A great summary of how the pot-in-a-pot method of creating evaporative cooling works. Check out what Zeer Pots are.
http://preparednessadvice.com/food_storage/how-to-make-and-use-zeer-pots-to-keep-your-foods-cold/
This is a companion discussion topic for the original entry at https://peakprosperity.com/zeer-pots/