Botkin Environmental Science Earth as Living Planet 8th txtbk

20.4 The Atmosphere 435
cold trap, on the troposphere because it is where almost
all remaining water vapor condenses.
Another important layer for life is the stratospheric
ozone layer, which extends from the tropopause to
an elevation of approximately 40 km (25 mi), with a
maximum concentration of ozone above the equator
at about 25–30 km (16–19 mi) (Figure 20.6). Stratospheric
ozone (O 3 ) protects life in the lower atmosphere
from receiving harmful doses of ultraviolet radiation
(see Chapter 21).
Atmospheric Processes: Temperature,
Pressure, and Global Zones of High
and Low Pressure
Two important qualities of the atmosphere are pressure
and temperature. Pressure is force per unit area.
Atmospheric pressure is caused by the weight of overlying
atmospheric gases on those below and therefore decreases
with altitude. At sea level, atmospheric pressure is 10 5
N/m 2 (newtons per square meter) (14.7 lb/in). We are
familiar with this as barometric pressure, which the
weatherman gives to us in units that are the height to
which a column of mercury is raised by that pressure.
We are also familiar with low- and high-pressure systems
in the atmosphere. When the air pressure is low, air tends
to rise, cooling as it rises and condensing its water vapor;
it is therefore characterized by clouds and precipitation.
When air pressure is high, it is moving downward, which
warms the air, changing the condensed water drops in
clouds to vapor; therefore high-pressure systems are clear
and sunny.
Temperature, familiar to us as the relative warmth
or coldness of materials, is a measure of thermal energy,
which is the kinetic energy—the motion of atoms and
molecules in a substance.
Water vapor content is another important characteristic
of the lower atmosphere. It varies from less than
1% to about 4% by volume, depending on air temperature,
air pressure, and availability of water vapor from
the surface.
The atmosphere moves because of the Earth’s rotation
and differential heating of Earth’s surface and atmosphere.
These produce global patterns that include
prevailing winds and latitudinal belts of low and high
air pressure from the equator to the poles. Three cells
of atmospheric circulation (Hadley cells) are present in
Polar high pressure
High pressure
Easterlies
3
60°
Low
Westerlies
Pressure
2
30°
High
0° Low
Horse latitudes
Trade winds
Doldrums
Trade winds
Pressure
Pressure
1
1
Three-cell
model of
circulation
30°
High
Horse latitudes
Pressure
Westerlies
2
60°
Low
Easterlies
High pressure
Pressure
Polar high pressure
3
FIGURE 20.7 Generalized circulation of the atmosphere. The heating of the surface of the Earth is uneven,
producing pressure differences (warm air is less dense than cooler air). There is rising warm air at the equator
and sinking cool air at the poles. With rotation of Earth three cells of circulating air are formed in each hemisphere
(called Hadley Cells after George Hadley who first proposed a model of atmospheric circulation in
1735). (Source: Samuel J. Williamson, Fundamentals of Air Pollution, Figure 5.5 [Reading, MS: Addison-Wesley,
1973]. Reprinted with permission of Addison-Wesley.)