494 



another case, on the Kaniksu National 

 Forest in northern Idaho, which usu- 

 ally averages about 1 fire a day from 

 man's carelessness, lightning set 207 

 fires on July 12 and 13, 1926. 



Such conditions made foresters real- 

 ize that their goal should be to employ 

 each day all the men they needed to 

 handle all the fires that might occur 

 that day but no more. Even before a 

 start could be made toward the goal, 

 however, some means had to be de- 

 veloped that would indicate the ap- 

 proximate number of fires that would 

 be fought each day and how much work 

 would have to be done to suppress 

 them. Consequently, methods of rating 

 the danger of forest fire were developed 

 to be used to guide fire-control action 

 before fires start. 



Eleven different systems for rating 

 fire danger are now in use in the United 

 States, each one having been adapted 

 for specific conditions of weather and 

 fuel in different parts of the country. 



The systems have five advantages. 

 They are based on scientific measure- 

 ments of the key factors that create fire 

 danger, and they eliminate such indefi- 

 nite opinions and guesses as "high 

 danger," "very bad conditions," "low 

 danger," and "not so bad." 



They focus attention on the really 

 important factors that influence fire 

 danger. 



They point out the small changes in 

 burning conditions that are frequently 

 overlooked but that may mean big sav- 

 ings in men, money, equipment, and 

 area burned. 



They prevent men from getting 

 caught in the off-season let-down or 

 in peak danger conditions something 

 that might happen even to experienced 

 men. 



They provide indexes that can be 

 translated into decisive preparedness 

 and action toward suppression. 



THE DIFFERENT SYSTEMS all recog- 

 nize certain key factors that have to 

 do with the chances of fires starting and 

 the speed with which they will spread. 

 Foremost are wind and fuel moisture. 



Yearbook of Agriculture 1949 



Other factors are condition of vegeta- 

 tion, the relative humidity, days since 

 last rain, amount of last rain, air tem- 

 perature, and time of year. 



Wind velocity, the amount of mois- 

 ture in the forest fuels, and condition 

 of vegetation are the most important 

 variables. 



Strong winds cause some of the most 

 dangerous conditions because they 

 spread fire rapidly and make control 

 difficult. 



Green vegetation retards fire. Dead 

 or dry vegetation spreads fire and in- 

 creases the likelihood that fires will 

 start : The degree of dryness of the for- 

 est fuels, more than any other item, 

 determines whether fires will start and 

 burn at all. But forest fuels are com- 

 plex. They vary from fine material 

 (such as dead grasses and pine needles, 

 which dry rapidly) to the slow-drying 

 fuels, like dead trees. 



Most systems use either direct meas- 

 urements of the moisture content of 

 a representative kind of fine fuel on 

 the surface or measurements of the 

 relative humidity and temperature of 

 the air, to which fuel moisture is closely 

 related. Some systems supplement 

 measurements of fine-fuel moisture 

 with humidity and other measure- 

 ments. Some recognize the dryness of 

 the larger, more slowly drying fuels by 

 including the amount of the last rain, 

 days since last rain, and time of year. 



Time of year also has other impor- 

 tant effects because the length of the 

 dry part of the day and the intensity 

 of the heat from the sun change from 

 month to month. 



The several components of fire dan- 

 ger are measured at stations that are 

 placed in strategic locations. A typical 

 station has various instruments, among 

 them an anemometer for determining 

 the velocity of the wind, a rain gage, 

 and basswood or ponderosa pine sticks, 

 which are carefully weathered and 

 calibrated and placed a few inches 

 above the forest floor, where they re- 

 spond to the same weather which de- 

 termines the wetness or dryness of the 

 natural forest fuels. Sensitive scales are 



