Water vapour act as a powerful greenhouse gas absorbing long-wave radiation. If the atmospheric water vapour concentration increases as a result of a global warming, then it is expected that it will enhance the greenhouse effect further. It is well known that the rate of evaporation is affected by the temperature and that higher temperatures increase the (saturated) vapour pressure (the Clausius-Clapeyron equation). This process is known as the water vapour feedback. One important difference between water vapour and other greenhouse gases such as CO2 is that the moisture spends only a short time in the atmosphere before being precipitated out, whereas the life time of CO2 in the atmosphere may be longer than 100 years.