A windcatcher - called a 'bâdgir' in Persian - usually looks like a square tower with open slits on several sides. Whichever direction the wind is blowing from, at least one side catches it and channels the air downward through a long shaft. The air rushes down, cools as it moves underground, and flows into the rooms of the house, carrying the heat away. It works a bit like blowing on soup to cool it down, but in reverse.
Some windcatchers are connected to underground water channels called 'qanats'. As the air passes over the cool water on its way down, it becomes even colder and slightly moist - natural air-conditioning that can make a room feel 15 degrees cooler than the blazing desert outside, even when the temperature reaches 40°C. Engineers today are studying these ancient designs to help build modern buildings that need less energy.
Yazd itself is a beautifully preserved desert city, mostly built from warm golden mud-brick, and the whole old city is a UNESCO World Heritage Site. Walking through its narrow winding lanes, you pass windcatcher towers on almost every corner. The city has been lived in for more than 7,000 years - it may be one of the oldest continuously inhabited cities in the world.