The Water Cycle

Part of the Water: H2O = Life exhibition.

Misty mountains
© Lydia Gigerichova / AGE Fotostock

The water on Earth today is all the water we will ever have, And Earth's water is in endless motion, on the planet's surface, in its depths and in the atmosphere above. Lakes, rivers, and oceans lose water to the air through evaporation. Plants draw water from the soil and return it to the air. Volcanoes release water once locked in rocks deep within the Earth. All that water rises and falls back to Earth as rain or snow. Eventually, it all finds its way back underground and to lakes, rivers, and the sea. The whole cycles then repeats itself.

Sea Smoke

Sea smoke isn't really smoke--it's rising fog, formed when cold air reaches a warmer ocean. Some of these fog droplets will turn back to a gas, or evaporate, and become part of the water vapor in the atmosphere. As much as a meter (three feet) of the world's oceans evaporates every year, returning to Earth as rain, snow, or hail.

Nothing Is Lost

Some of the rain that soaks the soil evaporates into the air; some finds its way underground, where it very slowly moves through sediments and rocks and may eventually seep into lakes and streams.

© Digital Vision/AGE Fotostock

Giving Back

Plants take up soil moisture as they grow and release it to the atmosphere when they breathe, as well as when they die and decay. The leaves in 0.4 hectares (one acre) of broad-leafed forest may release as much as 30,000 liters (8,000 gallons) of water a day to the atmosphere as water vapor through tiny pores in their leaves.

Heavy Weather

On average, molecules of water vapor spend only 11 days in the air before condensing into water droplets that may return to Earth as rain or snow. Most water droplets in clouds are less than a fraction of the width of a human hair. Many water droplets must combine, through as many as one million collisions, before becoming heavy enough to fall as rain.

On the Run

About one-third of the water that falls to Earth runs off the surface in streams rather than soaking into the soil--though the amount varies by region. A drop of water finding its way to this stream will spend two weeks there, on average, before rejoining the ocean.