The amount of water in the Earth's atmosphere is tiny compared to all other sources of water on our planet, fresh or otherwise. However, this tiny amount of water is fundamental to most aspects of human life. Clouds, for instance, contain about two orders of magnitude less water in condensed form than the water vapor in air, but this miniscule amount of water is an essential stage of those processes that deliver all freshwater upon which human life depends. This influence is complex and not entirely understood, yet exerts profound effects on climate and on those forces that affect climate change.

Thus understanding the moist processes that determine the supply of water to the atmosphere and back to the surface is of some importance. Consequently observations of the distribution and variability of clouds and precipitation have emerged as priority, even as our ability to observe the properties of clouds and precipitation by satellites presently orbiting Earth is unprecedented in the history of space-borne Earth observations. These new capabilities are now delivering important new insights on how water cycles through Earth's atmosphere and a firmer basis to predict how this water cycle evolves and thus how it may alter with climate change.