The constant movement of water between the atmosphere, the land surface, and underground is called the water cycle, or hydrologic cycle. This movement supplies all the water needed to support life.
The water cycle is made up of five processes — condensation, precipitation, infiltration, runoff, and evaporation. Water vapor condenses to form clouds, which result in precipitation when the conditions are suitable. Precipitation falls to the surface and infiltrates the soil or flows to the rivers and streams as runoff. Surface water evaporates, returning moisture to the atmosphere, which will again condense to form clouds.
Did you know?
- Water on Earth is recycled over and over again. In fact, there is about the same amount of water on the Earth now as there was when the dinosaurs roamed our world.
- Eighty-eight percent of all water entering the atmosphere originates from the ocean between 60 degrees north and 60 degrees south latitude. Most of the water evaporated from the ocean returns directly back to the ocean.
Condensation is the process of water changing from a vapor to a liquid. As the water vapor rises and cools, tiny water particles collect and form clouds.
You can see condensation whenever you take a cold soda from the refrigerator and set it in a room. Notice how the outside of the soda can “sweats?” The water doesn’t come from inside the can; it comes from the water vapor in the air. As the air cools around the can, water droplets form.
Precipitation is water being released from clouds as rain, sleet, snow, or hail. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy and falls.
A portion of the precipitation that reaches the Earth’s surface seeps into the ground through a process called infiltration. The amount of water that infiltrates the soil varies with the degree of the land slope; the amount and type of vegetation, soil and rock; and whether the soil is already saturated by water. The more porous, or loose, the soil at the surface is, the more infiltration occurs.
Precipitation that reaches the Earth’s surface but does not infiltrate the soil is called runoff. Runoff also comes from melted snow and ice. Runoff flows to the streams and rivers, and eventually to the ocean.
Evaporation occurs when radiant energy from the sun heats water, causing the water molecules to become so active that some of them rise into the atmosphere as vapor. On a lake such as Lake Conroe, evaporation can cause a water loss of up to 180 million gallons on a hot, summer day.