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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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Muno
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    Individual particles don't have a temperature. – ACuriousMind Dec 13 '15 at 17:09
  • Look up vapour pressure. Even ice evaporates. – Gert Dec 13 '15 at 17:16
  • @Gert Vapor pressure is the pressure formed above some liquid or solid as a result of some particles becoming excited enough to leave their "mass." This pressure is in an equilibrium, meaning that as many particles enter the cloud above the "mass" to contribute to pressure as return to the "mass." (if this were true, wouldn't no evaporation take place?) some particles need to have only been energetic enough to enter the cloud; the average of all particles need not be at 100 C. Is this fine? – Muno Dec 14 '15 at 01:25
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    @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. – Gert Dec 14 '15 at 01:38
  • @Gert Ah thanks. My understanding is that at BP, vapor pressure = atmospheric pressure. Even when vapor pressure isn't equal to atmospheric pressure, particles escape the liquid, because they have energy to do so. I would infer that it also doesn't make sense to think of these escaping particles as having a pressure greater than $P_{atmosphere}$? – Muno Dec 14 '15 at 07:47
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    @Muno: again, pressure is a stochastic property: it only works if you measure it for a large number of particles. – Gert Dec 14 '15 at 14:56
  • @Gert But mustn't there be some threshold that individual particles have to break (governed by temperature and pressure) in order to escape either a liquid or solid? – Muno Dec 14 '15 at 14:59
  • Even though, yeah, I now understand that pressure and temperature necessarily refer to collections of particles. – Muno Dec 14 '15 at 15:00
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    @Muno: the particle needs to have sufficient kinetic energy to break away. – Gert Dec 14 '15 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.

stuffu
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