The results of four balloon flights of the NOAA ultraviolet fluorescence stratospheric water vapor instrument are presented. A series of improvements in the instrument has brought results which are credibly free from contamination by outgassing. The results are in essential agreement with the extensive soundings by H.J. Mastenbrook. The minimum water vapor mixing ratio occurs 2–3 km above the tropopause in both tropical and temperature latitudes. Our measured minimum values were 2.6 ppmv over Brazil (5°S) and 3.6 ppmv over Wyoming (41°N), with an estimated total error of 20%. This degree of dryness permits the conclusion that the global circulation originally proposed by Brewer is correct; i.e., that air enters the stratosphere from the troposphere in substantial quantities only through the tropical tropopause. This general circulation must apply to all other trace gases of tropospheric origin as well. The carbon monoxide measurements of Seiler support the conclusion.
Present affiliation: National Center for Atmospheric Research, Boulder, CO 80303.