To cool the water in the summer heat, it is poured into vessels made of slightly fired clay

To cool the water in the summer heat, it is poured into vessels made of slightly fired clay, through which water slowly seeps. The water in such vessels is colder than the surrounding air. Why?

Water evaporating from the surface of such a vessel takes with it some of the heat, cooling the vessel. In the summer heat, and especially in dry climates, this process is especially intense and the coolness of the water in the vessel is clearly visible. You just need to take into account that the water in such a vessel is slowly decreasing.