Water supports life on Earth. It constantly cycles from evaporation to condensation, precipitation, and run-off over land. Since 70% of the Earth is covered in water, the ocean plays a key role in the water cycle. Having liquid water sustains life on Earth, and is a primary criteria for habitability, as defined by NASA in its search for other habitable planets.
Key learning points
About 86% of evaporation on Earth comes from oceans, 4% from lakes and rivers, the remaining 10% is due to plant transpiration
Evaporation increases with temperature (e.g. daytime, tropics) and wind speed (e.g. tropical storms)
Condensation happens when water vapor cools
78% of global precipitation is over oceans: most near the tropics, least near the poles
Run-off from rain and snow over land is influenced by gravity acting on topography and surface type