The Hydrologic Cycle

Where does the water go when rain falls to the earth?  Where does the rain water come from?  The hydrologic cycle answers these questions by explaining how water moves between the earth, oceans, and sky. 

The picture to the left gives an overview of the hydrologic cycle.  This lesson will explain the hydrologic cycle in more detail. 

The hydrologic cycle begins when the sun heats water in the oceans, lakes, or streams.  The heated water is able to evaporate, meaning that it turns into water vapor and enters the air.  Water can also enter the air when plants "breathe" in a process known as transpiration.  An average tree can transpire hundreds of gallons of water into the air every day. 

Condensation is the process through which water vapor in the air turns into a liquid and forms clouds.  Rain drops often form when water vapor comes in contact with dust in the air.  As you can see in the picture to the right, the particle of dust moves through the air, slowly gathering more water vapor molecules around it.  Eventually, a raindrop will have formed around the dust particle. 

A cloud is a mass of water or ice droplets in the air.  Although you might not realize it, you have probably walked through a cloud before - fog is an example of a cloud next to the earth's surface.  When you were last out in a fog, you might have noticed that the fog made you feel damp.  If you were riding in a car through a fog, droplets of water would have collected on the windshield as a result of the water's attraction to the glass.

The next step in the hydrologic cycle is precipitation in which water from clouds falls back to the earth as rain, snow, or sleet.  Water precipitates out of clouds when the air temperature drops.  The drop in air temperature may occur when air moves up the side of a mountain or when a mass of cold air comes in contact with the mass of warm air. 

The picture at the left shows what happens to rain once it hits the earth's surface.  The rain can either become surface water or groundwater, or it can evaporate back into the air. Surface water is any water found on the surface of the earth, including creeks, rivers, lakes, and oceans. Groundwater is water which sinks down into the earth and forms underground reservoirs known as aquifers.  Eventually, the rain water evaporates back into the air and the hydrologic cycle continues. 


Build Your Own Hydrologic Cycle

Fun Facts
  • Water is the only substance naturally found on earth in three states, as a solid (ice), a liquid (water), and a gas (water vapor.) 
  • A person can live for more than a month without food but for only about a week without water. 
  • The average person uses 50 gallons of water per day.  This costs us each about 25 cents daily. 
  • Water is constantly being recycled in the hydrologic cycle, so you could be drinking the same water which a dinosaur once drank. 

For more water trivia visit the EPA's Water Trivia Facts