3.4.2 Water Vapour
Water vapour is a key climate variable. In the lower troposphere, condensation of water vapour into precipitation provides latent heating which dominates the structure of tropospheric diabatic heating (Trenberth and Stepaniak, 2003a,b). Water vapour is also the most important gaseous source of infrared opacity in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies (Kiehl and Trenberth, 1997), and provides the largest positive feedback in model projections of climate change (Held and Soden, 2000).
Water vapour at the land surface has been measured since the late 19th century, but only observations made since the 1950s have been compiled into a database suitable for climate studies. The concentration of surface water vapour is typically reported as the vapour pressure, dew point temperature or relative humidity. Using physical relationships, it is possible to convert from one to the other, but the conversions are exact only for instantaneous values. As the relationships are nonlinearly related to air temperature, errors accumulate as data are averaged to daily and monthly periods. Slightly more comprehensive data exist for oceanic areas, where the dew point temperature is included as part of the ICOADS database, but few analyses have taken place for periods before the 1950s.
The network of radiosonde measurements provides the longest record of water vapour measurements in the atmosphere, dating back to the mid-1940s. However, early radiosonde sensors suffered from significant measurement biases, particularly for the upper troposphere, and changes in instrumentation with time often lead to artificial discontinuities in the data record (e.g., see Elliott et al., 2002). Consequently, most of the analysis of radiosonde humidity has focused on trends for altitudes below 500 hPa and is restricted to those stations and periods for which stable instrumentation and reliable moisture soundings are available.
Additional information on water vapour can be obtained from satellite observations and reanalysis products. Satellite observations provide near-global coverage and thus represent an important source of information over the oceans, where radiosonde observations are scarce, and in the upper troposphere, where radiosonde sensors are often unreliable.