the air reaches progressively lower latitudes (that is, it gets closer to the equa-
tor), the tangential speed of the earth’s surface, resulting from the rotation of the
planet, increases. Therefore, air flowing toward the equator is deflected toward
the west; the earth literally speeds up beneath it. We find prevailing easterly
winds in these regions, known as the torrid zones or the tropics, for this reason.
Between 30°N and 60°N, and also between 30°S and 60°S, the air at the sur-
face tends to flow away from the equator and toward the pole, from the semi-
permanent 30° high-pressure zone toward the semipermanent 60° low-pressure
zone. As the air near the surface moves poleward, the earth slows down under-
neath it. Thus, the air is deflected in the same direction the planet rotates. For
this reason, we find prevailing westerly surface winds in the mid-latitude regions,
which are called the temperate zones.
Between 60°N and the north pole, and also between 60°S and the south pole,
the air flow is generally away from the poles and toward the equator. The effect
here is the same as it is in the tropics, but of course at much colder temperatures.
The earth speeds up under the air near the surface as it flows away from the pole.
There are prevailing easterly surface winds in the arctic and antarctic regions,
which are called the frigid zones.
The prevailing winds of the torrid, temperate, and frigid zones sometimes
shift because of changes in the locations of the semipermanent high and low
pressure belts. In general, however, the torrid zones have easterly winds called
the trade winds, the temperate zones have westerly winds called the prevailing
westerlies, and the frigid zones have easterly winds called the polar easterlies.
The earth’s atmosphere is thus broken up into six major belts or zones that gir-
dle the planet.
SEMIPERMANENT PRESSURE REGIONS
Near the 30° and 60° latitude lines, both in the northern hemisphere and in the
southern hemisphere, the surface winds converge from, or diverge in, opposing
directions. This gives the air an impetus to spin clockwise at 30°N and 60°S,
counterclockwise at 60°N and at 30°S. The air also spins around the poles,
clockwise in the arctic and counterclockwise in the Antarctic. The eddies near
the 60th parallels tend to pull surface air inward, and the eddies near the 30th
parallels and the poles tend to push surface air outward.
Because of the tendency of the air to rotate near the 30th and 60th parallels and
the poles, we find persistent cyclonic (inward-spiraling) and anticyclonic (outward-
spiraling) atmospheric zones. The cyclonic zones are semipermanent low-pressure
systems, and the anticyclonic zones are semipermanent high-pressure systems.
CHAPTER 2 The Atmosphere
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