Page 265 - NS-2 Textbook

Wind and Weather

Air in motion is called willd. Winds blow because they direction of the wind to curve to the right in the North-
are attempting to achieve a balance in atmospheric pres- ern Hemisphere and to the left in the Southern Hemi-
sure. TI,e unequal distribution of atmospheric pressure is sphere. This curving or deflection effect continues until a
caused by the unequal heating of Earth's surface. Winds balance with other forces is reached.
blo"\v from high-pressure areas to low-pressure areas. At this point, we must again bring in the factor of at-
The strength of these winds depends on the distance of mospheric pressures in order to explain why there are
the high from the low and the difference in pressure (the different belts of prevailing prinlary winds on Earth.
gradient) between the two areas. Since various places on We know that air rises at the equator and begins
Earth's surface receive more heat than others, tempera- moving northward at high altihtdes. It eventually sinks
htres and strengths of winds differ from one area to an- and accumulates near the surface, forming a high-
other. pressure area. TIUs sinking and accumulating takes place
TI,ere is a continual flow of wind over the face of the in the area of 30 degrees north and south latitudes. These
Earth as the result of this uneven heating. From about 2» areas are called the Horse Latitudes.
to 3 miles above the surface to the tropopause, winds are Air must always flow outward from the center of a
westerly in direction at all degrees of latitude, from the high-pressure area; this is called divergence. Conversely,
equator to the poles. At the surface, a band of easterly air flnws in toward the center of a low-pressure area; this
winds called the trade winds extends from the equator to effect is convergence. It follows that when both high and
30 degrees, both north and south. Between 30 degrees low pressure areas are present, air flows from the high to
and 60 degrees, in both the Northern and Southern the low pressure area, thus creating wind.
Hemispheres, there are the prevailing westerlies. Finally,
between 60 degrees and both poles there are winds called
PREVAILING WINDS
the polar easlerlies.
Why are there so many different wind directions, The Doldrums. The equatorial belt of light and variable
and why are there differences in wind circulations in the converging winds is called the doldrums. They vary in
Northern and Southern Hemispheres? The answers to position and tend to move north and south of the geo-
these questions come from our knowledge of the mo- graphic equator with the Sun. In the doldrums the tem-
tions of Earth itself. peratures are high and excessive precipitation occurs.
Days go by without a breath of wind; thus, in the days of
sail, ships avoided this area, if possible. Severe tropical
WIND AND THE EARTH'S ROTATION
storms begin here.
Two motions of Earth affect the weather. The movement n'ade Winds. At the surface and on the pole-ward
of Earth around the Sun accounts for the seasonal sides of the doldrums there are bands of easterly winds
changes on Earth. We will talk briefly about this a bit
later. The other motion is the rotation of Earth on its axis.
This rotation causes night and day, with the consequent
heating and cooling effects on the atmosphere. It also
produces the major wind belts of Earth.
If Earth did not rotate, the warmer air over the equator
would rise and move north and south toward the poles,
high above Earth's surface. TI,e air would cool and sink
as it moved toward the poles. Later, it would move back An illustration of vertical convergence and divergence. Unequal
heating of the Earth's surface results in unequal distribution of pres-
toward the equator at a steady speed and direction.
sure. Warm air rises or converges above lows, and colder air falls or
Howevet; the Coriolis effect discussed in unit 2 causes the diverges around highs.

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