Re: Can lowering the partial pressure of water in the air create evaporation?

Dear Ben,

I feel, like you, that there are too many terms that confuse us. In addition, there are always at least two ways to look at (and thus “explain”) a piece of nature when there is really only nature. That can be confusing too if we try to combine the parts of two stories. I’ll assume that the gross evaporation refers to the actual rate of molecules leaving the surface and the net evaporation refers to the difference between those leaving and those returning from the gas state.

I’ll suggest that we look at your case on the molecular kinetic level. Let's simplify somewhat by considering pure water and a normal air mixture. Start with the surface; below we have a swarm of water molecules. They are moving about with a range of speeds (energies) and in random order. They have an attraction for each other. Above the surface there is a mixture of molecules that have a much greater separation (10x under standard conditions) and are also moving in random order including toward the surface.

The subset of interest are those molecules at or very near the surface. A molecule can only “evaporate” if it is at the surface and moving away from the liquid and with enough speed to escape the attraction of its neighbors below (not unlike the escape velocity from the earth). A molecule can only “condense” if it strikes the surface inelasticly and is slowed to the point that its attraction to the liquid will hold it.

In general, evaporation is a cooling process since it is the more energetic molecules that escape and carry their greater energy with them. Those remaining are statistically slower (cooler). On the other hand, condensation of the gas is the opposite and is a warming process.

Under normal conditions, these two processes are carried out to reach equilibrium. The number of water molecules in the air remains constant because the evaporation rate and the condensation rate are equal. If more molecules were to leave the surface, that would raise the number of free molecules and increase the rate of condensation. Thus the process has a natural feedback to remain at equilibrium. The relative number of water molecules in the air versus the nitrogen, oxygen, etc. constitutes the Partial Pressure. If the system were heated, more surface molecules would have the energy to escape and this would raise the number in the air and increase the “partial pressure”. Lets not even mention humidity and its cousins since that’s one of the other ways of talking about the same nature.

You indicate that you already know this much. So what about artificially lowering the partial pressure? This can be done by removing water vapor by means other than sending it back into the surface. There are chemical and mechanical traps that can do this. The little packet in so many things today is filled with a hygroscopic material, CaO or some gel, that absorbs the water. Fabrics like cotton do also. On the other hand, cold surfaces will collect water from the air. This is what we see on a cold glass and the principle behind dehumidifiers. In a confined area, this will lower the partial pressure of the water in the air and would allow for and encourage the Evaporation from some other surface. Find a way to lower the partial pressure of water and you encourage an increase in the evaporation elsewhere.

Let me share a personal example. In your gap between the mountains, you may not have experience with the high humidity of the Gulf influence. Where we suffer in the east, comfort is a combination of temperatures and partial pressures. An important point is that we are not so interested in the air temperature for comfort as we are in skin temperature. Since we are designed to perspire when warm, we expect/hope to be cooled by that moisture evaporating. I’ve been quite comfortable on days 90-100° when the partial pressure is low enough to make me cool by evaporation. In eastern states, we can be miserable in 80° weather when the partial pressure is so great that there is little or no NET EVAPORATION from our skin … thus no cooling.

On hot summer days, we carry a layer of moisture on our skin when outside. Upon entering an air conditioned building there is often an immediate sense that the room is too cold. After a few minutes, that feeling is gone and we may even think it could be cooler. The reason is that the room is not just cooler than outside; but it is dryer than outside. There is a flash evaporation from our skin upon entering that gives an immediate cold sensation. Once the perspiration is gone, no more rapid evaporation and no more sense of cold. High partial pressure outside and low partial pressure inside. We move to change the environment, but it has the same effect as what you were asking … I hope.

Keep up the thinking process; and if you have time, consider that Bernoulli’s Principle is nothing but Newton’s First Law.

Thanks for asking,

Gene Ewald