WINDSPEED AT THE TOP OF THE VEGETATION COVER 



To predict wildland fire behavior, the "standard" windspeed at 20 ft must be 



converted to a value representing the actual motion of the air in the vicinity of the 



fuel and flame structure. As a first step, we related the 20-ft windspeed (.^',q+\\'> 

 to the windspeed at the top of the vegetation (U^) . ~ * 



The relationship between U,, and U^^ ,, is then: 



U20+H ["-"o-TJh" ^ measured in feet. (2) 



Since 



= iUJK) [--^.-^yH"- 1 = 1-02 U^/K UJK (3) 



we can write equation (2) as follows: 



__V - _ "1 (4) 



U^^.M , 720+0. 36H\ 



20.H ^■"'--o-.TB-H-j 



WINDSPEED FOR PREDICTING FIRE SPREAD 

 IN SURFACE FUEL ( NO CANOPY CASE) 



With the profile of windspeed variation defined, we need only choose an appropriate 

 height range over which to average to obtain a relationship between "midflame" windspeed 

 and windspeed at 20 ft above the fuel surface. For this purpose here we assume that the 

 flame extends to some height above the surface vegetation and average the windspeed be- 

 tween the top of the vegetation and this height. 



3 



