Geoscience Reference
In-Depth Information
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Figure 6.5. Diagram of the three major circulation cells, the predominant surface pressure systems, and the
predominant surface wind systems on the Earth. H at the surface equals high pressure. H aloft is high altitude
at a given pressure above the surface, which is similar to saying it is high pressure at a given altitude above the
surface. L at surface equals low pressure. L aloft is low altitude at a given pressure above the surface (or low
pressure at a given altitude). “Lower” equals lower altitude at the same pressure as L aloft (or lower pressure
at the same altitude). “Lowest” equals lower altitude than “Lower” at the same pressure as L aloft (or lower
pressure at the same altitude). Thus, elevated high and low altitudes are relative to other altitudes at the same
pressure. This is similar to saying that elevated high and low pressures are relative to other pressures at the
same altitude. The circles with x sdenote winds going into the page (west to east). The circles with embedded
circles denote winds coming out of the page (east to west).
low pressure and diverge while flowing counterclock-
wise around the center of high pressure (right-hand
rule).
cells in each hemisphere shown in Figure 6.5 repre-
sent the meridional and vertical components of the
Earth's winds, averaged zonally (over all longitudes)
and over a long time period. The cells are symmet-
ric about the Equator and extend up to the tropopause
(Section 3.3.1.2), which is near 18 km altitude over the
Equator and near 8 km altitude over the poles.
Two cells, called Hadley cells ,extend from 0 N
to 30 N and 0 Sto30 Slatitude, respectively. These
cells were named after George Hadley (1685-1768), an
English physicist and meteorologist, who, in 1735, first
proposed the cells in his paper, “Concerning the Cause
of the General Trade Winds,” which was presented to
the Royal Society of London. Hadley's original cells,
however, extended between the Equator and the poles.
6.3. Global Circulation of the Atmosphere
Air pollution is affected by winds, winds are affected by
large-scale pressure systems, and large-scale pressure
systems are affected by the global circulation of the
atmosphere. Figure 6.5 shows features of the global
circulation, including the major circulation cells, the
belts of low and high pressure, and the predominant
wind directions.
Winds have a west-east ( zonal ), south-north ( merid-
ional ), and vertical component. The three circulation
 
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