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Towers That Catch the Breeze

Towers That Catch the Breeze

Look across the rooftops of Yazd, an old desert city in Iran, and you will see dozens of tall towers rising above the houses like chimneys. They are not chimneys. They are windcatchers, and for hundreds of years they have helped families in this hot, dry city keep their homes bearable through summer.

A windcatcher works by giving moving air somewhere useful to go. Openings near the top of the tower face the breeze. Air pushed into them is steered down a shaft inside the tower and into the rooms below. At the same time, warm air already inside the house rises and escapes through openings on the side facing away from the wind. Even on a calm day, the tower keeps working a little: warm air rises up the shaft by itself and draws cooler air in behind it through the doors and windows.

Some builders went further. Beneath parts of the city ran qanats, tunnels that carried water from distant hills without letting the desert sun reach it. Where a windcatcher was linked to a cool underground room near this water, the air was cooled further on its way into the house. Families stored food in these rooms, and on the hottest afternoons they rested there too.

None of this needed fuel, moving parts or anyone to switch it on. The builders used thick mud brick, which soaks up the day's heat slowly, so rooms stayed cooler until evening, and they built the towers to suit the local winds.

Today, architects in many hot countries are studying these towers again. Air conditioners cool rooms quickly, but they use a great deal of electricity, and they pump their heat out into the street, making the city outside hotter. A few new schools and offices now include tall shafts that work on the same idea as the old towers. They rarely cool a building as much as a machine can. Their designers argue that this is the wrong comparison: the real question is how much cooling a building can get for free before it needs a machine at all.