Water on the earth and in the earth's atmosphere is constantly cycling. It moves between land, sea, and air. Heat energy from the sun causes water in rivers, lakes, and seas to evaporate. This moist air rises.

As altitude (height above the earth's surface) increases, air temperature decreases. The moist air condenses into tiny particles of water or ice. We see these clusters of water droplets as clouds in the sky.

When the air temperature is cool enough, water precipitates (condenses and falls as rain or snow) out of the clouds and back to the earth's surface.

Most of the rain will fall back into the seas, lakes, and rivers. This is because two-thirds (2/3) of the earth's surface is covered with water (mostly oceans).

Some of the water that falls on land will soak into the ground. The rest will run downhill into streams and rivers. Rivers eventually flow out into seas. Also, some water that has soaked deep underground may eventually flow underground to the sea.