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Water molecules can evaporate off a surface without needing to boil (How does water evaporate if it doesn't boil?). This is because surface particles are sometimes energetic enough to overcome atmospheric pressure.

For these particles to overcome atmospheric pressure, are they at $T = 100$?

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    $\begingroup$ Individual particles don't have a temperature. $\endgroup$
    – ACuriousMind ♦
    Commented Dec 13, 2015 at 17:09
  • $\begingroup$ Look up vapour pressure. Even ice evaporates. $\endgroup$
    – Gert
    Commented Dec 13, 2015 at 17:16
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    $\begingroup$ @Muno: as ACM wrote, it's pointless to look at particles having temperature, unless you're looking at huge numbers of them. At below BP there are plenty molecules that have enough energy to leave the liquid (or solid). But as the temperature goes up the proportion of molecules that have sufficient energy to leave the liquid increases. At BP there's so many of them that the vapour pressure equals the pressure above liquid. $\endgroup$
    – Gert
    Commented Dec 14, 2015 at 1:38
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    $\begingroup$ @Muno: again, pressure is a stochastic property: it only works if you measure it for a large number of particles. $\endgroup$
    – Gert
    Commented Dec 14, 2015 at 14:56
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    $\begingroup$ @Muno: the particle needs to have sufficient kinetic energy to break away. $\endgroup$
    – Gert
    Commented Dec 14, 2015 at 15:05

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Water molecules can evaporate off a surface without needing to boil. This is because surface particles are sometimes energetic enough to overcome the surface tension, which is a technical term referring to the pull of other water molecules.

For these particles to overcome the surface tension, they need to be at something like T=600. (Not single molecules, but the set of molecules that are energetic enough to escape)

As the water particles fight their way against the pull of other water particles they lose so much energy that the temperature of the evaporated stuff becomes same as the temperature of the water.

What does atmospheric pressure have to do with this? Nothing, because there are huge holes between air molecules - or at least for a molecule those holes are huge.

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