowest pressure (50 to 60 degrees from
the equator) one sees the "direct" polar easterly circulation - up to a
point. At levels above the surface the cold air in the polar regions
results in a vortex (due to pressure falling more rapidly with height in the
dense cold air) which can "isolate" the polar circulation from the rest of
the planet.

This explanation is unlikely to win you many credits in your course,
however. Try searching for "Hadley" and "Ferrel" to get more detail.
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