Wind Catchers of Yazd

“The city of Yazd’s first mention in historic records predate it back to around 3000 years B.C. when it was related to by the name of Ysatis, and was then part of the domain of Medes, an ancient settler of Iran. In the course of history due to its distance from important capitals and its harsh natural surrounding, Yazd remained immune to major troops’ movements and destruction from wars, therefore it kept many of its traditions, city forms and architecture until recent times. One of the distinctive features of the cities of Yazd province which distinguish it from other cities is the existence of various wind catchers.

A wind catcher is a high structure on the roof under which, in the interior of the building, there is a small pool. It is capped and has several directional ports at the top (traditionally four). By closing all but the one facing away from the incoming wind, air is drawn upwards using the Coanda effect, similar to how opening the one facing the wind would push air down the shaft. This generates significant cooling ventilation within the structure below, but is not enough to bring the temperature below ambient alone. The key to generating frigid temperatures is due to the existence of a significant air gap above the pool. Completely shaded from the sun, the pool aggregates the cold, sinking air of the night, which is then trapped within, unable to rise up to the less dense surface air. A wind catcher, however, can create a pressure gradient which sucks at least a small amount of air upwards through a house. This cool, dry night air, being pulled over a long passage of water, evaporates some of it and is cooled down further.

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http://historicaliran.blogspot.com/2009/12/wind-catchers-of-yazd.html