You climb out of a swimming pool, and a breeze blows. Brrr! You shiver, even on a warm summer day.

Why? The water on your skin is quietly turning into a gas and flying away. It takes some of your warmth with it. Let’s find out how that works.

Solid, liquid or gas?

Everything around you is made of tiny particles, far too small to see. They are built from atoms. One particle of water, for example, is three atoms stuck together.

These particles never keep still. They jiggle all the time, and the warmer something is, the faster they jiggle. That is what temperature tells us.

How the particles move decides what a material is like:

  • In a solid, like ice, the particles hold on to each other tightly. They can only wiggle in place.
  • In a liquid, like water, they stay close together but slide past each other. That is why water can flow.
  • In a gas, like air, they fly around freely, far apart from each other.

Solid, liquid and gas are called the three states of matter. Water can be all three: ice, liquid water and water vapour. Water vapour is a gas, and it is invisible.

Water as a solid, a liquid and a gas Three boxes of water particles. In ice, a solid, the particles sit in neat rows. In liquid water they stay close together but are jumbled. In water vapour, a gas, a few particles fly far apart in different directions. An arrow along the bottom shows that adding heat turns ice into water and water into vapour. solid liquid gas ice water water vapour add heat
The same water particles in three states. In ice they sit in neat rows, in liquid water they slide around each other, and in water vapour they fly far apart.

What is evaporation?

Imagine you could see the particles at the surface of a puddle. They bump and jostle all the time. Now and then, one of them gets a kick big enough to break free from its neighbours. It shoots off into the air as a particle of gas.

This slow escape is called evaporation. Particle by particle, the whole puddle turns into water vapour and mixes into the air. Water evaporates at any temperature, so even on a cool day, puddles and wet laundry slowly dry.

Why does evaporation make things cold?

Only the fastest particles can break free. When they fly off, they take their energy with them. The slower particles stay behind — and slower particles mean a lower temperature. So the water that is left gets colder.

The cooled water then takes heat from whatever it touches, such as your skin. Evaporation soaks up heat from its surroundings, the way a sponge soaks up water. That is how it cools things down.

You can feel this cooling in many places:

  • Sweat. When you run and play, your body makes sweat. As the sweat evaporates, it carries heat away from your skin. This stops you from getting too hot.
  • A wet swimsuit in the wind. The wind blows the water vapour away, so more water evaporates, faster. That is why you shiver after a swim on a windy day.
  • A clay jug. In hot countries, people keep drinking water in jugs made of unglazed clay. A little water seeps through the clay and evaporates on the outside. This keeps the water inside cool, without any fridge.

Try it yourself: wet the back of one hand and keep the other one dry. Blow gently on both. The wet hand feels much colder.

Evaporation or boiling?

As water heats up in a pot, it evaporates faster and faster. Then, at about 100 °C, something new happens. Bubbles of water vapour form deep inside the water and race to the top. This is boiling.

Evaporation is slow. It happens at any temperature, but only at the surface. Boiling is fast and happens all through the liquid — but only once the liquid is hot enough. That temperature is called the boiling point.

What is condensation?

Evaporation can also run backwards. When water vapour touches something cold, its particles slow down and cling together again. They turn back into tiny drops of liquid water. This is called condensation.

You have seen it many times:

  • A glass of cold lemonade gets wet on the outside on a warm day. That water comes from the air, not from inside the glass!
  • In the morning, grass is often covered in dew. During the night, the grass cooled down, and water vapour from the air condensed on it.
  • After a hot shower, the bathroom mirror fogs up.

Here is the important part: condensation gives back all the heat that evaporation soaked up. So it warms up whatever the water vapour condenses on.

Evaporation cools, condensation warms Left: a drop of water on skin. Water particles fly up out of the drop, and red arrows show heat flowing from the skin into the drop, so the skin cools. Right: a glass of cold water with ice cubes. Water vapour particles from the air move towards the glass and form drops on its outside, and red arrows show heat flowing into the glass, so the glass warms up. evaporation skin soaks up heat condensation gives heat back
Evaporating water soaks up heat, so it cools your skin. Condensing water vapour gives that heat back, so it warms the cold glass.

The secret inside fridges and heat pumps

Evaporation soaks up heat in one place. Condensation gives it back in another. Put the two together, and you can carry heat from one place to another!

That is exactly what a fridge does. A special liquid called a refrigerant flows through its pipes. It evaporates inside the fridge, soaking up heat from the food. Then it condenses in pipes at the back or in the sides, giving that heat to the kitchen.

That is why the back or the sides of a running fridge often feel warm. Air conditioners and heat pumps use the same trick to cool or heat whole homes.

Go deeper: Hidden heat for grown-ups and the extra curious

The heat soaked up in evaporation, and given back in condensation, is called latent heat. “Latent” means hidden, because this heat does not change the temperature. Water boiling in a pot stays at 100 °C even though the stove keeps heating it. All that heat goes into pulling the particles apart.

And it is a lot of heat. Warming 1 kg of water from 0 °C to 100 °C takes about 420 kilojoules (kJ):

Q=m⋅c⋅ΔT=1 kg⋅4.19 kJkg ⋅ ∘C⋅100 ∘C≈420 kJQ = m \cdot c \cdot \Delta T = 1\ \text{kg} \cdot 4.19\ \tfrac{\text{kJ}}{\text{kg}\,\cdot\,{}^\circ\text{C}} \cdot 100\ {}^\circ\text{C} \approx 420\ \text{kJ}

Here QQ is the heat, mm is the mass of the water, cc is its specific heat capacity (how much heat 1 kg of water needs to warm up by 1 °C) and ΔT\Delta T is how much warmer the water gets.

Turning that same kilogram of boiling water into steam takes about 2,260 kJ — more than five times as much:

Q=m⋅L=1 kg⋅2,260 kJkg=2,260 kJQ = m \cdot L = 1\ \text{kg} \cdot 2{,}260\ \tfrac{\text{kJ}}{\text{kg}} = 2{,}260\ \text{kJ}

LL is the latent heat of vaporisation of water at 100 °C. When the steam condenses again, the same 2,260 kJ come back out. That is why a steam burn is so bad: the skin gets the latent heat first, and then the heat of the hot water.

Sweating uses the same effect. Evaporating one litre of sweat carries away about 2,400 kJ from your body, because at skin temperature the latent heat is even a little larger.

Melting works like this too. Melting 1 kg of ice at 0 °C takes about 334 kJ, and freezing gives those 334 kJ back.

Go deeper: Where the picture of escaping particles is too simple for grown-ups and the extra curious
  • Particles do not decide to leave. They bump into each other at random. Now and then, a particle at the surface gets a kick strong enough to escape the pull of its neighbours.
  • Particles of water vapour from the air also land on the water and stay there. A puddle dries only while more particles leave than arrive. In very humid air the two almost balance, so wet things dry slowly, and sweat cools you poorly on a muggy day.
  • The playground picture suggests that the water left behind keeps getting colder forever. In reality, it cools only until the heat flowing in from its surroundings matches the heat carried away by the escaping particles.

Check yourself

Why do you feel cold in a wet swimsuit on a windy day?

Show the answer

The water on your skin evaporates and takes heat from your body — Evaporating water soaks up heat from your skin. The wind blows the water vapour away, so the water evaporates even faster.

Check yourself

Where do the drops on the outside of a glass of cold lemonade come from?

Show the answer

Water vapour from the air condenses on the cold glass — The air around us always holds some invisible water vapour. When it touches the cold glass, it condenses into tiny drops.