Geoscience Reference
In-Depth Information
100
100
Thermosphere
90
Mesopause
90
80
80
70
Mesosphere
70
60
60
50
50
Stratopause
40
40
Ozone
layer
Stratosphere
30
30
20
20
Tropopause
10
10
Troposphere
0
0
180
200
220
240
260
280
300
Temperature (K)
Figure 3.3. Temperature structure of the Earth's lower atmosphere, ignoring the boundary layer.
surface because too few molecules are present in the
moon's atmosphere to transfer energy away from its
surface. Similarly, neither convection nor advection can
occur in the moon's atmosphere.
the air by precipitation or gravitational sedimentation.
The boundary layer differs from the background tropo-
sphere in that the temperature profile in the boundary
layer responds to changes in ground temperatures over
a period of less than an hour, whereas the tempera-
ture profile in the background troposphere responds to
changes in ground temperatures over a longer period
(Stull, 1988).
Figure 3.4 shows a typical temperature variation with
height in the boundary layer over land during the day
and night. During the day (Figure 3.4a), the boundary
layer consists of a surface layer, a convective mixed
layer, and an entrainment zone. The surface layer ,
which comprises the bottom 10 percent of the bound-
ary layer, is a region of strong change of wind speed
with height (wind shear). Because the boundary layer
depth ranges from 500 to 3,000 m, the surface layer
is about 50 to 300 m thick. Wind shear occurs in the
surface layer simply because wind speeds at the ground
are zero and those above the ground are not.
The convective mixed layer is the region of air
just above the surface layer. When sunlight warms the
ground during the day, some of the energy is trans-
ferred from the ground to the air just above the ground
by conduction. Because the air above the ground is now
warm, it rises buoyantly as a thermal. Thermals origi-
nating from the surface layer rise and gain their max-
imum acceleration in the convective mixed layer. As
thermals rise, they displace cooler air aloft downward;
thus, upward and downward motions occur, allowing
air and pollutants to mix in this layer.
The top of the mixed layer is often bounded by a tem-
perature inversion ,which is an increase in temperature
3.3. Temperature Structure
of the Atmosphere
The bottom 100 km of the Earth's atmosphere, the
homosphere ,isaregion in which major gases are well
mixed. The homosphere is divided into four layers in
which temperatures change with altitude. These are,
from bottom to top, the troposphere , stratosphere ,
mesosphere , and thermosphere .Figure 3.3 shows an
average profile of the temperature structure of the homo-
sphere.
3.3.1. Troposphere
The troposphere is divided into the boundary layer
(ignored in Figure 3.3) and the background troposphere.
These regions are briefly discussed as follows.
3.3.1.1. Boundary Layer
The boundary layer extends from the topographical
surface up to between 500 and 3,000 m above the sur-
face. All people live in the boundary layer, so it is
this region of the atmosphere in which air pollution
buildup is of most concern. Pollutants emitted near the
ground accumulate in the boundary layer. When pollu-
tants escape the boundary layer, they can travel hori-
zontally long distances before they are removed from
 
 
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