CLOUDS dominate the albedo of the Earth and hence have a vital role in the global radiation balance. They are one of the most important physical properties of the atmosphere but the most difficult to parameterise. The forecasting of short-term climatological excursions (weather) requires numerical integration of complex dynamical models. However, over longer time periods it may be possible to include cloud cover without resorting to explicit atmospheric dynamics. Here we suggest that over evolutionary time periods (108-109 yr) the Earth's percentage cloud cover has remained approximately constant. This is in general agreement with present ideas about the stability of the Earth's evolution1-3. Over medium-term climatological periods (10 4-107 yr) we have found that the position of large cloud masses may be directly related to the changing surface configuration, caused, for example, by continental drift. Global cloud cover fluctuates about a mean, which is near the present-day value and reinforces albedo changes caused by surface configuration; this could be highly significant for theories of climatic change.