2.1. Water vapour
It is anticipated that atmospheric warming will lead to more rainfall. The
amount of water potentially available for rainfall increases because warmer air is
capable of holding more moisture. This is derived from the Clausius-Clapeyron
relation. In this figure, qsat is the specific humidity of saturated air, defined as
the ratio between the mass of water vapour and the mass of the saturated air in
a given volume (in g/g). Around T=20oC, the increase is about 6% per degree
Celsius. Clearly, qsat increases exponentially with temperature, causing the
amount of water vapour to rise even higher at higher temperatures. There is
evidence that this mechanism does take place in nature. Note the enhanced
convectional rainfall in the tropics, where relative humidity is high (compared
to middle latitudes).
Indeed, upward trends in the amount of precipitable water over the ocean have
been reported by a number of authors. For example, satellite observations
(Stephens, 1990) have revealed that the amount of water vapour increases with
sea surface temperature. Flohn et al. (1990) observed increases in the
evaporation rate from parts of the ocean. These findings* are consistent with the
measured warming of the North Atlantic Ocean.
Footnotes:
* It has been argued that increased convection dries the middle and upper
troposphere due to a compensatory subsidence of air. This is not confirmed by
observations: in the tropics, Hense et al. (1988) measured increases in both
temperature and moisture in the mid-troposphere. Studying satellite data on
atmospheric water vapour, Rind et al. (1993) found that increased convection leads
to an increased water vapour above 500 mb. Return