Zeer pot
A zeer pot is a container that keeps contents cold for extended periods of time. It is an electricity-free alternative to a conventional refrigerator but requires an area with low humidity and some airflow. The device is unsuited for contents needing very low or constant temperatures. A root cellar provides a larger storage area that needs less maintenance.
Zeer pots are made from two nestled terracotta pots with sand filling the space between. The inner pot can be glazed or unglazed but the outer pot must be unglazed. Water is added to the sand which soaks through the outer pot. A lid, or wet cloth, is laid on top of the inner pot. As dry air flows over and around the pot it draws out moisture from the outer pot, causing the water to evaporate which creates a cool space in the inside pot. As the sand dries out, more water is added.
Evaporation cools the zeer pot and thermal mass allows it maintain its temperature. A large outer pot, ideally raised on a support to expose all sides including underneath, creates a larger surface area to allow for more evaporation. A smaller inside pot allows for more sand to increase the thermal mass to maintain the lower temperatures while also allowing less frequent waterings. Maintaining an inner pot that is full also helps increase the thermal mass.