Passage
After a rainstorm, shallow puddles on pavement disappear over the next few hours. Two students agreed that the water leaves a puddle by evaporation — turning from liquid into vapor that mixes into the air — but they disagreed about what mainly controls how fast a puddle dries.
Student 1: Temperature is the main factor. Warmer water evaporates faster, because its molecules move more energetically and escape the surface more readily. On a hot, sunny afternoon a puddle may vanish within an hour, while on a cold day the same puddle can last until evening. Of two otherwise identical puddles, the one in the warmer spot will always dry first, whatever the breeze.
Student 2: Air movement is the main factor. As water evaporates, vapor builds up in the still air just above the puddle; if that humid layer is not carried away, evaporation slows almost to a stop. Moving air sweeps the humid layer aside and lets drier air take its place, so evaporation continues. A puddle on a breezy but cool day will dry faster than one on a warm but perfectly still day.
