Environmental Engineering Reference
In-Depth Information
where C p , m is the heat capacity per unit mass of air at constant pressure. It is defined
as C v , m + R/M . The term Γ adia = g/C p , m is called the dry adiabatic lapse rate .
The student is urged to work out Problem 2.5 to get a feel for the magnitude of
temperature changes in the lower atmosphere. To quantify the atmospheric stability
(the capacity of air to disperse pollutants is related to this property), one compares the
prevailing (environmental) lapse rate, Γ env =− d T/ d h , to the dry adiabatic lapse rate.
Figure2.2showsthecharacteristicprofilesforunstable,stable,andneutralatmospheres.
It can be seen that for an unstable atmosphere, Γ env > Γ adia , whereas for a stable
atmosphere, Γ env < Γ adia . For an unstable atmosphere, the less dense air parcel at the
higher altitude continues to rise, and that at the lower altitude is denser and continues
to sink. The condition is unstable since any perturbation in the vertical direction is
enhanced. Pollutant dispersal is rapid in an unstable atmosphere.
Neutral
G env = G adia
Stable
G env < G adia
h
Unstable
G env >
G adia
Dry adiabatic
T
FIGURE 2.2 Dry adiabatic and environmental lapse rates related to atmospheric
stability. Neutral, stable, and unstable atmospheric conditions are shown.
E XAMPLE 2.2 H URRICANE AS A H EAT E NGINE
Emanuel (2005) described the hurricane as Nature's steam engine that obeys Carnot's
energy cycle, which is the basis for the second law of thermodynamics. The hurricane
can be considered to obey a four-step cycle, described in Figure 2.3, where a cross-
section through a hurricane is shown. The axis on the left side is the central axis of the
storm. At point A near the sea surface, the air is at least hundreds of kilometers from
the storm center. The air slowly spirals inward toward the eye wall (point B). The A to
 
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