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Why does drying water make things cooler?

When water dries up, it takes some heat away with it. That is why a wet cloth in a breeze feels cool, and why sweat cools you down on a hot day.

Pure, your Wing Force guide
Pure says“Beep! Drying water takes heat with it. Let’s test it!”

⏱ About 15 minutes. 💧 A little wet. Grown-up helps: a room fan, if you use one.


2 pages: the experiment, then a page to write your results.

What you need

  • 2 small washcloths or squares of paper towel, the same size
  • A bowl of room-temperature tap water
  • A sheet of paper to fold into a fan (or a room fan a grown-up turns on)
  • 2 plastic cups
  • Optional: a kitchen thermometer, if you have one

Steps

  1. Fold the paper back and forth like an accordion to make a hand fan.
  2. Dip one cloth in the water and squeeze it out so it is damp, not dripping. Leave the other cloth dry.
  3. Wrap the damp cloth around one cup and the dry cloth around the other cup.
  4. Fan both cups for 2 minutes. Fan them the same amount.
  5. Touch each cloth with the back of your hand. Which one feels cooler?
  6. Now try it on you. Dip one finger in the water and keep one finger dry. Blow gently on both, or fan them.
  7. If you have a thermometer, a grown-up can help you tuck it in each cloth for a minute and read the numbers.

What you will see

The damp cloth feels cooler than the dry one, even though the water started at room temperature. Your wet finger feels much cooler than your dry finger when you blow on it. More breeze makes the cool feeling stronger, and the cloth dries faster.

Why it works

Water is made of tiny pieces called molecules (MOL-uh-kyools). They are always wiggling. The fastest, most energetic ones can jump off the water and float into the air as a gas. This is called evaporation (ee-vap-or-AY-shun).

To jump away, those molecules need energy, and they take it as heat from whatever they are touching: the cloth, the cup or your skin. So the things left behind get cooler. The breeze helps by carrying the wet air away, so more water can evaporate.

That is how sweating works. On a hot day your body makes a little water on your skin. When it evaporates, it carries heat away and you feel cooler. A fan helps you feel cooler because it helps that water evaporate. A fan does not make the room itself colder.

How your house uses this

Your air conditioner uses the same idea, but with a special fluid instead of water. This fluid is called refrigerant (ree-FRIJ-er-ent). It lives inside sealed metal pipes.

  • Inside the house, the refrigerant evaporates in the cold coil. Just like your wet cloth, that takes heat away, so the coil gets cold.
  • A fan blows your house air across the cold coil, and the air comes out cooler.
  • The refrigerant carries that heat through pipes to the outdoor unit, which lets the heat out into the air outside.
  • Then it comes back inside to do it again, over and over.

Your fridge works the same way. Learn more in How does air conditioning work?

Fun fact: On a humid, sticky day, sweat dries slowly because the air already holds lots of water. That is part of why a muggy day feels hotter than a dry day with the same temperature.

If you use a room fan, a grown-up turns it on, and fingers stay away from the blades.

Try this next

  • Breeze or no breeze. Make two damp cloths. Fan one and leave one still. Which dries first?
  • Sun or shade. With a grown-up, hang two damp cloths outside, one in the sun and one in the shade. Which dries first?
  • Muggy day test. Try the lab on a sticky day and a dry day. Does the wet finger feel as cool both times?
  • Puddle race. Put the same spoonful of water on a plate and in a narrow cup. Which one disappears first? Hint: more open surface means faster drying.

Share what you found

Ask your grown-up to blow on your wet finger and dry finger and guess which feels cooler. Then explain why.


2 pages: the experiment, then a page to write your results.

Keep exploring