We live at the bottom of a gaseous envelope—the atmosphere—that is bound
gravitationally to the planet Earth. The circulation of our atmosphere
is a complex process because of the Earth’s rotation and the tilt of its
axis. The Earth’s axis is inclined 23½° from the ecliptic, the plane of
the Earth’s orbit around the Sun. Due to this inclination, vertical rays
of the Sun strike 23½° N. latitude, the Tropic of Cancer, at summer
solstice in late June. At winter solstice, the vertical rays strike 23½°
S. latitude, the Tropic of Capricorn. In the Northern Hemisphere, the
summer solstice day has the most daylight hours, and the winter solstice
has the fewest daylight hours each year. The tilt of the axis allows
differential heating of the Earth’s surface, which causes seasonal
changes in the global circulation.
[Illustration: The circulation pattern of the Earth’s atmosphere. Most
of the nonpolar deserts lie within the two trade winds belts.]
On a planetary scale, the circulation of air between the hot Equator and
the cold North and South Poles creates pressure belts that influence
weather. Air warmed by the Sun rises at the Equator, cools as it moves
toward the poles, descends as cold air over the poles, and warms again
as it moves over the surface of the Earth toward the Equator. This
simple pattern of atmospheric convection, however, is complicated by the
rotation of the Earth, which introduces the Coriolis Effect.
To appreciate the origin of this effect, consider the following. A stick
placed vertically in the ground at the North Pole would simply turn
around as the Earth rotates. A stick at the Equator would move in a
large circle of almost 40,000 kilometers with the Earth as it rotates.
The Coriolis Effect illustrates Newton’s first law of motion—a body in
motion will maintain its speed and direction of motion unless acted on
by some outside force. Thus, a wind travelling north from the equator
will maintain the velocity acquired at the equator while the Earth under
it is moving slower. This effect accounts for the generally east-west
direction of winds, or streams of air, on the Earth’s surface. Winds
blow between areas of different atmospheric pressures.
The Coriolis Effect influences the circulation pattern of the Earth’s
atmosphere. In the zone between about 30° N. and 30° S., the surface air
flows toward the Equator and the flow aloft is poleward. A low-pressure
area of calm, light variable winds near the equator is known to mariners
as the _doldrums_.
Around 30° N. and S., the poleward flowing air begins to descend toward
the surface in subtropical high-pressure belts. The sinking air is
relatively dry because its moisture has already been released near the
Equator above the tropical rain forests. Near the center of this
high-pressure zone of descending air, called the “Horse Latitudes,” the
winds at the surface are weak and variable. The name for this area is
believed to have been given by colonial sailors, who, becalmed sometimes
at these latitudes while crossing the oceans with horses as cargo, were
forced to throw a few horses overboard to conserve water.
The surface air that flows from these subtropical high-pressure belts
toward the Equator is deflected toward the west in both hemispheres by
the Coriolis Effect. Because winds are named for the direction from
which the wind is blowing, these winds are called the northeast trade
winds in the Northern Hemisphere and the southeast trade winds in the
Southern Hemisphere. The trade winds meet at the doldrums. Surface winds
known as “westerlies” flow from the Horse Latitudes toward the poles.
The “westerlies” meet “easterlies” from the polar highs at about 50-60°
N. and S.
Near the ground, wind direction is affected by friction and by changes
in topography. Winds may be seasonal, sporadic, or daily. They range
from gentle breezes to violent gusts at speeds greater than 300
kilometers/hour.
[Illustration: These dunes in the Algodones Sand Sea of southeastern
California move as much as 5 meters per year. The dunes in this
photograph, looking south, move toward the east (left) (photograph by
Peter Kresan).]
[Illustration: Searles Lake, California (photograph courtesy of
Kerr-McGee, Inc.).]