Gusts and Wind Steadiness Set Variability a Spread Forecast Cannot Resolve
A reported wind is an average, and fires respond to the extremes inside it. A gust is the peak of a short burst, conventionally a three-second maximum, and the gust factor is that peak divided by the sustained mean, typically \( 1.3 \)–\( 1.8 \) over rough ground and higher near terrain or a convective cell. Direction is variable too: a steady synoptic wind may vary by \( 10^{\circ} \), while a light terrain-driven wind can swing through \( 90^{\circ} \) within minutes. Because fire spread responds strongly and non-linearly to wind, the mean wind gives the mean spread but the gusts give the runs, the spot fires and the sudden flank flare-ups. The confusion this resolves is reading a spread forecast built on average wind as a description of the next hour rather than as an average over it. After this Concept you can state why an average wind speed under-describes the range of fire behaviour that wind will actually produce.
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A reported wind is an average, and fires respond to the extremes inside it. A gust is the peak of a short burst, conventionally a three-second maximum, and the gust factor is that peak divided by the sustained mean, typically \( 1.3 \)–\( 1.8 \) over rough ground and higher near terrain or a convective cell. Direction is variable too: a steady synoptic wind may vary by \( 10^{\circ} \), while a light terrain-driven wind can swing through \( 90^{\circ} \) within minutes. Because fire spread responds strongly and non-linearly to wind, the mean wind gives the mean spread but the gusts give the runs, the spot fires and the sudden flank flare-ups. The confusion this resolves is reading a spread forecast built on average wind as a description of the next hour rather than as an average over it. After this Concept you can state why an average wind speed under-describes the range of fire behaviour that wind will actually produce.
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