Well, I decided to go a bit further.
First, the science behind the whole thing is that of the wet-bulb thermometer. You know: the one that measures the dewpoint? The gist is that terrestrial air has some amount of moisture in it and that vaporizing (liquid) water to dissolve it in the (gaseous) air requires energy which comes from the heat that is present in the air. Because heat allows the transfer, a thermometer with a "wet bulb" will read a cooler temperature than a one with a "dry bulb". That is, something that is wet is always a little cooler than something that is dry. All because of evaporation.
Second, how can one convert this to something practical? Like, say, the ever present problem of keeping one's beer ... er ... beverage cool on a hot day.
So I developed the idea of a terra cotta beverage cooler. It works like this: you add water to elicit evaporation which causes cooling which keeps your beverage cool. Even on a hot day. And it's not just passive: it actually reduces the temperature of a warm beverage to something cooler.
The only catch is (as I found out) that it requires a low dewpoint.
The cooler didn't work all that well out in the desert. The used flower pots I started with had very little porosity, presumably all the pores had filled with minerals and dirt over time. Also, the sand wasn't easy to use, so I'd recommend trying small pebbles instead. You could even fill it with ice just to set the height of the concrete cap.