Scorch height

n. 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).

 

  • Discussion

 

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:

  • W = in-stand wind speed, mi/hr

  • I = fireline intensity, BTU/ft/s

  • T = ambient air temperature, oF

  • Hs = maximum height of lethal scorch, ft

 

Fireline intensity can be calculated from the following equation:

 

 

Where:

  • I = rate of heat release per unit of fire edge, BTU/ft/s

  • IR = reaction intensity, rate of energy release per unit area in the actively flaming front, BTU/ft2/min

  • D = depth of flaming zone, as calculated by D = Rt. Where R equals the rate of spread and t equals the residence time of active flaming (minutes).

 

  • Units

     

    Preferred units for scorch height are m in technical reports and papers. Fire Management in the United States still uses the English units ft. See the following units conversion table for conversion factors.

     

  • See Also

  • References

    • Albini, F. A. 1976. Estimating wildfire behavior and effects. Gen. Tech. Rep. INT-30. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 92 p.

    • Van Wagner, C. E. 1973. Height of crown scorch in forest fires. Canadian Journal of Forest Research. 3: 373-378.

     

  • Notes

    • Author 

      Sharon Hood, Forester

      Rocky Mountain Research Station