Evaporation cools

Aavi's mother buys a new matka (earthen pot) for drinking water, in place of the steel pot. When Aavi comes home from school and drinks from it, the water is cold! It never got cold in the steel pot.

Why? Clay has tiny pores. Water seeps through the surface of the earthen pot and evaporates from the outside. To evaporate, water takes heat from the pot and the water inside, so they get cooler. Evaporation causes cooling.

More examples:

• In summer people sprinkle water on the floor or the roof to cool it, for example before sleeping on the roof on a hot night.
• We feel cooler under a fan because the moving air helps our sweat evaporate, and that cools us.

Make a pot-in-pot cooler, with no electricity:

• Put a layer of sand in the bottom of a large earthen pot.
• Stand a smaller earthen pot in the centre and fill the gap between them with more sand.
• Pour water into the sand.
• Cover the small pot with a lid or a wet jute sack.

After 4 to 5 hours it cools down, and it keeps vegetables and fruits fresh, as long as you keep the sand moist.

The surahi is another clay pot used in summer to keep water cold.