Zeer pots have been used for thousands of years and today are still helping people keep food fresh in the heat of saharan and sub-saharan Africa. They are made up of two earthenware pots of different sizes, placed one inside the other. You fill the space between the pots with damp sand and keep it moist by adding water - you fill the smaller pot with food. The top is covered over with a damp cloth and as the water in the sand evaporates the temperature of the pot is lowered from the ambient temperature by several degrees. A zeer pot can keep 10kg of food fresh for about 20 days.
I have seen Omani air coolers that work on a similar principle - clay pots filled with water and hung up in windows where the water evaporates and cools the surrounding air.
Clever, frugal and beautiful.
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