Chimneys, Saddles and Narrow Canyons Channel Wind and Accelerate Fire
Terrain shapes the wind the fire actually feels. Air forced through a constriction speeds up, so saddles and gaps between peaks concentrate the flow; narrow canyons steer it along their axis regardless of the regional wind direction; and steep, narrow drainages known as chimneys combine confinement with slope so that a fire in the bottom preheats fuel up both walls and runs vertically at extreme speed. Wind-terrain alignment is the case to watch: when the regional wind runs up the same drainage the slope favours, the two effects multiply, and this alignment marks the terrain most likely to produce a rapid run. Local circulations add a daily rhythm, with upslope and up-valley winds by day and downslope drainage winds at night. The confusion this resolves is reading a regional wind forecast as the wind in a canyon; below ridge height, terrain frequently wins. After this Concept you can pick out the terrain features on a map where a fire is most likely to accelerate.
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Terrain shapes the wind the fire actually feels. Air forced through a constriction speeds up, so saddles and gaps between peaks concentrate the flow; narrow canyons steer it along their axis regardless of the regional wind direction; and steep, narrow drainages known as chimneys combine confinement with slope so that a fire in the bottom preheats fuel up both walls and runs vertically at extreme speed. Wind-terrain alignment is the case to watch: when the regional wind runs up the same drainage the slope favours, the two effects multiply, and this alignment marks the terrain most likely to produce a rapid run. Local circulations add a daily rhythm, with upslope and up-valley winds by day and downslope drainage winds at night. The confusion this resolves is reading a regional wind forecast as the wind in a canyon; below ridge height, terrain frequently wins. After this Concept you can pick out the terrain features on a map where a fire is most likely to accelerate.
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