Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 133

the heaviest and most dispensable component of which
was often horses. Thus, the 30 ◦ Nlatitude band was also
known as the horse latitudes .

relatively strong. West-east wind speeds also increase
with increasing height at the polar fronts. At the
tropopause in each hemisphere, they culminate in the
polar front jet streams .Although the subtropical jet
streams do not meander to the north or south over great
distances, the polar front jet streams do. Their predom-
inant direction is still from west to east.

6.3.4. Trade Winds
At the surface at 30 ◦ N and 30 ◦ S, descending air diverges
both equatorward and poleward. Most of the air mov-
ing equatorward is deflected by the ACoF to the right
(toward the west) in the Northern Hemisphere and to
the left (toward the west) in the Southern Hemisphere,
except that friction reduces the extent of ACoF turning.
The resulting winds in the Northern Hemisphere are
called the northeast trade winds because they origi-
nate from the northeast (Figure 6.5). Those in the South-
ern Hemisphere are called the southeast trade winds
because they originate from the southeast. Sailors from
Europe have used the northeast trades to speed their
voyages westward since the fifteenth century. The trade
winds are consistent winds. The northeast and south-
east trade winds converge at the Intertropical Conver-
gence Zone (ITCZ; Figure 6.5), which moves north of
the Equator in the Northern Hemisphere summer and
south of the Equator in the Southern Hemisphere sum-
mer, generally following the direction of the sun. At the
ITCZ, air convergence and surface heating lead to the
rising arm of the Hadley cells.

6.3.6. Westerly Winds Aloft at Midlatitudes
One might expect air in the elevated portion of the Ferrel
cell to move equatorward and for the ACoF to deflect
such air to the west, creating easterly winds (from east
to west) aloft at midlatitudes in both hemispheres. In
fact, winds aloft in the Ferrel cell are generally west-
erly (from west to east), although they meander between
south and north. Par t of the reason for the westerly
winds aloft in the Ferrel cell is that heights of constant
pressure (or pressures at a constant height) decrease
between the Equator and the poles in the upper tro-
posphere ,asshowninFigure 6.5. Winds tend to start
flowing down the height (or pressure) gradient, toward
the poles in the Ferrel cell aloft. The Coriolis force then
acts on this moving air, turning it toward the east in
both hemispheres, creating westerly winds aloft in both
hemispheres. The reason heights of constant pressure
(or pressures at a constant height) decline from equator
to pole is that temperatures transition from warm to cold
between the Equator and pole. Warm air rises and cold
air sinks; thus, near the Equator, the rising air pushes up
the height of a constant pressure level (or increases the
pressure at a constant height), and near the pole, sinking
air pushes down the height of a constant pressure level
(or decreases the pressure at a constant height).
The second reason for westerly winds aloft in the
Ferrel cell relates to the presence of centers of low and
high pressure. Descending air at 30 ◦ N and 30 ◦ Screates
centers of surface high pressure, and rising air at 60 ◦ N
and 60 ◦ Screates bands or centers of surface low pres-
sure. Figure 6.7b shows an example of surface high-
and low-pressure centers over the Pacific Ocean in the
Northern Hemisphere between 10 ◦ N and 80 ◦ N. Surface
winds moving around a Northern Hemisphere surface
high-pressure center travel clockwise (diverging away
from the center of the high), and surface winds mov-
ing around a surface low-pressure center travel coun-
terclockwise (converging into the center of the low)
(Sections 6.2.3 and 6.2.4). Indeed, these characteris-
tics are seen in Figure 6.7b, which shows winds travel-
ing clockwise around the highs and counterclockwise
around the lows. The positions of the highs and lows

6.3.5. Subpolar Low-Pressure Belts
As surface air moves poleward in the Ferrel cells, the
ACoF turns it toward the right (east) in the North-
ern Hemisphere and left (east) in the Southern Hemi-
sphere. However, surface friction reduces the extent
of turning, so that near-surface winds at midlatitudes
(30 ◦ Nto60 ◦ N and 30 ◦ Sto60 ◦ S) are generally west-
erly to southwesterly (from the west or southwest) in
the Northern Hemisphere and westerly to northwest-
erly in the Southern Hemisphere. In both hemispheres,
poleward-moving near-surface air in the Ferrel cell
meets equatorward-moving air from the polar cell at
the polar front ,which is a region of sharp tempera-
ture contrast between these two cells. Converging air
at the surface front rises and diverges aloft, reducing
surface air pressure and increasing air pressure aloft
relative to pressures at other latitudes. The surface low-
pressure regions at 60 ◦ N and 60 ◦ Sarecalled subpo-
lar low-pressure belts .Regions of rising air and sur-
face low pressure are associated with storms. Thus,
the intersection of the Ferrell and polar cells is asso-
ciated with stormy weather. Unlike at the Equator, sur-
face pressure gradients and winds at the polar front are