Unglazed. Earthenware porous water jar. As the water evaporates on the outside the water starts cooling. By hanging the jar in a "windy" open window the process is further enhanced.
In his book "Topsy Turvy Land, Arabia pictured for children" Ref 1 S.M. Zwemer writes in 1920 : "Oh what a blessing those jars are to all the people of this hot and dry country. We have no ice in Arabia and so no refrigerators; the wells are never very deep and the water comes a long distance. So if it were not for the crockery man and his water-jugs we could never drink cold water. But just pour the water in one of these pots and hung it in the wind and then in a few minutes the water gets cold. We missionaries always have such water jars hanging or standing in our windows to catch the breeze"
(The pottery is made deliberately porous, the evaporation of water on the outside of jar causes the jar to cool down, similar to the effect of sweating on the human skin) Height 25 cm. The Zwemer brothers worked as missionaries of the Dutch Reformed Church of America in Muscat and Bahrain. 
Water Jars, just out of the Kliln in Bahla (1980's)