This curve matches the line from equation 1 from 2 to 12 

 mi/h (3.2 to 19.3 km/h) windspeeds. 



Because it is generally difficult to identify the fire starting 

 point and establish the centerline of the fire shape, it is recom- 

 mended that the total fire length and width be considered for 

 field applications. This also conforms to the general use of the 

 simple ellipse as is currently being done. However, it must be 

 remembered that the distance, d, is from the focus opposite the 

 head of the fire and not the major axis of a simple ellipse (see 

 fig. 1). 



The application of the results of this analysis must consider 

 the average wind on the flame. This may require calculating 

 that value from wind measurements made at some other height. 

 The variability of the wind along with the array of fuels and 

 topography that a fire may encounter are probably the most 



LENGTH/ WIDTH RATIO 

 Figure 7.— Wisconsin's Fire Suppression Hand- 

 book examples of fire size and shape with and 

 without good control action. 



significant factors in establishing a fire's size and shape. How 

 well the double ellipse or simple ellipse matches field observa- 

 tions will depend on how wisely an observer selects a windspeed 

 and a fuel bed description so reasonable inputs are made to the 

 mathematical models. 



Initial tests of the model and the assumptions used have been 

 made with historical data and are tabulated in appendix II. 

 These are checks for the reasonableness of the model and pro- 

 vide indications of how wind, fuels, and topography must be 

 considered as validation opportunities become available. The 

 documented values from field observations in appendix II are 

 compared to computed values from the double-ellipse 

 mathematical model and the quadratic equation approach to 

 the single ellipse model. 



Interpretation of the combinations of fire size and shape 

 parameters must be given careful consideration if historic fires 

 are to be used as data for developing aids to fire management. 

 An example is the Wisconsin Fire Suppression Handbook,'' 

 which contains examples of fires with no control and good con- 

 trol. In figure 7 the data from appendix II of this report are 

 plotted to show the differences. Fire "A" was not controlled 

 and shows that for given weather and fuel conditions the //w 

 ratio remains constant while the area and perimeter continue to 

 increase. Fire "B" shows the resuhs of good flanking fire con- 

 trol and pinching off attack on the fire front. The area and 

 perimeter are increasing at a decreasing rate while the i/w ratio 

 increases rapidly. When utilizing historic fires it must be 

 recognized that either good fire control or a sudden increase in 

 wdndspeed or a change in wind direction can change //w ratios. 



Comparison to Field Observations of WOdfires 



The following five fires had enough documentation to 

 show how fire shape and size relationships change with 

 time. The time histories of fire shape and size, graphically 

 presented in figure 8, show that Hornby's (1936) approach is 

 reasonable. Time since fire start is in the direction the arrows 

 point. When the wind is stable and fuels are constant, the fires 

 tend to orient vertically in figure 8. 



Close agreement can be achieved between the field observa- 

 tions and the mathematical computation when the wind profile 

 is considered. Albini and Baughman (1979) present a procedure 

 that is used to depict the average wind on the flame by reduc- 

 ing the windspeed at 20 ft (6 m) above the vegetation cover to 

 what would be present at the location of the flame front. 

 Winds may be reduced to the value at the vegetation upper sur- 

 face or to a point within the cover depending on whether a 

 crown fire or a surface fire occurred. 



Following are brief summaries of each example. These com- 

 parisons are valid only to the extent that effective suppression 

 action or major fuel changes have not taken place, restricting 

 fire growth. 



Excerpts provided by Jim Miller, Fire Staff Specialist, Rhinelander, Wis. 



8 



