I am having a doubt regarding what is the exact mechanism of cooling during evaporation. At one place it is said that as surface water molecules evaporate it takes heat from surroundings and hence both temperature of water and surrounding air decreases.In another book it is explained that surface water molecules take latent heat from water and goes as vapour into air ,but then again as temperature of air is more energy from air flows back into water....this happens until a thermal equilibrium is reached at temperature lower than initial. So what is exactly happening?


What exactly is happening?

First it helps to state whether the evaporation is taking place to the open atmosphere or to the air in a closed environment (e.g., a closed container). Let's assume it is to the open atmosphere.

The temperature of a mass of water is the average translational kinetic energy of the molecules. The kinetic energies of individual molecules is distributed around the average. Some are higher, some are lower.

At the surface of the water some molecules may have sufficiently high kinetic energy such that they escape the surface and become gaseous $H_{2}0$ in the atmosphere. These molecules remove their kinetic energy from the surface thereby lowering the average kinetic energy of the remaining molecules at the surface, and thus the temperature at the surface. Heat then transfers from the layers below to the layers above as well as from the air to the surface.

Insofar as the effect of evaporation on the overall average temperature of the water is concerned, that depends on the mass of water as well as its configuration (e.g., ratio of surface area to volume). If the evaporation is occurring at the surface of a deep lake, for example, the effect on the average temperature of the lake is probably insignificant. On the other hand, if we are dealing with a layer of water on the surface of a road, for example, the effect is significant.

Hope this helps.

  • $\begingroup$ Well tell me if I am wrong but I guess what you are saying is that since surface energy is always greater than bulk , this very fact is the driver for heat transfer and subsequent temperature lowering ( however insignificant it might be in some cases)? $\endgroup$ – C Ray Apr 16 '19 at 15:26
  • $\begingroup$ I’m not saying the surface energy is greater than the bulk. I’m saying throughout the water some molecules have higher kinetic energy than the average. Those that happen to be at the surface having higher kinetic energy than the average thave the opportunity to escape the surface taking their energy with them thus lowering the temperature at the surface. The lowering of the surface temperature drives heat transfer to the surface from the bulk and air near the surface. $\endgroup$ – Bob D Apr 16 '19 at 16:00
  • $\begingroup$ All right, so extending the same phenomenon if excess K.E for particles at surface is the sole factor that triggers this process then how do we explain on energy terms the level of difficulty for evaporation with increasing humidity in the air above; given that the energy of molecules is independent of the state of air above $\endgroup$ – C Ray Apr 16 '19 at 18:14
  • $\begingroup$ It is not the sole factor that influences the rate of evaporation. While molecules are escaping the surface some molecules return. The higher the relative humidity of the air above the surface the greater the rate of returning molecules and the lower the net rate of evaporation. At 100 percent relative humidity the air is saturated and the net rate of evaporation is zero. Ad to this the effect of air movement, i.e., wind will increase the rate of evaporation all other things being equal. But the phenomena of evaporation is still due to higher energy molecules escaping the surface. $\endgroup$ – Bob D Apr 16 '19 at 18:25

The bulk liquid is a bunch of molecules that, on average, don't have enough energy to overcome the forces binding them together. However, the energies are spread out in a distribution (e.g. Boltzmann) so a small fraction do have a sufficiently high energy and can escape. Once they do, that lowers the average energy of the bulk. Average energy at the molecular level is what we call temperature at the bulk level, so the bulk cools down.

If the air above the liquid becomes saturated (high humidity), that makes it more difficult for molecules to escape so the heat loss is slower.

If there is a draught blowing, those escaped molecules will be removed so this effect will not occur. That's why it's easier to dry things in a breeze.


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