How does evaporation affect temperature

One of the main functions of this process in plants is to transport water needed by plant tissues in other parts of the plant besides the roots. But this evaporative cooling effect also benefits the plant. This keeps the plant—which might very well be exposed to direct, intense sunlight—from overheating. And this also explains why, on a hot day, if we enter a forested area, we feel considerably cooler. Part of that is due to the shade, but part is also due to the evaporative cooling effect from the trees through this process of transpiration.

Wind increases the effect of evaporative cooling, and this is a familiar concept. Anyone who's ever been swimming and has come out of the water into a calm environment, versus one that's windy, can attest to it feeling colder in the wind. The wind increases the evaporation rate of the liquid water from our skin surface and accelerates the amount that's being converted to vapor.

Incidentally, this process also causes so-called wind chill. Even in colder conditions, when we're outside and our skin is exposed to the elements, a certain amount of perspiration occurs. When it's windy, more evaporative cooling takes place from exposed skin.

This explains the basics behind the so-called wind-chill factor. As a result, the liquid molecules that remain now have a lower average kinetic energy. As evaporation occurs, the temperature of the remaining liquid decreases.

You have observed the effects of evaporative cooling. On a hot day, the water molecules in your perspiration absorb body heat and evaporate from the surface of your skin. The evaporating process leaves the remaining perspiration cooler, which in turn absorbs more heat from your body.

A given liquid will evaporate more quickly when it is heated. The Figure below shows the kinetic energy distribution of liquid molecules at two temperatures. The numbers of molecules that have the required kinetic energy to evaporate are shown in the shaded area under the curve at the right.

The higher temperature liquid T 2 has more molecules that are capable of escaping into the vapor phase than the lower temperature liquid T 1. Kinetic energy distribution curves for a liquid at two temperatures T 1 and T 2. Yes, there is also water particles that become a gas.

Typically, we call this water vapor. In the gas phase, the particles of water are the same as in the liquid. The difference is that they are not really interacting that much with other water particles in the gas phase. The water vapor particles are much farther apart. If one of those water particles had enough energy, it could break out of the liquid water phase and become a gas.

This is exactly what happens during evaporation. Of course, not every water particle has enough energy to break free of the liquid state. But those that do are the highest energy particles. By removing these higher energy particles, you reduce the average energy of all the remaining particles.

This average kinetic energy of the particles is essentially proportional to the temperature of the liquid. You might think that once the highest energy particles leave, that would be it - but it isn't. The particles in the water are always interacting with each other. This means that some of them interact to slow down and some interact to speed up. Even though the average kinetic energy decreases, there will still be some of these water particles with enough energy to escape - but just not as many.

What the heck is a one dimensional liquid? I don't know, but I am going to make one anyway. Suppose that I have a whole bunch of particles that can only move in the x-direction either in the positive or negative direction. But what about the distribution of speeds? As a guess, I will say that the speeds are normally distributed. If I randomly pick 10, particles and plot their speed, it might look like this. But what about the kinetic energy? I will assume all the particles have the same mass so that the only thing that matters is the velocity.

Here I square these speeds and call it kinetic energy which is a partial lie and I get this distribution. As you might expect, there are a few of these particles with very high kinetic energies. However, most of them are very low. Let me go ahead and point out something that might be obvious: a one dimensional liquid is NOT the same as a 3D liquid.

What if I made a plot of the distribution of kinetic energies in 3D? Since KE is a scalar quantity, wouldn't the shape look the same? These two factors are in conflict, so the answer is non-trivial to me. Let's not consider long term factors like convection eventually cooling the air. Considering only the action of a molecule escaping the surface of water through evaporation in a closed system, how does that affect the temperature of the gas it escapes into?

If one goes to the wiki article on evaporation one sees that. For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.

Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid, thus, decreases.

This is due to the statistical distribution of the kinetic energy of the molecules, the molecules in the tails have enough energy to escape the surface tension of the liquid.

In a closed container energy should be conserved and the higher kinetic energy molecules released in the gas should increase the temperature while the liquid surface and the contact part of the gas will be cooling. In open systems with convection the contact of the gas with the liquid is continually renewed and thus a cooling of the gas can be obtained as in the evaporative coolers.

Convection by continually replacing the air keeps the humidity low, which also allows higher evaporation rates. As the water cools ,its means the air molecules carrying the extra energy transfers it into the atmosphere.

And this has little to do with surface tension, as surface tension is a result of the top most layer of the liquid which more or less remains the same. For evaporation to heat up the air, the newly escaped molecules from the liquid would have to have higher kinetic energy than the average air molecule.