Evaporative cooling: clay pot cooler and wet cloth

For water to change from liquid to gas, it needs energy. It takes this energy, the heat of evaporation, from its surroundings – from the clay, the sand and therefore also from the inside of the pot. As long as water is evaporating, heat is constantly being drawn out of the cooler.

Unglazed clay is porous, meaning it has tiny pores through which water slowly seeps outwards. On the outside it evaporates. Glazed pots do not work, because the glaze seals the pores. How strong the cooling is depends mainly on the air: dry air takes up a lot of water vapour, and moving air carries the vapour away and makes room for more evaporation.

There is a natural limit: an evaporative cooler can never get colder than a certain value that depends on air temperature and humidity. In very dry air this value is well below the air temperature; in muggy air it is hardly below it. That is why this entry gives no temperatures – the effect varies greatly with the weather.

You need two unglazed clay pots that fit inside each other with a few fingers' width of space all round. If the pots have drainage holes, seal them first. The gap is filled with clean sand and soaked thoroughly without letting water run into the inner pot. A wet cotton cloth serves as a lid.

The location decides success or failure. The cooler belongs in the shade in an airy place, for example under a canopy where a breeze blows. In the sun it heats up; in a closed room the moist air builds up and evaporation comes to a standstill.

The sand must always stay damp – usually this means re-wetting it once or twice a day. Check the contents every day and remove anything that turns soft or rotten, otherwise it spoils its neighbours. The pots are cleaned regularly so that no mould takes hold. A simpler version is a basket covered with a wet cloth whose ends hang in a dish of water.

An evaporative cooler makes sense in heat and dry air: during a long power cut in summer, for a garden harvest without a fridge, or in regions with a hot, dry climate. Fruit and vegetables stay crisp for longer and wilt more slowly in it, because they are kept cooler and dry out less.

It makes little sense in damp, muggy weather, because then hardly anything evaporates and the cooling effect collapses. It is also the wrong choice when water is scarce: it uses water every day, and drinking water always comes first. Use only water that would not be drunk anyway.

Perishable foods – meat, fish, dairy, cooked leftovers and infant food – need real, reliable refrigeration. An evaporative cooler does not reliably reach those temperatures and varies with the weather. Anything stored in it therefore counts as unrefrigerated. When in doubt, it is thrown away.

Germs that make you ill often cannot be detected by smell or appearance. So with perishable foods, do not rely on them “still seeming fine”. Wash fruit and vegetables from the cooler with safe water before eating them.

The damp cooler can attract insects and is a good place for mould. So keep it covered and clean and remove anything spoiled straight away.

Clay pot coolers have long been used in hot, dry regions to keep food fresh without electricity; the pot-in-pot cooler is also known as a “zeer”. The same principle is at work in unglazed water jugs, whose contents stay pleasantly cool on hot days.

Your body uses evaporative cooling too: when you sweat, water evaporates on the skin and cools you down. That is why heat often feels more bearable in dry air than in muggy air – there, sweat evaporates less well.