The water in your glass may once have been part of a cloud, a glacier or the ocean. Earth's water goes round and round in the water cycle.
Key idea
Evaporation: the Sun warms water and it rises as invisible water vapor. Condensation: high up, the vapor cools into tiny drops that make clouds. Precipitation: drops join, grow heavy and fall as rain, snow, sleet or hail. Collection: water gathers in oceans, lakes, rivers and under the ground, and the cycle starts again.
Plants add water vapor to the air too, from tiny holes in their leaves. Most of Earth's water, about 97 out of every 100 drops, is salty ocean water.
Why it works
When seawater evaporates, the salt stays behind. That is why rain is fresh water even though so much of it came from the salty ocean.
Worked example: Name the step
On a cold morning, drops of water form on the outside of a glass of ice water. Which step is that?
- The drops came from water vapor in the air.
- The cold glass cooled the vapor into liquid drops.
Answer: Condensation.
Common mistake
Thinking clouds are made of water vapor or smoke. Clouds are made of tiny drops of liquid water or bits of ice, so small they float.
Try it at home
- Put a little warm water in a clear bowl and cover it with plastic wrap. Set a few ice cubes on top of the wrap.
- Watch for a few minutes. Drops form under the wrap (condensation) and drip back down (precipitation).
Going further. The Water Cycle in Earth Science.