Page 266 - NS-2 Textbook

METEOROLOGY 261

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EARTH'S ROTATION

0' EQUATORIAL
OOLDRUMS

The general circulation pattern of air over the rotating Earth.

called the trade winds. TI1e northeast trades were a pop- Polar Front Zone. The belt of low pressure known as
ular route for sailing vessels, and aircraft traveling west the polar front zone lies in the area of 60 degrees north
in the Northern Hemisphere are favored ·with a tail ,vind and south latihldes. In the north it is called the Arctic
and clear skies if they fly near 30 degrees north latihlde. Semipermanent Low, and in the south it is called the
The winds come from the southeast in the Southern Antarctic Permanent Low .. These two areas are noted for
Hemisphere, and thus are called the southeast trades. their bad weather because the westerlies and the polar
Subtropical High-Pressure Belt. These are the Horse easterlies converge in them.
Latitudes. Because of sinking wind from aloft and di- Polar Easterlies. TIUs is a zone of poorly developed
verging winds at the surface, these areas generally have surface winds created by outflow from the high pressure
fair weather. The diverging winds cause the trade winds at the poles. TI1ey have a northeasterly direction in the
found on the equatorial side of this high-pressure belt. Northern Hemisphere and a southeasterly direction in
The Horse Latihldes tend to be cloudless and calm, with the Southern Hemisphere.
weak, tmdependable winds. The term "Horse Latitudes"
comes from the fact that, in the days of sail, ships carry-
WIND AND THE EARTH'S REVOLUTION
ing horses from Europe often were becalmed here. When
this happened, the horses died for lack of food and water, We have discussed in some detail the effects of the ro-
so the dead animals "vere thrown overboard to prevent tational movement of Earth on weathel; and particu-
the spread of disease. larly its effect on winds. Another important movement
Prevailing Westerlies. These winds are found on the is the revolution of our planet around the Stm. TIUs
pole-ward side of the subtropical highs and are created movement, combined with Earth's inclination, causes
by the diverging winds of these highs. They blow from the seasons.
the southwest in the Northern Hemisphere and from the Earth is inclined at an angle of 23)0 (23.5) degrees
northwest in the Southern Hemisphere. The prevailing from the perpendicular to the plane of its orbit of revolu-
westerlies provide most of the air flow over the United tion, called the plane of the ecliptic. This simply means
States. that Earth tips at this angle all the time, like a top as it