The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, DC, and Boulder, Colorado

Stratospheric water vapor concentrations measured at two midlatitude locati ons in the northern hemisphere show water vapor amounts have increased at a rate of 1-1.5% yr(-1) (0.05-0.07 ppmv yr(-1)) for the past 35 years. At Wa shington, D.C., measurements were made from 1964-1976, and at Boulder, Colo rado, observations began in 1980 and continue to the present. While these t wo data sets do not comprise a single time series, they individually show i ncreases over their respective measurement periods. At Boulder the trends d o not show strong seasonal differences; significant increases are found thr oughout the year in the altitude range 16-28 km. In winter these trends are significant down to about 13 km.