The hydrologic cycle (also called the water cycle) refers to the continuous motion of water between the oceans and seas, land, and atmosphere. It may be divided into three main phases: evaporation, precipitation, and runoff.
- Evaporation is the movement of water from the oceans to the atmosphere. Along with strict evaporation, it includes transpiration from plants; thus, it is sometimes called evapotranspiration.
- Atmospheric moisture may condense into clouds, and fall from them as precipitation. This usually occurs as rain, but snow and other forms are also seen. Precipitation seldom falls in the oceans, because under normal circumstances, mountain ranges are needed to induce condensation and the formation of clouds.
- Runoff is the variety of ways in which water on the land finds its way back down to the oceans. The water flows through rivers, some of them subterranean, and may be delayed in lakes and aquifers. Not all water completes the runoff phase; some of it evaporates before it reaches the oceans.
