A FOREST firefighter has three main enemies: low humidity, high temperatures and strong winds. The most dangerous of these is wind. A blaze in the aspen forest north-east of Tucson, Arizona, which burnt up 350 square kilometres (including 333 houses) in less than 5 weeks before it was contained last month, was driven chiefly by the local Santa Ana winds.
The Santa Ana is typical of a particular type of wind that begins over the sea and blows onto coastal mountain ranges before plunging into valleys on the other side. The best-known example is the chinook, which blows across the Rocky Mountains.
Do chinook-type winds feel any different? Yes, spectacularly so. They are famous for causing dramatic changes in temperature. As the wind is forced up the coastal face of the mountain range, it cools and loses its moisture. Once over the top, it tumbles at up to 160 kilometres per hour into the leeward valleys, where the resulting compression causes it to warm by 1 掳C for every 100 metres it falls.
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How dramatic are the temperature swings? The fastest recorded was in the Black Hills of South Dakota on 22 January 1943, when the air went from 鈭�20 掳C to +8 掳C within 2 minutes of a chinook striking. This can have startling effects: chinooks have caused car windscreens to break. They are well known for vaporising thick snow 鈥� the name comes from a Native American word meaning 鈥渟now-eater鈥�.
Chinook-type winds occur in many of the world鈥檚 mountain ranges and have a host of local names: the f枚hn in the Alps, the Mono in northern California. So if you see the telltale arch of wispy clouds hugging the peaks of coastal mountains, take cover 鈥� or get out your shorts and T-shirt.