Tropospheric Water Vapor Climatology and Trends over North America: 1973–93

Here 21 years of radiosonde observations from stations in the Western Hemisphere north of the equator were analyzed for trends in tropospheric water vapor. Mean fields of precipitable water and relative humidity at several levels we shown. Annual trends of surface-500 mb precipitable water were generally increasing over this region except over northeastern Canada. When trends were expressed as a percentage of the climatological mean at each station, the trends south of ∼45°N represent a linear rate of increase of 3%–7% decade−1. Trends in the upper portion of this layer, 700–500 mb, were as large or larger than those of the middle (850–700 mb) or lower layer and were consistent in sign. Annual trends in dewpoint generally agree in sign with trends in temperature. However, the dewpoint trends tended to be larger than those of temperature. This was consistent with the annual increases found in relative humidity over this period. Relative humidity increased except in Canada, Alaska, and a few stations in western mountainous areas. Largest percentage increases of relative humidity were in the Tropics. Seasonal trends of precipitable water varied spatially more than the annual trends and fewer were statistically significant. More stations had significant trends in summer than in other seasons and these were located over the central and eastern United States and the Tropics. Spring trends were largest over the western United States, while the largest winter trends were along the Gulf Coast. The one area where significant water vapor increases were found in all four seasons was the Caribbean.