The sources and sinks of water vapor in the stratosphere

The average water vapor concentration in the lower stratosphere is about 2.5 ppmm according to the most reliable measurements. There is an average annual north-to-south gradient of less than 0.5 ppmm, a maximum mixing ratio over the Intertropical Convergence Zone (ITCZ) of the 3 to 4.5 ppmm, and an increase of about 50 percent in the mixing ratio with increasing altitude up to the stratopause. The mother-of-pearl clouds occasionally observed in high latitudes are evidence that higher mixing ratios sometimes occur. Removal of straospheric moisture during the polar night by the formation of ice particles is the only sink mechanism which is specific for water vapor. The north-to-south gradient in stratospheric water vapor is due to the lower temperatures and the resulting greater effectiveness of the Antarctic sink.