Liquid water is made up of molecules of H2O attracted to one another by intermolecular forces known as ‘hydrogen bonds’. These are relatively weak, and there are always some H2O molecules whizzing around with enough energy to break free of their neighbours, even at temperatures well below 100°C. These can then escape – ‘evaporate’ – into the air. Even more evaporation will take place if the surrounding air pressure is reduced, for example by going to high altitude.
Robert is a science writer and visiting professor of science at Aston University.