The variable and its history

Observations of the extent, height and type of cloud-cover are important for many purposes, including meteorology and aviation, and also now for climatology, since clouds have a considerable influence on the energy budget of the Earth. At present, climate modellers have a problem in predicting change in part because of the difficulty of representing clouds in the models. Low, liquid-water clouds tend to cool the climate, the higher ice clouds to warm it, but there is the complication of supercooled cloud when the drops remain liquid well below the freezing point, for they reflect almost half the incoming solar radiation and so lead to cooling while the models might assume from their temperature that they are made of ice. At present all the effects appear to be nearly in balance, but with a slight net cooling. To improve the models it is important to compare what they foretell with what actually happens. The measurement of clouds thus takes on a new important role.