The study of the earth's climate requires reliable global data sets to validate numerical simulation models and to identify regional and global fluctuations and trends. This book presents ways to obtain such data from space-borne and ground-based measurements, both passive and active, over the entire electromagnetic spectrum. It describes the basics of such methods together with the most recent advancements and spans the field from clouds and the planetary radiation budget to surface processes and ocean properties. Each subject is backed by extensive reference lists to enable readers to probe more deeply.

Water is the most effective agent in the climate system to modulate energy transfer by radiative processes, through its exchanges of latent heat and within cascades of chemical processes. It is the source of all life on earth, and once convective clouds are formed, it enables large vertical transports of momentum, heat and various atmospheric constituents up to levels above the tropical tropopause. Water triggers very complex processes at the earth's continental surfaces and within the oceans. At last, water in its gaseous phase is the most important greenhouse-gas! Numerical modelling and measurements of the state of the present climate system needs a very thorough understanding of all these processes and their various interactions and forcings. This is a prerequisite for more substantial forecasts of future states in all scales of time, from days to centuries. Therefore, the management of the World Climate Research Programme established in 1988 the new programme GEWEX (Global Energy and Water Cycle Experiment). GEWEX is specifically defined to determine the energy and water transports in the fast components of the climate system with the presently available modelling and measurement means and to provide new capabilities for the future. Research in GEWEX must further develop methods to determine the influence of climatic anomalies on available water resources.