Fire Behaviour Needs Midflame Wind, Not the Wind at Standard Measurement Height
Wind speed is meaningless without a height, because wind slows sharply towards the ground. Fire weather observations are standardised at \( 6.1 \) m (20 feet) above the vegetation in the United States and \( 10 \) m in most of the rest of the world, averaged over ten minutes. A surface fire, however, feels the wind at the height of its own flames — often under a metre, and often under a forest canopy. Practitioners therefore multiply the standard wind by a wind adjustment factor to obtain midflame wind: roughly \( 0.4 \)–\( 0.6 \) in the open, and as low as \( 0.1 \)–\( 0.3 \) beneath a closed canopy. The confusion this resolves is feeding a forecast wind straight into a spread calculation, which can overstate spread rate by several times in timber and is one of the commonest amateur errors. After this Concept you can convert a forecast wind into the wind a flame front actually experiences, and say what the conversion depends on.
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Wind speed is meaningless without a height, because wind slows sharply towards the ground. Fire weather observations are standardised at \( 6.1 \) m (20 feet) above the vegetation in the United States and \( 10 \) m in most of the rest of the world, averaged over ten minutes. A surface fire, however, feels the wind at the height of its own flames — often under a metre, and often under a forest canopy. Practitioners therefore multiply the standard wind by a wind adjustment factor to obtain midflame wind: roughly \( 0.4 \)–\( 0.6 \) in the open, and as low as \( 0.1 \)–\( 0.3 \) beneath a closed canopy. The confusion this resolves is feeding a forecast wind straight into a spread calculation, which can overstate spread rate by several times in timber and is one of the commonest amateur errors. After this Concept you can convert a forecast wind into the wind a flame front actually experiences, and say what the conversion depends on.
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