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

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

Answer: Water from porous wall of earthen potevaporates continuously, which lowers the temperature of water kept in the earthen pot. 

In summer moisture level is very low in the atmosphere, which increases the rate of evaporation as evaporation is inversely proportional to the moisture level in atmosphere. 

That is why in summer water kept in earthen pot becomes cool.