Why exactly does water cool down by evaporation? What happens in this process?

How much does it cool down? What's the formula to describe this process?

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    $\begingroup$ I'll answer this later if no one else beats me to it, but in a nutshell the water requires energy to change state from liquid to gas and that energy comes from the surrounding air, cooling the air. $\endgroup$
    – casey
    Apr 20 '16 at 11:45
  • $\begingroup$ Water cools down in part by evaporation but only if it's warmer than the air to begin with. Stuff warmer than their surroundings cool down by a few processes, conduction, radiation, sometimes convection, sometimes evaporation. The process for water is somewhat complicated, see madsci.org/posts/archives/2005-10/1128610141.Ph.r.html and physics.stackexchange.com/questions/121516/… I don't want to try to tackle the formula part. It would vary based on air pressure, humidity, sunlight, wind. $\endgroup$
    – userLTK
    Apr 22 '16 at 2:59
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    $\begingroup$ I'm voting to close this question as off-topic because it belongs on Physics $\endgroup$
    – Jan Doggen
    Apr 22 '16 at 15:38
  • $\begingroup$ @JanDoggen, a liquid that is cooler than the gas it is in contact with can still cool from evaporation if the gas is not saturated with the liquids vapor, ie very dry. For instance, a sweating person in the hot desert will cool from evaporation of sweat. $\endgroup$
    – Eubie Drew
    Apr 22 '16 at 18:22

Someone (Casey?) can probably provide a fuller answer, but the basic idea is that ice molecules are kind of locked into place by the molecular structure of the ice crystal, and can't vibrate very much. Increase the energy, and the molecules vibrate more until the molecular attraction is disrupted to become water, when they vibrate a lot. Increase the energy still further and the individual molecules are flying about like crazy, thereby turning into a gas. Water is an extraordinary compound. The energy required to break the attraction between molecules in the liquid state is a whopping 2258 kilo-joules per kilogram - a handy property that brought about the age of steam, and the industrial revolution.

It is this phase transition energy (latent heat) that produces the cooling. The principle of conservation of energy, as applied in nature, requires that to balance the energy gain by the water vapour, there has to be an equivalent energy loss somewhere - hence the cooling effect on the liquid.

  • 1
    $\begingroup$ Should note that this is a property of all materials (AFAIK, anyway, and anything that behaved differently would be really wierd :-)). Phase transitions from solid to liquid, or liquid to gas, take energy to break the bonds between molecules. $\endgroup$
    – jamesqf
    Apr 20 '16 at 19:10
  • $\begingroup$ Thank you for your comprehensive answer so far! However, if you assume that the additional energy, needed to convert liquid water to water vapour, for instance is added by solar radiation - why does the remaining liquid water cool down? And at least, does it? This is often mentioned e.g. when describing conditions for the formation of hurricanes: The water temperature has a higher threshold for initiating correspondent movements because it is cooling down itself by evaporation. One more would be the example of cooling a bottle in the sun by wrapping a wet towell around it. $\endgroup$
    – Arne
    Apr 20 '16 at 22:26

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