INTRODUCTION 



The behavior of fire in wildland fuels is influenced strongly by the wind acting 

 on the fire. Predicting fire behavior for fire control planning or preparing burning 

 prescriptions must include this influence. Rothermel (1972) gives a mathematical model 

 for predicting the rate of spread of a surface fire in wildland fuels. This model uses 

 an average windspeed "at midflame height" to account for the influence of wind on the 

 rate of spread. But windspeed is usually measured or forecast at a standard height 20 

 ft (6 m) above the vegetation (Fischer and Hardy 1972) , and varies with height and veg- 

 etation cover. The poorly defined "midflame" windspeed can be approximated by using a 

 spatially averaged value of the windspeed over an appropriate height range. 



Presented here is a means of estimating the windspeed over various wildland fuel 

 types from the "standard" windspeed at 20 ft above the vegetative cover. The report 

 is divided into two major sections. The first deals with the wind above a vegetative 

 cover that is a single-stratum fuel (grass, brush, etc.). The second part deals with 

 wind under a forest canopy. The windspeed at the top of the forest canopy is found by 

 use of a logarithmic wind profile. Windspeed beneath the canopy layer is estimated from 

 the canopy-top value by use of a model based on mechanical force balance. 



This research report deals only with the steady, undisturbed windfield and its in- 

 fluence on fire in surface fuels. No account is taken of the influence of the fire on 

 the windspeed, wind direction, or the profile of windspeed with height. Furthermore, 

 flat terrain and uniform continuous vegetation cover are assumed. 



These restrictions clearly limit the applicability of the results, but even the 

 extent and severity of the restrictions is uncertain. Rothermel 's model implicitly 

 accounts for local (near-flame) influence of the fire on the windfield. At what inten- 

 sity or size the fire's influence on the prevailing windfield becomes significant can 

 only be estimated by order-of-magnitude arguments at present. Future research should 

 help to clarify this situation. 



The restriction to flat terrain can probably be relaxed to "smooth" or "slightly 

 uneven" terrain, but numerical descriptions of adequate "flatness" are not presently 

 available. Similar considerations apply to the uniformity and continuity of vegetation 

 cover required, and these matters are under study in the field of applied meteorology 

 (Bergen 1976). 



With these restrictions in mind, this effort is seen as a small first step in the 

 direction of a more complete description of the complex phenomena with which we are 

 deal ing . 



LOGARITHMIC WIND PROFILE 



The v^;indspeed above a vegetative cover was determined by using the logarithmic wind 

 profile in the following form (Sutton 1953, p. 239) : 



■ ^ ■■(?) 



(1) 



1 



