The water, or hydrological, cycle, describes the processes that move water from one area to another on the Earth’s surface. A signifi cant part of the water cycle involves the transformation of water from its liquid state on the Earth’s surface to the gaseous state in the atmosphere (evaporation) and then back to its liquid state (precipitation). Although the amount of water that can be stored at any time in the atmosphere is only a fraction of the amount that is stored on land or in the ocean (Table 1),[1] it can transport water great distances, and therefore is a vital component of the water cycle. An additional component of the water cycle is the liquid water transported via rivers, or from melting of frozen water in glaciers and icesheets (runoff).