Abstract
The atmosphere contains only about 0.001 % of the water available on our planet. Despite this small amount its horizontal and vertical distribution plays a key role in the global water cycle and the Earth’s climate. The atmosphere has direct connections to most of the other reservoirs and steers the redistribution of water between them with an average turnover time of about 10 days. Evaporation over the oceans exceeds precipitation and over land evapotranspiration amounts only to 2/3 of the precipitation reaching the ground. Consequently, there is a net flux of water from the oceans towards the continents, of course via the atmosphere, which has the largest overall volume of fluxes. Water is present in the atmosphere as solid, liquid, or gas. Water vapour is the
most important greenhouse gas in the atmosphere and, in addition, changes of water phase and cloud-radiation interaction contribute strongly to the global energy cycle. Water is also a physically and chemically integral part of other biogeochemical cycles. Although there have been large efforts and improvements in recent years, uncertainties in quantifying the components of the atmospheric water cycle still exist. Observational capabilities on the global scale are not satisfactory at present, but the advent of new satellites devoted to the global observation of precipitation and cloud systems along with dedicated modelling projects certainly will improve the situation.
