The cycle starts when the sun's heat provides energy to evaporate water from the surface. Then, winds lift the water vapor from the ocean over the lands into the atmosphere. The water vapor condenses to form clouds, and when the conditions are just right, the clouds release water as rain or snow. About 80% of the rain falls in the oceans, but the rest falls onto land. Rivers and streams collect water from the ground and return it to the ocean so the whole cycle can start all over again. The water cycle never ends because the salty ocean water constantly supplies fresh water to the continents.

One process which tranfers water from the ground back to the atmosphere is evaporation. Evaporation is when water passes from a liquid phase to a gas phase. Rates of evaporation of water depend on factors such as solar radiation, the temperature, humidity, and wind.

Water that is held in lakes and rivers evaporates directly into the atmosphere, but some of the water in the groung may also be returned to the atmosphere by way of

evaporation through the soil surface. Of course, the ocean is the greatest source for water evaporated into the atmosphere.

Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets.

Precipitation is the main way for transporting water from the atmosphere to the surface of the earth. There are several forms of precipitation, the most common of which for the United States is rain. Other forms of precipitation include; hail, snow, sleet, and freezing rain.

Groundwater is all the water that has gone through the earth's surface and is found in one of two soil layers. The one nearest the surface is the "zone of aeration", where gaps between soil are filled with both air and water. Below this layer is the "zone of saturation", where the gaps are filled with water.

Transpiration is the process by which plants return water to the atmosphere. After absorbing water from the ground, plants release water through their leaves. Transpiration helps plants stay cool, in the same way perspiration keeps humans and animals cool.

Runoff is the movement of land water to the oceans, mostly in the form of rivers, lakes, and streams. Runoff consists of precipitation that hasn’t evaporated, transpired or penetrated the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide.