The process of evaporation is the conversion of liquid into a gaseous state. An earthen pot or a matka is porous in nature. Vaporisation is the process that takes place in every part of the environment. Similarly, the vaporisation that helps water kept in an earthen pot (matka) become cool during summer.
Water placed in earthen pots is evaporated from minuscule pores it has as it is made of mud particles. There are a large number of extremely small pores in an earthen pot through which the water kept inside the pot keeps on evaporating and takes the latent heat required for vaporization from the earthen pot and remaining water. The earthen pot and water hence loses heat and this makes the water inside the pot cool. This evaporation of water produces a cooling effect. Some of the heat energy that is generated is used in the process of evaporation. Hence, water stored in earthen pots tends to become cooler in summer.