When particles break away from the surface of a liquid and form a vapour, the process is known as evaporation. The more energetic molecules of the liquid escape from the surface. This reduces the average energy of the molecules remaining in the liquid and so the liquid cools down.

Evaporation causes cooling. The evaporation of sweat helps to keep a body cool in hot weather. The cooling obtained in a refrigerator is also due to evaporation of a special liquid inside the cooling panel at the back of the compartment.

Evaporation is increased at higher temperatures and it is also increased by a strong flow of air across the surface of the liquid, as in this way the evaporating molecules are carried away quickly. A certain amount of water will also evaporate more quickly if you increase its surface area. A bowl of soup will cool down much more quickly than a mug of soup, because the large surface area of the bowl allows more evaporation.

Clouds are formed from invisible water vapour that evaporates from the sea and is carried away by the wind. When the water vapour cools at high altitude, it turns back into the small drop lets of water that you can see as these clouds.



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