air brought down into the qanat tunnel is cooled by coming into contact with the cool water and / or cool earth and the cool water running through the qanat. The air is also evaporatively cooled when some of the water in the qanat evaporates as the hot, dry surface air passes over it; the heat energy in the air is absorbed as energy of vaporization. The dry air is thus also humidified before entering the building. The cooled air is drawn up through the house and finally out the windcatcher, again by the Coandă effect. On the whole, the cool air flows through the building, decreasing the structure's overall temperature. A is a type of fountain with a thin sheet of flowing water, shaped to maximize surface area and thus evaporative cooling. Windcatchers are often used with salasabils may be used to maximize the flow of unsaturated air over the water surface and carry the cooled air to where it is needed in the building. Wetted matting can also be hung inside the windcatcher to cool incoming air. This can reduce flow, especially in weak winds. However, it can also produce a downdraft of cool air in windless conditions. The evaporative cooling within a windtower causes the air in the tower to sink, driving circulation. This is called passive downdraught evaporative cooling (PDEC). It may also be generated using spray nozzles (which have a tendency to get blocked if the water is hard) or cold-water cooling coils (like hydronic underfloor heating in reverse). Regional use Africa Egypt In Egypt windcatchers are known as , pl. . They are generally shaped as right triangular prisms with the vertical side left open and facing directly up or down wind (one of each per building). They work best if oriented within 10 degrees of wind direction; larger angles allow the wind to escape. Windcatchers were used in traditional ancient Egyptian architecture, and only started to fall out of use in the mid-1900s C.E.. Their use is now being re-examined, as air conditioning accounts for 60% of Egypt's peak electrical power demand (and thus the need for 60% of its generating capacity). Windcatchers in Egypt are often used in conjunction with other passive cooling elements. Middle East and Asia Windcatchers are common feature across many Middle Eastern countries influenced by the spread of culture under Islamic rule. Iran In Iran, a windcatcher is