Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 132

form clouds of great vertical extent. Condensation of
water vapor releases latent heat, providing the air with
more buoyancy. Over the Equator, the air can rise up to
about 18 km before it is decelerated by the stratospheric
inversion. Once the air reaches the tropopause, it cannot
rise much farther, so it diverges to the north and south.
At the surface on the Equator, air is drawn in horizon-
tally to replace the rising air. As long as divergence aloft
exceeds convergence at the surface, surface air pressure
decreases and the altitude of a given air pressure aloft
increases over the Equator (relative to altitudes at the
same pressure level aloft but at other latitudes). The
surface low-pressure belt at the Equator is called the
equatorial low-pressure belt .Because pressure gradi-
ents are weak, winds are light, and the weather is often
rainy over equatorial waters, this region is also called
the doldrums .

6.3.2. Winds Aloft in the Hadley Cells
As air diverges toward the north in the elevated part of
the Northern Hemisphere Hadley cell, the ACoF force
deflects much of it to the right (to the east), giving rise to
westerly winds aloft (winds are generally named after
the direction that they originate from). Westerly winds
aloft in the Northern Hemisphere Hadley cell increase
in magnitude with increasing distance from the Equator
until they meet equatorward-moving air from the Ferrel
cell at 30 ◦ N, the subtropical front .Thefront is a region
of sharp temperature contrast. The winds at the front
are strongest at the tropopause, where they are called
the subtropical jet stream .Winds aloft in the Southern
Hemisphere Hadley cell are also westerly and culminate
in a tropopause subtropical jet stream at 30 ◦ S.

Figure 6.6. William Ferrel (1817-1891). National
Oceanic and Atmospheric Administration Central
Library.

In 1855, William Ferrel (1817-1891; Figure 6.6), an
American school teacher, meteorologist, and oceanog-
rapher, published an article in the Nashville Journal of
Medicine pointing out that Hadley's one-cell model did
not fit observations so well as did the three-cell model
shown in Figure 6.5. In 1860, Ferrel went on to pub-
lish a collection of papers showing the first application
of mathematical theory to fluid motions on a rotating
Earth. Today, the middle cell in the three-cell model is
called the Ferrel cell .AFerrel cell extends from 30 ◦ N
to 60 ◦ N and from 30 ◦ Sto60 ◦ S, whereas a polar cell
extends from 60 to 90 degrees in each hemisphere as
well.

6.3.3. Subtropical High-Pressure Belts
As air converges at the subtropical fronts at 30 ◦ N and
30 ◦ S, much of it descends. Air is then drawn in hori-
zontally aloft to replace the descending air. As long as
inflow aloft exceeds outflow at the surface, surface air
pressure builds up. The surface high-pressure belts at
30 ◦ N and 30 ◦ Sarecalled subtropical high-pressure
belts .Because descending air compresses and warms
evaporating clouds, and because pressure gradients are
relatively weak around high-pressure centers, surface
high-pressure systems are characterized by sunny skies
and light winds. Sunny skies and the lack of rainfall at
30 ◦ N and 30 ◦ Saretworeasons why many deserts of
the world are located at these latitudes. The light winds
forced some ships sailing at 30 ◦ Ntolighten their cargo,

6.3.1. Equatorial Low-Pressure Belt
Circulation in the three cells is controlled by heating
at the Equator, cooling at the poles, and the rotation
of the Earth. In the Hadley cells, air rises over the
Equator because the sun heats this region intensely.
Much of the heating occurs over water, some of which
evaporates. As air containing water vapor rises, the air
expands and cools, and the water vapor recondenses to