How do wet clothes dry?

1. A wet shirt goes up on the clothesline, heavy and dripping. A few hours later it is light and dry. Nobody wiped the water away. So where did it go?
2. Zoom in. The cloth is woven from threads, and each thread is a bundle of fine fibers. Water clings to the fibers and fills the tiny gaps between them.
3. Water is made of tiny particles called molecules, always jiggling. At the surface, the fastest ones break free and fly off into the air. This is evaporation. The water has become a gas you cannot see: water vapor.
4. When the sun warms the shirt, its water molecules move faster. More of them have the speed they need to escape, so the shirt dries more quickly.
5. In still air, the escaped water hangs around the shirt, and some of it lands back on the cloth. A breeze sweeps that damp air away and brings drier air in its place.
6. Air can only hold so much water vapor. On a damp day it is nearly full, so almost as many water molecules land back on the shirt as leave it. Drying slows right down.

Tap the things in the garden that help the shirt dry. Then tap the tiny things in the magnifying glass.

• Wet shirt. A wet shirt is darker and heavier than a dry one, because water fills the gaps between its fibers.
• Line and pegs. Hanging clothes spread out lets air reach both sides. A shirt left in a heap dries only on the outside, and very slowly.
• Sunshine. Sunlight warms the shirt. Warmer water molecules move faster, so more of them escape into the air every second.
• Wind. Moving air carries off the water vapor that collects around the shirt and replaces it with drier air.
• Humidity. Humidity is the amount of water vapor already in the air. The dial reads low on a dry day and high on a damp one. The damper the air, the slower clothes dry.
• Threads and fibers. Each thread is twisted from many hair-thin fibers. The tiny gaps between them soak water up like a sponge.
• Liquid water. In liquid water the molecules stay close together and cling to each other, but they are always moving.
• Water vapor. Water vapor is water as a gas: single molecules flying freely through the air. It is invisible. It is the same water, in a different state.

Hang up a wet shirt and change the weather: the wind, the warmth and how damp the air is. Find the fastest drying day, and the slowest.

Experiment: It is warm, but the air is very damp. How will the shirt dry compared with a warm, dry day?

1. Hang up the shirt on a warm, dry, still day and watch the clock.
2. Hang a fresh wet shirt and make the air very damp.
3. Now try a cool day with dry air and plenty of wind.

What we found: On the damp day the shirt dried much more slowly, even though it was warm. The air was already nearly full of water vapor, so almost as much water landed back on the shirt as left it. Wind and dry air can matter more than warmth.

Turn on JavaScript to try it yourself.

The shirt must be dry before the game. Each round, tap the forecast card with the best drying weather, then hang the shirt out.

When clothes dry, the water in them does not disappear. It changes from a liquid into a gas called water vapor and drifts away in the air. This change is called evaporation. You cannot see water vapor, which is why the water seems to vanish.

Water is made of tiny molecules that never stop moving. In the liquid they cling to each other. But at the surface, the fastest ones break away and fly off. Warmth makes all the molecules move faster, so more of them escape. That is why clothes dry quickly in the sun or near a radiator.

Air can only hold a certain amount of water vapor. How full it is, is called humidity. On a dry day there is plenty of room, and clothes dry fast. On a damp or foggy day the air is nearly full, and molecules land back on the cloth almost as fast as they leave. Wind helps because it blows away the damp air next to the cloth and brings fresh, drier air.

So the best drying day is warm, dry and breezy, and it helps to hang clothes spread out so the air reaches all of the cloth. The puddle comparison leaves one thing out: in cloth, the water is spread over thousands of fine fibers, so it meets far more air than a puddle does.

Key words

Evaporation

A liquid slowly turning into a gas at its surface.

Water vapor

Water as an invisible gas, mixed into the air.

Humidity

How much water vapor is in the air. Damp air has high humidity.

Saturated

Full: air that holds as much water vapor as it can. Its relative humidity is 100 percent.

Condensation

The opposite of evaporation: water vapor turning back into liquid, as on a cold window.

Molecule

The smallest piece of a substance that is still that substance.

Check your thinking

1. What happens to the water in clothes as they dry?

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   It evaporates into the air as water vapor. The water changes from a liquid to an invisible gas and mixes into the air.

2. Which weather dries washing fastest?

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   Warm, dry and breezy. Warmth helps molecules escape, dry air has room for them and a breeze carries them off.

3. Why does washing dry so slowly on a foggy day, even if it is not cold?

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   The air is already nearly full of water. Damp air can take very little extra water vapor, so the water mostly stays in the cloth.

4. Why should you hang a towel spread out, not scrunched up?

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   So the air can reach much more of it. Water can only escape where the cloth touches the air. Spreading it out makes that area as big as possible.

5. You step out of a pool on a warm, windy day and feel cold. Why?

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   Evaporating water takes heat from your skin. The fastest molecules leave first and carry their energy away, cooling what is left behind. Wind speeds the evaporation up.

6. A room is at 15 degrees Celsius and 80 percent humidity. A heater warms it to 25 degrees, and no water is added or removed. What happens to the relative humidity?

   Show the answer

   It falls, because warmer air can hold more vapor. The amount of vapor is unchanged, but the limit is higher at 25 degrees, so the air is now further from full.