Page 300 - Fundamentals of Air Pollution

256 17. The Physics of the Atmosphere

E. Effect of Mixing
The mixing of air in a vertical layer produces constant potential tempera-
ture throughout the layer. Such mixing is usually mechanical, such as air
movement over a rough surface. In Fig. 17-8 the initial temperature structure
is subadiabatic (solid line). The effect of mixing is to achieve a mean potential
temperature throughout the layer (dashed line), which in the lower part
is dry adiabatic. The bottom part of the layer is warmed; the top is cooled.
Note that above the vertical extent of the mixing, an inversion is formed
connecting the new cooled portion with the old temperature structure above
the zone of mixing. If the initial layer has considerable moisture, although
not saturated, cooling in the top portion of the layer may decrease the
temperature to the point where some of the moisture condenses, forming
clouds at the top. An example of this is the formation of an inversion and
a layer of stratus clouds along the California coast.

F. Radiation or Nocturnal Inversions
An inversion caused by mixing in a surface layer was just discussed
above. Inversions at the surface are caused frequently at night by radiational
cooling of the ground, which in turn cools the air near it.

G. Subsidence Inversions

There is usually some descent (subsidence) of air above surface high-
pressure systems. This air warms dry adiabatically as it descends, decreas-
ing the relative humidity and dissipating any clouds in the layer. A subsi-
dence inversion forms as a result of this sinking. Since the descending air
compresses as it encounters the increased pressures lower in the atmo-

Fig. 17-8. Effect of forced mixing (dashed) on the environmental subadiabatic lapse rate
(solid). Note the formation of an inversion at the top of the mixed layer.