Water is practically everywhere on Earth. It is in the air, on the land, in the ocean and underground. The water cycle is the endless process that connects all of it, as water moves through the atmosphere, into and out of the oceans, over the land and underground.

Start with the sun. Energy from the sun helps power the cycle. It gives water at the surface of the ocean, rivers and lakes the energy to become vapor. Evaporation needs energy because heat breaks the bonds that hold water molecules together.

Nearly 90 percent of the moisture in the air comes from evaporation over oceans, seas, lakes and rivers. Most of the rest comes from plants, through transpiration. Evapotranspiration is the name for both of these together.

The air holds very little of Earth's water. Only about 0.001 percent is water vapor. Yet about 500,000 cubic kilometers of water move through the air and fall as precipitation each year.

Air gets cooler as it rises higher. Warmer air can hold more vapor than cooler air. When air cools below the dew point, vapor condenses into liquid water. It condenses more easily onto particles such as dust, salt and smoke.

Cloud droplets grow as more vapor condenses onto them. Rain falls when they get too heavy to stay in the cloud. Falling ice crystals may land as snow. Or they may melt on the way down and land as rain. Hailstones grow in thunderstorm clouds when liquid drops freeze onto them.

Where does it all land? Most precipitation falls as rain, and most lands in the oceans. On land, water can stay in the soil for a month or two. Snow can be stored as snowpack or ice until it melts. Some water seeps down to become groundwater.

Here is the balance. Worldwide, about as much water evaporates as falls back as precipitation. Runoff, rivers, streams and groundwater return water to the ocean. Wells can tap groundwater when they are drilled deep enough to reach an aquifer.

Even the salty ocean feeds the sky. Salt does not evaporate with the water, so it is left behind. The water that rises is fresh. Then it falls, runs down, and starts the trip again.

Words to know

• evapotranspiration: land and plants
• aquifer: underground store
• precipitation: falling water
• runoff: flowing water