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What is condensation?

The short answer

Condensation is when water vapor, the invisible water in the air, cools down and turns back into liquid drops. You see it on the outside of a glass of ice water, on a bathroom mirror after a shower and on a window on a cold morning. The drops do not leak through anything: they come out of the air.

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Mia and Leo, in the kitchen

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The longer answer

There is always some water in the air, spread out as a gas too thin to see. Warm air can hold more of it than cold air.

When warm, damp air touches something cold, the air right next to it cools down. It can't hold all its water any more, so some of the vapor turns back into tiny drops on the cold surface.

Clouds are condensation too. Air rising high into the sky cools down, and its water vapor condenses into tiny drops that float together as a cloud. When the drops grow heavy, they fall as rain.

Three things to know

  • Clouds are made of tiny water drops floating in the air. The drops are so small that the air holds them up, until they bump together and grow heavy enough to fall as rain.
  • The same water goes round and round. Water evaporates from seas and lakes, condenses into clouds and falls as rain, over and over again.
  • Your breath makes a cloud on a cold day. Warm breath carries water, and when it meets cold air the water condenses into tiny drops you can see.

Try it at home: Catch condensation

  1. Fill one glass with ice water and another with water from the faucet.
  2. Dry the outside of both glasses and stand them side by side.
  3. Wait and watch. Which glass gets wet on the outside?
  4. Touch the inside of that glass. It's dry, so where did the outside water come from?

Breathe on a cold mirror or window and watch your own condensation appear and fade.

Other questions people ask

Why does a cold glass get wet on the outside?

The cold glass cools the air touching it, and the water vapor in that air condenses into drops on the glass. The water came from the air, not from inside the glass.

What is the difference between condensation and evaporation?

Evaporation is liquid water turning into invisible vapor, like a puddle drying in the sun. Condensation is the opposite: vapor turning back into liquid drops.

Quick quiz

Watch the episode first, then tap an answer.

  1. Mia had two glasses side by side. Which one got wet on the outside?

  2. The water in the air was invisible. Why did it turn back into drops?

  3. The sky borrows water from us. What does it give back?

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