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Temperature Inversion

The little known killer

Quamrul Haider, Ph.D
NORMALLY, air temperature decreases with altitude, from roughly 20 degrees Centigrade at sea level to - 60 degrees Centigrade up to an altitude of 10 kilometers. During this normal cycle, a parcel of warm air will float upward in an ambient atmosphere of colder air. If temperature drops steeply, the atmosphere becomes unstable, causing air from near the surface to rise quickly and be replaced by cooler air sinking from above. In the process, the air will carry pollutants with it and disperse them at higher altitudes. The unstable condition is desirable because it leads to a relatively pollution-free lower atmosphere. Occasionally, air temperature will increase with altitude, resulting in colder air near the ground and warmer air above it. This is called temperature inversion - collusion between pollution and weather that creates warm-air lid over cooler air. It can occur anywhere from ground level up to few thousands of feet into the atmosphere
Temperature inversions are of great concern. In an area experiencing inversion, the warm-air lid prevents ground-level air from rising. Hence, the cool, dense ground air cannot mix vertically. The air is so stable that it is quiescent and pollutants become trapped below the warm layer of air, creating dirty air with dangerous concentrations of noxious pollutants. A hazy sky or a crimson colored sunset is an indication of inversion somewhere in the lower atmosphere. In areas with heavy pollution, problems with smog will be exacerbated by the presence of an inversion layer. If humidity in the cooler layer is high, thick ground-level fog will form. Temperature inversions are generally witnessed during winter months, when nights are long and cold. Inversion begins to form a few hours before the Sun sets. During
the day, the ground heats up more quickly than air. As the day ends, the air near the ground cools faster than the air above it. Making matters worse, inversions also coincide with the evening commute when pollution emissions from automobiles peak. The situation is reversed in the morning when sunlight strikes the Earth and vertical mixing begins. However, if there is a cloud cover, the ground does not warm up and the pollutants remain trapped. The duration of inversion can vary from few hours to several days. Temperature inversions can create extreme weather conditions. One example is freezing rain. Since inversion blocks an area's normal convection patterns of the air, it can give rise to intense thunderstorms and tornadoes. Temperature inversions in summer can be more troubling because sizzling temperatures will aggravate the already elevated level of pollution. Inversions are also common in mountainous regions where the Sun is obstructed by the mountains and therefore unable to warm the ground to stimulate vertical mixing. They can occur in coasted areas too
where upwelling of cold water can decrease surface air temperature and the cold air mass stays under warmer ones. If inversion condition in an area persists for long periods, it will have severe impact on human health. It can cause respiratory problems, burning eyes, and even death. Studies by scientists at the Paris Cardiovascular Research Center found that for a rise in the concentration of pollutants by 10 micrograms per cubic meter, the risks of heart attack in a healthy person during episodes of temperature inversion increases by 5 percent. The most devastating incident of temperature inversion occurred in London on December 4, 1952. Sulfur dioxide and other particulates from burning coal, and the usual pollutants of a big city accumulated and remained trapped for four days. The noxious, lethal smog was so thick that people were unable to see their feet as they walked. "Now yellow, now amber, now black, the great killer held London in its grip, and by early evening, only twelve hours after its onset, the first of the city's inhabitants began to die." The deadly impact of inversion was realized when funeral homes ran out of coffins and florists out of flowers. At the end of the four-day period, an estimated 4000 thousand people died. In addition, thousands became seriously ill and died monthslater.
Finally, recent dense fogs in Bangladesh have an eerie resemblance to the Donora Death Fog of 1938 in Pennsylvania. It is attributed to temperature inversion and claimed 20 lives. We have a propensity to forget that "if the pollutants we put into the air are harmful to us during a temperature inversion then they are harmful to everyone all of the time."
The writer is a Professor in the Department of Physics & Engineering Physics, Fordham University, New York.