Water circulates around the Earth and atmosphere, partly as a way of transporting latent heat in an effort to create global energy balance (recall that radiatively, the tropics and the poles are out of radiative balance and need weather systems and the ocean to transport heat poleward).

An important step in this energy transport is the evaporation of water from the Earth's surface, the transport of this vapor upward, and the subsequent condensation of the vapor into clouds, which then precipitate the water back to the Earth's surface. Concepts of humidity control the rate of evaporation and the time and place that condensation of the cloud water occurs. Later, we'll look at other factors that control the formation of the clouds (stability of the atmosphere leading to varying degrees of vertical motion, resulting in different types and amounts of cloud) and of precipitation.