Re: Is it possible for wind, through evaporation, to lower the temp of water?

The answer given in the question about windchill is mostly right, but its statement suggesting that water will not freeze when the temperature is above freezing is a bit misleading because it does not pay enough attention to the cooling effect of evaporation.

Energy is required to turn water from liquid to gaseous form. Suppose you're walking around outside in 110-degree (F) weather. The hot air is trying to warm your body up to 110 degrees: this direct warming effect is called "sensible heat transfer". As the sweat evaporates off your skin, it carries heat away with it, keeping you cool. This indirect cooling effect by evaporation is called "latent heat transfer".

"Sensible" refers to the fact that the heat carried to your body by the air can be measured, or sensed, with a thermometer. "Latent" refers to the fact that the energy contained in water vapor is hidden until the vapor turns back into liquid.

The same thing happens when it's very cold. There are formulas for calculating how large sensible and latent heating are over a large area of water (like the ocean or a lake).

But there's another thing to consider. Water can only evaporate if the air can receive more water vapor. The more humid the air, the more slowly water can evaporate into it, and so the less heat can be carried away. This is why humid days feel hotter: the sweat can't evaporate as fast, so you're not losing as much "latent heat".

Using the formulas for heat transfer, plus some information on maximum humidity at different temperatures, I compute that evaporation can suck away heat fast enough to make water cool to freezing even when the air is at 12 degrees celsius (54 F), provided that the air is perfectly dry. The more humid the air is, the slower evaporation sucks away heat, and so the colder the air must be to allow freezing. For a humidity of 50% (fairly dry), air temperatures must be colder than 4 C (39 F) to freeze.

I have seen a picture in National Geographic or some similar magazine, showing people who live in the Sahara desert making ice on cool nights by evaporation, exactly as I've described, even though the temperature never falls below 32 F. I regret that I can't find the source where I saw this.