Stratopause

The stratopause is one of the layers into which the atmosphere is divided. It is the buffer region of the atmosphere that lies between the stratosphere and the mesosphere, from a height of about 31 to 34 mi. (50 to 55 km) above the Earth’s soil. The atmospheric pressure is about 1/1000th of the pressure at sea level. In the stratopause, the temperature reaches a peak because of the heating generated by the absorption of ultraviolet radiation by ozone molecules in the stratospheric ozone layer. In this region, the catalytic cycles, which are less efficient at colder temperatures because of reduced O density, produce a significant ozone increase (~15 percent). Because of the considerable ozone presence in the stratopause, the understanding of this region is considered crucial to understanding the changes in climate and in the composition of the ozone layer. Above the stratopause, the temperature starts to decrease again with height as a result of the reduced solar heating of ozone.

The depletion of the ozone layer resulting from the emission of halogen atoms and the photodissociation of chlorofluorocarbon compounds is of particular concern to scientists, as the layer prevents the most harmful ultraviolet-B wavelengths from passing through the Earth’s atmosphere. Near the stratopause, the ozone reduction is slightly smaller in the drier stratosphere because of the stronger temperature dependence of the drier atmosphere.

Studies of the temperature in the stratopause have also been important to assess the validity of global circulation models. For example, a study published in 2002 by the University of Illinois at Urbana-Champaign and the High Altitude Observatory of the National Center for Atmospheric Research in Boulder, Colorado, showed that wintertime warming caused by sinking air masses was not as strong as the models had assumed.

The study employed lidar laser measurements and balloon observations made at the Amundsen- Scott South Pole Station from December 1999 to October 2001. These measurements and observations were then used to calculate the monthly mean winter temperature profiles from the surface to about 63 mi. (110 km). The measured temperatures during midwinter in both the stratopause and mesopause regions were 20–30 degrees Kelvin colder than current model predictions. These differences were caused by weaker than expected compressional heating associated with subsidence over the polar cap. The study showed that the greatest difference occurred in the month of July, when

the measured stratopause temperature was about 0 degrees F (minus 18 degrees C) compared with the about 40 degrees F (4.4 degrees C) predicted by the models.