Evapotranspiration is a key part of the hydrologic cycle. Some water evaporates directly from soils and water bodies, but much is returned to the atmosphere by transpiration (a word combining transport and evaporation) from plants via openings in the leaves called stomata. Within the same climates, forests and lakes yield about the same amount of water vapor. The amount of evapotranspiration is dependent on energy inputs of heat, wind, humidity, and the amount of stored soil water. In climate studies this term is used to indicate levels of surplus or deficit in water budgets. Aridity may be defined as an excess of potential evapotranspiration over actual precipitation, while in humid regions the amount of runoff correlates well with the surplus of precipitation over evapotranspiration.