How Water Enters the Stratosphere

摘要

The air in Earth's stratosphere (the atmospheric layer from ～11 to ～50 km) is extremely dry. The causes of this aridity have been studied for over half a century. Brewer provided a general explanation in 1949, but the devil remains in the details. New measurements reported on page 1742 of this is sue by Webster and Heymsfield shed new light on the underlying mechanisms. In the 1940s, aircraft measurements showed unexpectedly low amounts of stratospheric water vapor over England. The water vapor content was below 5 ppmv (parts per million by volume), much less than if the air were saturated at the temperature of the local mid-latitude tropopause (the boundary between the stratosphere and the underlying troposphere). Brewer concluded that all air en-tering the stratosphere must be freeze-dried near the tropical tropopause, where temperatures are regularly below 195 K.