How does a puddle evaporate?


How does a puddle evaporate even though the surrounding temperature is nowhere near 100 °C?

Liquids contain molecules with a range of molecular speeds. Fast moving molecules near the surface are constantly leaving surface giving rise to a ‘vapour pressure’ above the liquid. This is called evaporation.

The rate of evaporation increases with temperature however it is not zero, even for solids. So for example, ice evaporates – albeit slowly. However as more molecules accumulate in the region above the liquid the rate at which molecules return to the liquid also increases. At any temperature there is a balance of these processes that determines the vapour pressure above the liquid surface.

In a puddle there is typically open air above the puddle and so as the fast moving molecules leave the liquid they are swept away by breezes, and the rate at which molecules leave the liquid to join the vapour exceeds the rate at which the molecules leave the vapour to join the liquid – hence the puddle disappears.