Question: How does a puddle evaporate

Ciarán O'Brien answered on 8 Nov 2014:

More or less the same way as any other water. As long as the temperature of water is above freezing, it’s going to evaporate. Heat energy (It may not seem like there’s much heat in a puddle outside to you, but there’s enough!) makes molecules jiggle about. When water is frozen, there’s not enough jiggle energy for water molecules to break free of their bonds with other water molecules, so they stay rigid and solid as ice. Above 0 degrees, they’re jiggling enough to break free of most of the bonds holding them together, so they can slide around and flow like water. Some molecules will be jiggling faster than others as they bump into each other, and the ones with the most energy break free of the rest and float into the air as evaporation (this does happen with frozen water too, but really, REALLY slowly because the bonds holding the molecules together are way stronger then the jiggling energy most of them have at that temperature).

You’ve all seen how hotter water will evaporate faster, but that’s not the only thing controlling evaporation. If there’s already a lot of water in the air, there won’t be much room for more, so evaporation is slowed down by high humidity. If the air isn’t moving very much around a puddle, the air closest the puddle will get more humid as water evaporates into it, and the puddle will evaporate more slowly. If there’s wind blowing though, all the humid air will be swept away from the puddle, leaving less humid air for the puddle to evaporate into, and you’ll speed up the puddle’s evaporation. Hand dryers in school toilets, cafes etc. use a combination of hot air and wind to evaporate water from your hands as fast as possible.

A lot of water from puddles just soaks into the ground though and from there into nearby streams and rivers, so unless the puddle is on a waterproof surface, evaporation is not the only reason puddles eventually disappear!