Relative humidity, percent 



Figure 10. --Short-term retardant-- 

 an example of effective holding 

 time. Assume: Windspeed = 5 

 m.p.h. at 1-ft. level, Temp.= 

 90° F., Fuel loading = 14.5 

 tons/acre. 



result from solar radiation or the close proximity of a fire. Equation (7) will, therefore, pro 

 duce the longest holding time that could be expected, and the effective times shown in figure 10 

 would be considerably shortened in direct sunlight or near a fire. 



Burning Test 



Rate of spread, rate of weight loss, and the radiant flux to an overhead radiometer fro 

 each fire were measured. Rate of spread, however, was the only variable used in analyzing the 

 results of the fires. Rate of weight loss and irradiance supported the rate -of- spread data, but 

 tended to be misleading because of the afterburning that often occurred well behind the leading 

 edge of the fire. A "t" test was used to check the significance of changes in environmental con- 

 ditions, amount of retardant, and drying time on all of the data. Complete results of the "t" 

 test are given in tables 8-11 (Appendix). 



Tables 5 and 6 summarize the rate-of-spread data. Each number cited in the table is the 

 average of three or more tests conducted at the stated combination of environmental condition, 

 retardant amount, and degree of dryness. Using this information plus the initial fuel moisture 

 and the water content in the retardant mixture, we calculated the total amount of moisture 

 remaining in the fuel bed at the time of ignition . 



22 



