Radiosonde-Based Northern Hemisphere Tropospheric Water Vapor Trends

Trends in tropospheric water vapor at Northern Hemisphere radiosonde stations are presented for two periods;1973–95 and 1958–95. Stations with incomplete or inhomogeneous temporal records were identified and excluded from the analysis. For the 1973–95 period, trends in surface–500-mb precipitable water and in specific humidity, dewpoint, and temperature at the 850-mb level are shown. At most stations in this analysis, precipitable water, specific humidity, and dewpoint temperature have increased along with temperature over the period. An exception is Europe, over which temperature increased but humidity slightly decreased. Water vapor increases are larger, more uniform, and more significant over North America than over Eurasia, and the differences in trend magnitude and sign between the two regions may be attributable to changes in the late 1970s that affected North America more than Eurasia. Seasonal and annual correlations of surface–500-mb precipitable water with temperature, dewpoint temperature, and specific and relative humidity at the surface, 850, and 700 mb indicate a strong and relatively geographically invariant relationship between 850-mb specific humidity and surface–500-mb precipitable water. Specific humidity at 850 mb is then used as a surrogate for the surface–500-mb precipitable water over the 1958–95 period to avoid data quality problems in the pre-1973 precipitable water time series. Generally, 850-mb specific humidity trends at a small set of stations for 1958–95 show that only small increases occurred and that most of the overall increase probably occurred since 1973.