Most of our water came as a result of Earth’s cooling after its creation. While new water is still formed on a small scale, we basically use the same water over and over again.
The water cycle, also known as the hydrological cycle, is Earth’s natural system for recycling water.
As a result of heat from the Sun, ocean water evaporates. Evaporation causes the pure liquid water (most of its salts remain on Earth) to expand and turn into vapor, or gas. The expanded molecules cool as the vapor rises into the atmosphere.
The change in temperature causes the vapor to condense back into liquid, in the form of tiny droplets. These droplets combine until they become too large to remain suspended in the air. They fall back to Earth as rain, snow, or other forms of precipitation.
Where the precipitation falls back onto the ocean, the process immediately begins again.
Not all rain or snow falls hack into the ocean, however. A small amount will evaporate from the surface of Earth again. Some will fall into lakes or rivers that ultimately find their way back to the ocean.
Still other amounts of precipitation will become groundwater, either gradually combining with streams headed toward the ocean or going through a longer process—as drinking water, irrigation, or water in a car wash, for example.
Sooner or later, though, all water makes its way back to the air through evaporation.
About 3 trillion tons (2.7 million metric tons) of ocean water evaporate a day.
The water cycle is a continous process. Air absorbs water from oceans, lakes, and rivers. Air also absorbs the water vapor exhaled by plants and animals.
When the air rises, it cools, and the water vapor condenses to form a cloud. As soon as the cloud becomes too heavy with liquifying vapor, precipitation sends the water back to Earth.