PRESSURE OVER THE GLOBE
The unequal heating of Earths surface due to its
tilt, rotation, and differential insolation, results in the
wide distribution of pressure over Earths surface.
Study figures 3-3A and 3-3B. Note that a low-pressure
area lies along the intertropical convergence zone
(ITCZ) in the equatorial region. This is due to the
higher temperatures maintained throughout the year in
this region. At the poles, permanent high-pressure areas
remain near the surface because of the low
temperatures in this area throughout the entire year.
Mainly the "piling up" of air in these regions causes the
subtropical high-pressure areas at 30°N and S latitudes.
Relatively high or low pressures also dominate other
areas during certain seasons of the year.
ELEMENTS OF CIRCULATION
differences, which in turn cause air movements. The
following sections show how air movements work and
how they evolve into the various circulationsprimary,
secondary, and tertiary.
To explain the observed wind circulation over
Earth, three basic steps are used. The first step is to
assume Earth does not rotate and is of uniform surface;
that is, all land or all water. The second step is to rotate
Earth, but still assume a uniform surface. The third step
is to rotate Earth and assume a non-uniform surface.
For now, we deal with the first two steps, a non-rotating
Earth of uniform surface and a rotating Earth of
The circulation on a non-rotating Earth is referred
to as the thermal circulation because it is caused by the
difference in heating. The air over the equator is heated
and rises (low pressure); while over the poles the air is
circulation was shown in figure 3-1.
In thermal circulation, the assumption was made
that the Earth did not rotate, but of course this is not
true. The rotation of Earth causes a force that affects
This rotation results in the
deflection to the right of movement in the Northern
Hemisphere, and to the left of the movement in the
Southern Hemisphere. This force is called the Coriolis
force. The Coriolis force is not a true force. It is an
apparent force resulting from the west-to-east rotation
of Earth. The effects, however, are real.
Arctic rivers cut faster into their right banks than
their left ones. On railroads carrying only one-way
traffic, the right hand rails wear out faster than the
left-hand rails. Artillery projectiles must be aimed to
the left of target because they deflect to the right.
Pendulum clocks run faster in high latitudes than in
lower latitudes. All these phenomena are the result of
the Coriolis force, which is only an apparent force. The
most important phenomena are that this force also
deflects winds to the right in the Northern Hemisphere.
Therefore, it is important to understand how this force
As Earth rotates, points on the surface are moving
eastward (from west to east) past a fixed point in space
at a given speed. Points on the equator are moving at
approximately 1,000 miles per hour, points on the poles
are not moving at all, but are merely pivoting, the points
somewhere between are moving at speeds between
1,000 and zero miles per hour depending upon their
relative position. Refer to view A in figure 3-4.
Figure 3-4.Coriolis force.