Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 135

located at 30 ◦ Sinthe South Pacific, South Atlantic, and
Indian Oceans.
The subpolar low-pressure belt in the Northern Hemi-
sphere is dominated by semipermanent surface low-
pressure centers .The subpolar low-pressure belt in
the Southern Hemisphere is dominated by a band of low
pressure. In the Northern Hemisphere, the two semiper-
manent low-pressure centers are the Aleutian low (in
the Pacific Ocean) and the Icelandic low (in the Atlantic
Ocean). These pressure systems tend to move north
in the Northern Hemisphere summer and south in the
winter, but generally stay between 40 ◦ N and 65 ◦ N. In
Figure 6.7b, the Aleutian low is the surface low-pressure
center at

of sand over clay and the preferential heating of land
over water are important factors giving rise to thermal
low-pressure centers.
When a region of soil warms, the air above the soil
warms, rises, and diverges horizontally aloft, creating
low pressure at the surface. Because the low-pressure
system forms by heating, it is called a thermal low-
pressure system .Thermal low-pressure centers form
in the summer over deserts and other sunny areas (e.g.,
the Mojave Desert in Southern California, the plateau of
Iran, the north of India). These areas are all located near
30 ◦ N, the same latitude as the semipermanent highs.
Descending air at 30 ◦ N due to the highs helps form the
thermal lows by evaporating clouds and thus clearing
the skies. The descending air also keeps the thermal
lows shallow (air rising in the thermal lows diverges
horizontally at a low altitude). In some cases, such as
overthe Mojave Desert, the thermal lows do not produce
clouds due to the lack of water vapor and shallowness
of the low. In other cases, such as over the Indian con-
tinent in the summer, rising air in a low sucks in warm,
moist surface air horizontally from the ocean, produc-
ing heavy rainfall. The strong sea breeze due to this
thermal low is the summer monsoon, where a monsoon
is a seasonal wind caused by a strong seasonal variation
in temperature between land and water.
During the winter, when temperatures decrease over
land, air densities increase, causing air to descend.
Air from aloft is drawn in horizontally to replace the
descending air, building up surface air pressure. The
resulting surface high-pressure system is a thermal
high-pressure system .Inthe Northern Hemisphere,
two thermal high-pressure systems are the Siberian
high (over Siberia) and the Canadian high (over the
Rocky Mountains between Canada and the United
States). As with thermal low-pressure system, the ther-
mal high-pressure systems are often shallow.

148 ◦ W, 5 3 ◦ N. −

6.5. Thermal Pressure Systems
Although semipermanent surface high- and low-
pressure centers exist over the oceans all year, thermal
surface high- and low-pressure systems form over land
seasonally. Thermal pressure systems form in response
to surface heating and cooling, which depend on proper-
ties of soil and water, such as specific heat. Specific heat
(J kg − 1 K − 1 )isthe energy required to increase the tem-
perature of 1 g of a substance 1 K. Soil has a lower spe-
cific heat than does water, as shown in Table 6.1. Dur-
ing the day, the addition of the same amount of sunlight
increases the temperature of soil more than it increases
water temperature. During the night, the release of the
same amount of thermal-IR energy decreases soil tem-
perature more than it decreases water temperature. As
such, land heats during the day and cools during the
night more than does water. Similarly, land heats dur-
ing the summer and cools during the winter more than
does water.
Specific heat varies not only between land and water,
butalso between different soil types, as shown in Table
6.1. Because sand has a lower specific heat than does
clay, sandy soil heats to a greater extent than does clayey
soil during the day and summer. The preferential heating

6.6. Effects of Large-Scale Pressure
Systems on Air Pollution
Semipermanent and thermal pressure systems affect air
pollution. Table 6.2 compares characteristics of such
pressure systems, including their effects on pollution.
Semipermanent low-pressure systems are associated
with cloudy skies, stormy weather, fast surface winds,
and low penetrations of solar radiation to the surface.
Thermal low-pressure systems, which are often shal-
low and occur in desert areas, may or may not produce
clouds. Air rises in both types of low-pressure systems,
dispersing near-surface pollution upward. When clouds

Table 6.1. Specific heats of four media at 298.15 K

Specific heat
(J kg −1 K −1 )

Substance

Dry air at constant pressure 1,004.67

Liquid water 4,185.5

Clay 1,360

Dry sand 827