Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 130

1000 hPa 1012 hPa

Surface wind

99 6 hP a 1 016 hP a

FF

PGF+
ACfF

ACoF

L H

PGF ACfF

ACoF

FF

Surface
wind

(a) (b)

Figure 6.4. Surface winds (in green) around centers of (a) low and (b) high pressure in the Northern Hemisphere
and the forces (in blue) affecting them. The pressures given represent those along the circular isobars (in red).

90 degrees to the right of the wind), the resulting wind
must flow counterclockwise around the center of low
pressure.
Around a high-pressure center aloft in the Northern
Hemisphere, the PGF and ACfF point opposite from the
center of high. To balance the sum of these two forces,
the ACoF must point toward the center of high. Because
the wind is always pointed 90 degrees to the left of the
ACoF, the resulting gradient wind must flow clockwise
around the center of high (Figure 6.3b).
In the Southern Hemisphere, the right-hand rule
is used. It indicates that the gradient winds flow clock-
wise around low-pressure centers and counterclockwise
around high-pressure centers, thus opposite in direction
from their respective flows around lows and highs in
the Northern Hemisphere.

pressure is called convergence ,whereas the flow of
air away from a center of high pressure is divergence .
Air converges into surface centers of low pressure and
diverges from surface centers of high pressure in both
hemispheres.
In the Northern Hemisphere, surface winds con-
vergewhile flowing counterclockwise around the cen-
ter of low pressure and diverge while flowing clock-
wise around the center of high pressure (left-hand
rule). Figure 6.4 shows the force balances and result-
ing winds in the presence of a surface (a) low-pressure
system and (b) high-pressure system in the Northern
Hemisphere.
In the low-pressure case, the PGF is balanced by the
sum of the ACoF, ACfF, and FF. The resulting wind
converges counterclockwise into the center of the low.
The converging air rises, and the rising air expands and
cools. If sufficient cooling occurs, clouds form. As such,
surface cyclones are frequently associated with stormy
weather.
In the high-pressure case, the sum of the PGF and
ACfF is balanced by the sum of the FF and ACoF. The
resulting wind diverges clockwise out of the center of
the high. The diverging air pulls more air downward in
the center of the high. The descending air compresses
and warms, potentially evaporating clouds. As such,
surface high-pressure centers, or anticyclones, are often
associated with sunny skies and warm weather.
In the Southern Hemisphere, surface winds con-
vergewhile flowing clockwise around the center of

6.2.4. Surface Winds along Curved Isobars
The large-scale circulation of the wind around a sur-
face low-pressure center is called a cyclone , and that
around a surface high-pressure center is called an anti-
cyclone . Cyclonic flow is flow around a low-pressure
center (either at the surface or aloft), and anticyclonic
flow is flow around a high-pressure center (either at the
surface or aloft).
Near the Earth's surface, the friction force slows and
turns the wind toward the center of low pressure in a
cyclone and away from the center of high pressure in
an anticyclone. The flow of air into a center of low