Inversions
Under normal circumstances, air temperature tends to decrease with increasing elevation above the surface through the troposphere. Under particular conditions the lapse rate of temperature is reversed and temperatures increase with increasing elevation creating an inversion or inverted lapse rate of temperature.
Figure 5.9 Radiation inversion
A radiation inversion commonly occurs when the evening air is still and there are no clouds to trap heat. Surface temperature drops as longwave radiation emitted by the Earth escapes to space. Air in contact with the surface cools, but that at a higher elevation and not in direct contact with the earth remains somewhat warmer creating the inversion. Cool air will collect in depressions as air density increases and gravity pulls it down slope.
Figure 5.10 Sea breeze inversion
Sea breeze inversions occur along windward coasts bordered by cold ocean currents. The bottom layer of a warm maritime air mass originating over the ocean becomes cooler upon coming in contact with the cooler water bordering the coast. This creates cooler air near the surface with warmer air aloft.
Figure 5.11 Subsidence inversion
Areas dominated by high pressure are also subject to inversions. Subsidence inversions form when subsiding air undergoes adiabatic heating aloft, while air in contact with the surface remains cooler. Subsidence and subsidence inversions in association with high pressure are discussed in Chapter 6.
The effects of a temperature inversion are shown in Figure 5.12. During the late fall and early winter months, cold air slides down the Himalaya, coming to rest over northern India. The cold air nestles under warmer air at the base of the mountains over the northern Ganges Plain creating a temperature inversion. The inversion allows haze to build as pollutants trapped in the cold air are unable to disperse. Much of this air pollution is from human activities that produce aersols rich in sulfates, nitrates, organic and black carbon, and fly ash. Not only hazardous to human health, these pollutants affect the region's climate and hydrologic cycle.
Figure 5.12 Haze trapped in an inversion at the base of the Himalaya.
Source NASA EOS.
Assess your understanding of the concepts just covered by "Looking Back at Air Temperature", or continue reading.