Central Iran shows large diurnal temperature variation with an arid climate. Most buildings are constructed from thick ceramic with high insulation values. Towns centered on desert oases tend to be packed very closely together with high walls and ceilings, maximizing shade at ground level. The heat of direct sunlight is minimized with small windows that face away from the sun. The windcatcher's effectiveness had led to its routine use as a refrigerating device in Persian architecture. Many traditional water reservoirs are built with windcatchers that are capable of storing water at near freezing temperatures during summer months. The evaporative cooling effect is strongest in the driest climates, such as on the Iranian plateau, leading to the ubiquitous use of windcatchers in drier areas such as Yazd, Kerman, Kashan, Sirjan, Nain, and Bam. A small windcatcher is called a shish-khan in traditional Persian architecture. Shish-khans can still be seen on top of ab anbars in Qazvin and other northern cities in Iran. These seem to function more as ventilators than as the temperature regulators seen in the central deserts of Iran.
Windcatchers in Egypt
Windcatchers were used in traditional ancient Egyptian architecture. A painting depicting such a device has been found at the Pharaonic house of Neb-Ammun, Egypt, which dates from the 19th Dynasty, c. 1300 BC. In Egypt the windcatchers are known as malqaf pl. malaaqef.
Structure and architecture
Windcatchers tend to have one, four, or eight openings. In the city of Yazd, all windcatchers are four- or eight-sided. The construction of a windcatcher depends on the direction of airflow at that specific location: if the wind tends to blow from only one side, it is built with only one downwind opening. This is the style most commonly seen in Meybod, 50 kilometers from Yazd: the windcatchers are short and have a single opening.
Function
The windcatcher can function in three ways: directing airflow downward using direct wind entry, directing airflow upwards using a wind-assisted temperature gradient, or directing airflow upwards using a solar-assisted temperature gradient.
Downward airflow due to direct wind entry
One of the most common uses of the windcatcher is to cool the inside of the dwelling; it is often used in combination with courtyards and domes as an overall ventilation and heat-management strategy. It is essentially a tall, capped tower with one face open at the top. This open side faces the prevailing wind, thus "catching" it, and brings it down the tower into the heart of the building to maintain air flow, thus cooling the building interior. It does not necessarily cool the air itself, but rather relies on the rate of airflow to provide a cooling effect. Windcatchers have been employed in this manner for thousands of years.
Upward airflow due to temperature gradient
Wind-assisted temperature gradient
Windcatchers are also used in combination with a qanat, or underground canal. In this method, the open side of the tower faces away from the direction of the prevailing wind. When only the leeward side is left open, air is drawn upwards using the Coandă effect. The pressure differential on one side of the building causes air to be drawn down into the passage on the other side. The hot air is brought down into the qanat tunnel and is cooled by coming into contact with the cool earth and cold water running through the qanat. The cooled air is drawn up through the windcatcher, again by the Coandă effect. On the whole, the cool air flows through the building, decreasing the structure's overall temperature. The effect is magnified by the evaporative cooling of water vapor when the air passes through the qanat water canal, as the water that evaporates in the canal has a large enthalpy of vaporization and, besides, the dry air is humidified by the evaporated water from the canal before entering the building.
Solar-produced temperature gradient
In a windless environment or waterless house, a windcatcher functions as a solar chimney. It creates a pressure gradient which allows hot air, which is less dense, to travel upwards and escape out the top. This is also compounded significantly by the diurnal cycle, trapping cool air below. The temperature in such an environment cannot drop below the nightly low temperature. When coupled with thick adobe that exhibits good resistance against heat transmission, the windcatcher is able to chill lower-level spaces in mosques and houses in the middle of the day to frigid temperatures. Directing airflow upwards using wind-assisted or solar-produced temperature gradients has gained some ground in Western architecture, and there are several commercial products using the name windcatcher.
Modern applications
The windcatcher approach has recently been utilized in Western architecture, such as in the visitor center at Zion National Park, Utah, where it functions without the addition of mechanical devices in order to regulate temperature. Using aluminum for the windcatcher provides a more efficient capturing system, allowing for wind capture from multiple directions. The Kensington Oval cricket ground in Barbados and the Saint-Étienne Métropole's Zénith both use this method.
