Scorch heightn. The maximum vertical height at which lethal scorching of foliage occurs. Below this height, all needles are brown and dead; above it, live and green (Albini 1976).
Scorch height is the maximum height, measured vertically from the base of the tree to the height in the tree crown at which needles have the survived the fire. This effect can be detected one to two weeks after the fire when below the scorch height all needles are brown and dead and above it they are live and green (Albini 1976). The height above the flaming front at which lethal foliage temperature (60oC for 1 minute) is reached or exceeded is dependent on fireline intensity, ambient air temperature, and wind speed (Van Wagner 1973).
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Van Wagner (1973) developed an equation to predict crown scorch height based on these three factors. The equation predicts declining scorch height with increasing wind speed due to cooling of the hot plume by entrained ambient air, given a fixed fireline intensity value. However, fireline intensity increases with increasing wind speed, which increases the temperature in the convective plume. The wind speed’s influence on fireline intensity is the dominant effect; therefore, the equation predicts scorch height to increase as the wind speed increases. Van Wagner’s equation for predicting maximum height of lethal scorch in English units is (Albini 1976):
Where:
Fireline intensity can be calculated from the following equation:
Where:
