Zeer pots are a simple cooling device that is used in third world counties to store fruits, vegetables and other foods. Now they are not going to freeze your foods or replace your refrigerator, they will keep foods cooler than the ambient air temperatures. Since they work on the principal of evaporative cooling, they work best in a climate with low humidity.
Often they are used in third world countries to transport fresh vegetable to market. Depending on the humidity there have been reports of them cooling foods by as much as twenty degrees.
Making a zeer pot is very simple, all you need is two porous clay pots, one larger than the other, as well as some sand, water, and a cloth. First plug the holes in the bottoms of the pots with a waterproof material; you don’t want the water leaking out. Then, you just put one pot inside the other, and fill up the space between them with wet sand. You will also need a thick wet towel to put over the top to keep the warm air and light from getting in.
The pot cools as the water evaporates, allowing food stored in the inner pot to stay fresh longer. The pot should be placed in a shaded, dry, ventilated space for the water to evaporate effectively. Evaporative coolers perform poorly or not at all in climates with high ambient humidity, since the water is not able to evaporate well under these conditions.
You can use undrinkable water to wet the sand without contaminating the food if the inner pot has a waterproof glaze. You need to replenish the water in the sand at least twice a day. Add as much water as you can without the inner pot starting to float.
Depending on the temperature and humidity in the area in which you live, reports indicate that the temperature difference between the inside and outside temperatures can be over 20° f. If you are a diabetic this could make a big difference in how long your insulin stays usable.