Climate Number: 195 Kelvin (-108.67 degrees Fahrenheit)
Commercial airline flights spend most the time in the lower reaches of the stratosphere, which is the second layer of the atmosphere beginning at five to six miles up in the air. The air in the stratosphere is thin and cold, making it inhospitable, but it is also less turbulent than the air in the troposphere near the Earth’s surface, which is part of the reason pilots like to fly there. The stratosphere is also where a protective layer of ozone sits. While ozone near the Earth’s surface poses a health hazard to people, ozone in the stratosphere has been protecting life for billions of years. Depletion of stratospheric ozone happens during the respective winters at both poles, as temperatures dip below 195 Kelvin allowing polar stratospheric clouds to form. Within these clouds, conditions are just right for a series of chemical reactions to take place, with the result being the destruction of ozone molecules. This ozone destruction process is more intense around Antarctica, leading to the formation of the famous “ozone hole” around that continent. Ozone concentrations reach their minimum during the Southern Hemisphere spring (September to December), following the peak (June through August) winter ozone destruction season. The same process happens in the Arctic, but not to the same extent. This past winter of 2010-2011, however, featured the lowest ever recorded stratospheric ozone values in the Arctic, although these values are higher than the values regularly experienced over Antarctica during its ozone minimum season. Temperatures at 12.5 miles in altitude over the Arctic remained below 195 Kelvin from mid-December 2010 through the end of March 2011. One climatic trend that may have influenced this first “Arctic ozone hole” is the cooling of the lower stratosphere by 0.5 Kelvin per decade and the cooling of the upper stratosphere by 1.0 Kelvin since 1979. This cooling is related to the warming of the troposphere that occurred during the same period.
For comparison: If 195 Kelvin (-108.67 degrees Fahrenheit) sounds cold, it is! Water boils at 373.16 Kelvin (212 degrees Fahrenheit) and freezes at 273.16 Kelvin (32 degrees Fahrenheit). It is not quite as cold, however, as the lowest surface temperature ever recorded, which was 184.0 Kelvin (-128.6 degrees Fahrenheit) at Antarctica’s Russian Vostok Station in July of 1983.
Seasons: Winter, Spring, Summer, Fall
Sources: Garcia, RR. “An Arctic ozone hole?” Nature 478 (2011): 462-463 and Manney, GL et al. “Unprecedented Arctic ozone loss in 2011.” Nature 478 (2011): 469-475.