I have noticed that water cools in matka(earthern pot) and people give it as a reason of evaporation of water at pot surface and further conduction of heat of water to cool down pot.
But evaporation will continue all the time upto the point pot has water, so water should be cooled upto freezer temperature, but this doesn't happen in real life, water in it cool only upto some extend.
Why?
Energy and matter tend to move to eliminate gradients, meaning spatial differences. Even as matter is moving—evaporating—to eliminate a chemical potential difference between the liquid and gas phases (causing a temperature gradient), energy is moving to eliminate that thermal gradient. An approximate temperature equilibrium is reached when the thermal energy removed by boiling matches that added by heat transfer from the environment. These two mechanisms are different, so you can maximize the cooling effect by using a thermally insulative barrier that’s still porous.
The amount of cooling you get by evaporating water from a surface (like the earthenware pot you mention) depends on the humidity of the ambient air. If the air is already carrying a lot of water vapor in it (very humid) then the rate of evaporation is almost the same as the rate of condensation on the surface, and the net rate of water evaporation falls towards zero- and so does the amount of heat transferred out of the pot by evaporation.
Let's say the water inside the earthen pot was at 30 degree celsius right after water is poured into it. Due to the latent heat of vaporisation, the water cools to lets say $28^@C$ and then gradually say $25^@$. At this stage the water is too cool for more evaporation to take and the temperature remains constant. Also an extremely fine layer of water molecules in contact with air evaporates which provides very little cooling which is counter balanced to some extent by the considerable temperature difference between the water and the surrounding environment
