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butitpasses easily through the inversion layer. Because
the background troposphere above the inversion is sta-
ble, the parcel ultimately comes to rest above the
inversion.
Inversions form whenever a parcel of air becomes
colder than the air above it. Common inversion types
include the radiation inversion, the large-scale subsi-
dence inversion, the marine inversion, the frontal inver-
sion, and the small-scale subsidence inversion. These
are discussed next.
2
1.5
Strength (
o
C)
1
Top
Thickness (km)
0.5
Base
Mixing depth
0
5
10
15
20
25
Temperature (
o
C)
6.6.1.4. Radiation Inversion
Radiation (nocturnal) inversions
occur nightly as land
cools by emitting thermal-IR radiation. During the day,
land also emits thermal-IR radiation, but this loss is
exceeded by a gain in solar radiation. At night, thermal-
IR emissions cool the ground, which in turn cools
molecular layers of air above the ground relative to
air aloft, creating an inversion. The strength of a radi-
ation inversion is maximized during long, calm, cloud-
free nights when the air is dry. Long nights maximize
the time during which thermal-IR cooling occurs, calm
nights minimize downward turbulent mixing of energy,
cloudfree nights minimize absorption of thermal-IR
energy by cloud drops, and dry air minimizes absorp-
tion of thermal-IR energy by water vapor. The morning
temperature profile in Figure 6.12 shows a radiation
inversion. Radiation inversions also form in the winter
during the day in regions that are not exposed to much
Figure 6.10.
Characteristics of a temperature
inversion.
which are increases in air temperature with increasing
height. Inversions are always stable, but a stable temper-
ature profile is not necessarily an inversion (e.g., Profile
3inFigure 6.9). Inversions are important because they
trap pollution near the surface to a greater extent than
does a stable temperature profile that is not an inversion.
An inversion is characterized by its strength, thick-
ness, top/base height, and top/base temperatures. The
inversion strength
is the difference between the tem-
perature at the inversion top and that at its base. The
inversion thickness
is the difference between the inver-
sion's top and base heights. The
inversion base height
is the height from the ground to the bottom of the inver-
sion. It is also called the
mixing depth
because it is
the estimated height to which pollutants released from
the surface mix. In reality, pollutants often mix into the
inversion layer itself. Inversion layer characteristics are
illustrated in Figure 6.10.
Stable air and inversions, in particular, trap pollu-
tants, preventing them from dispersing into the back-
ground troposphere and causing pollutant concen-
trations to build up near the surface. Figure 6.11
illustrates how such trapping occurs. It shows two air
parcels with different initial temperatures released at
the surface under an inversion. Suppose the parcels rep-
resent exhaust plumes that are initially warmer than
the environment. Due to their buoyancy, both parcels
rise, expand, and cool at the dry adiabatic lapse rate
of near 10
◦
Ckm
−
1
.Theparcel released at 20
◦
Crises
and cools until its temperature approaches that of the
air around it. At that point, the parcel decelerates and
then comes to rest after oscillating around its final alti-
tude. The path of this parcel illustrates how an inversion
traps pollutants emitted from the surface. It also illus-
trates that pollutants often penetrate into the inversion
layer. The parcel released at 30
◦
Calso rises and cools,
3
2.5
2
1.5
e
1
0.5
0
0
5
10
15
20
25
30
Temperature (
o
C)
Figure 6.11.
Schematic of pollutants trapped by an
inversion and stable air. The parcel of air released at
20
◦
Crisesat the unsaturated adiabatic lapse rate of
10
◦
Ckm
−1
until its temperature equals that of the
environment. This parcel is trapped by the inversion.
The parcel of air released at 30
◦
C also rises at 10
◦
C
km
−1
.Itescapestheinversion, but stops rising in
stable free-tropospheric air, where the environmental
lapse rate is 6.5
◦
Ckm
−1
.
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