How does the water kept in an earthen pot (matka) become cool during summer?

Evaporation of water molecule from its surface needs energy and this energy is obtained from the current temperature of the water column. Hence every time a water molecule escapes or evaporates it consumes the energy of the water provided by its temperature and hence brings down the temperature a little. By keeping water in the earthen pot we expose a large area of water column to the air (You might have noticed, the outer surface of matka remains wet as the water reaches its outer surface through capillary action) ,from where the evaporated water molecule can escape, hence the rate of evaporation increases, and by the time a water molecule escapes another water molecule takes its place through capillary action. Therefore the process of evaporation through a large exposed surface of the earthen pot continues, bringing down the temperature of the water bit by bit resulting in cooling it. read less