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[Science] . |
Operational Awareness for Advanced Firefighters & Fire Behaviour Analysts | ||||||
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As wind speeds increase, the headfire leans downwind more strongly. When winds are blowing upslope this can bring the headfire close to the surface fuel layer. It is possible for the flames to then attach to the surface, as it is easier for convection to push upslope then vertically. Typically this requires a slope of at least 30Ý, and strong winds. This attachment results in extensive sheets of flame. In a forest this may stay underneath the canopy, as may its smoke. Further, the headfire rate of spread may continue to accelerate, ultimately exceeding the egress speed of any crews on foot. The speed of onset is such that even pre-arranged egress routes have been jeopardied. These outbreaks may start from a minor breakaway, and typically reach the ridgetop in a matter of minutes. The deep flaming makes survival on foot in a burn-over difficult. There will typically be no time for a watchout to be issued. A canyon typically consists of a steep floor and precipitous sidewalls. The geometry of this is significant, as it constricts the attached flame, assisting the eruptive acceleration. . |