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Warm and windy beats damp and still

Why Do Puddles Dry Slower Some Days?

The same puddle can vanish by lunchtime or sit there all day — the air decides which.

A puddle in themorning Molecules escapinginto the airNo boiling required
Nothing drained away. It left one molecule at a time.

  1. Step 1 of 5

    A puddle is losing and gaining water at the same time.

    The puddle Air above itSome go up, some come back
    Drying is the difference between two flows, not a single one.

    Molecules leave the surface, and molecules already in the air drop back in. Drying is not one thing happening — it is a race, and the puddle shrinks only while leaving is winning.

  2. Step 2 of 5

    Warmth puts more molecules over the edge.

    Cold waterfew are fastenoughMild daymany more makeit outHot pavementescaping allover the surface

    Only the fastest molecules can break free. Warming the water does not speed up one molecule in particular — it widens the crowd that is fast enough to go.

  3. Step 3 of 5

    Wind clears the ones that already left.

    1 Vapour gathersabovethe return flowbuilds2 A breeze sweepsit awaythe crowd iscleared3 Fewer land backinso the puddleshrinks faster

    Vapour piles up in the still air just above a puddle, and that crowd keeps landing back in. A breeze carries it off, so fewer molecules return.

  4. Step 4 of 5

    Damp air sends back as many as leave.

    Leaving versus landingEscaping into the airFalling back in from damp air

    If the air is already near its water limit, every molecule that escapes is matched by one falling in. The puddle stops shrinking without ever being full or frozen.

    This is why washing dries so badly on a muggy day even when it is warm — the air has no room for the water.

  5. Step 5 of 5

    So the same puddle can take an hour or a day.

    Still, cold, damp Still there at duskWarm, windy, dry Gone by lunchtime

    Nothing about the water changed. Warm, moving, thirsty air empties a puddle by lunchtime; cool, still, damp air leaves the identical puddle sitting there at dusk.

The short version

A puddle dries as its fastest molecules escape one at a time, faster when it is warm and windy, slower when the air is already damp.

Try it yourself

Put two identical water spots on a windowsill and blow a fan at one. The difference in a quarter of an hour is obvious.

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