Page 473 - Environment: The Science Behind the Stories

Vertical mixing allows pollutants in the air to be carried
upward and diluted, but thermal inversions trap pollutants near
the ground. Both Los Angeles and Mexico City suffer their
worst pollution when thermal inversions prevent pollutants
from being dispersed. Both metropolitan areas are encircled
by mountains that promote inversion layers, interrupt air flow,
Warm air and trap pollutants. In Mexico City in 1996, a persistent ther-
mal inversion sparked a five-day crisis that killed at least 300
people and sent 400,000 to hospitals. Desperate for a solution,
Cool air
Warm some people proposed dynamiting a hole in the mountains and
front installing immense fans to blow out the air.
Light
precipitation The worst such crisis occurred back in 1952 in London,
England, when a thermal inversion spawned a “killer smog.”
A high-pressure system had settled over the city for several
days, trapping pollutants from factories and coal-burning
(a) Warm front
stoves. This created foul conditions that killed 4000 people—
and by some estimates up to 12,000. Such dramatic events
are by no means a thing of the past: In 2013, Beijing, China,
suffered many days of abysmal air quality in an event dubbed
an “airpocalypse” (p. 481). Across the world today, inversions
frequently concentrate pollution over major metropolitan

Warm air
Cold air High Cool

Heavy
precipitation
Cold front Vertical
Sun mixing
Altitude surface Heat radiates Temperature
warms
(b) Cold front up from
surface
Figure 17.7 Fronts occur where air masses meet. When a
warm front approaches (a), warmer air rises over cooler air, caus-
ing light or moderate precipitation as moisture in the warmer air
condenses. When a cold front approaches (b), colder air pushes Low Warm
beneath warmer air, and the warmer air rises, resulting in conden-
sation and heavy precipitation. (a) Normal conditions

High Warm
In a low-pressure system, warmer air rises, drawing air inward
toward the center of low atmospheric pressure. The rising air
expands and cools, and clouds and precipitation often result.
Under most conditions, air in the troposphere becomes
cooler as altitude increases. Because warm air rises, vertical
mixing results (Figure 17.8a). Occasionally, however, a layer of Altitude Temperature
cool air may form beneath a layer of warmer air. This departure
from the normal temperature profile is known as a temperature Pollution
trapped
inversion, or thermal inversion (Figure 17.8b). The band of air
in which temperature rises with altitude is called an inversion
layer (because the normal direction of temperature change is Low Cool
inverted). The cooler air at the bottom of the inversion layer is
denser than the warmer air above, so it resists vertical mixing (b) Thermal inversion
and remains stable. Thermal inversions can occur in different Figure 17.8 Thermal inversions trap air and pollutants.
ways, sometimes involving cool air at ground level and some- Under normal conditions (a), air becomes cooler with altitude and
times producing an inversion layer higher above the ground. air of different altitudes mixes, dispersing pollutants upward. In
One common type of inversion (shown in Figure 17.8b) occurs a thermal inversion (b), dense cool air remains near the ground,
in mountain valleys where slopes block morning sunlight, and air warms with altitude within the inversion layer. Little mixing
472 keeping ground-level air within the valley shaded and cool. occurs, and pollutants are trapped.

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