Hydrologic cycle is the continuous movement of water from the earth"s surface to the atmosphere and back to the surface, then to the atmosphere. * basic cycle involves the processes of evaporation & transpiration (evapotranspiration), condensation, precipitation, runoff. * transpiration refers to the water released to the atmosphere by vegetation during the process of photosynthesis. * evapotranspiration is the combined amount of water lost by evaporation and transpiration. Know these & the processes that change water from one state to another: melting, evaporation, condensation, freezing, sublimation, deposition. Also, know whether heat energy is absorbed, or released during each process. Change of state process energy released or absorbed solid to liquid melting absorbed liquid to gas evaporation absorbed gas to liquid condensation released liquid to solid freezing released solid to gas sublimation absorbed gas to solid deposition released. Humidity is a measure of the amount of water vapor in the air.