SEASONAL TRENDS IN FIRE WEATHER 



Perhaps the reason for the greater-than-ex- 

 pected rate of spread of fire in black spruce is 

 due to the fuel itself; perhaps it is due to the 

 method of determining burning index; or per- 

 haps it is due to a weather-length if day com- 

 plex. The reader should note that such weather 

 factors as relative humidity, fuel moisture, and 

 air temperature, do not approach the normally 

 expected critical points reached in such States 

 as Montana and Idaho. 



The percent of total frequency of each meas- 

 ured fire-weather factor during the 1958 season 

 is shown in figure 39 — relative humidity, wind 

 velocity, fuel moisture (both stick and slat), and 

 the resultant burning indexes. An overall pattern 



becomes apparent that all factors, except wind, 

 have an increasing percent of occurrences on the 

 severe side of the line according to this order of 

 stations: Anchorage, Fort Yukon, Priest River Ex- 

 perimental Forest, and Fort Howes. 



Generally speaking, burning indexes are 

 slightly higher in Interior Alaska in May and 

 June than in July and August. The reverse is 

 true for Montana and northern Idaho (table 13 

 and fig. 40). Statistics on fire occurrence and 

 burned area follow the same trend as the burn- 

 ing index data (refer to figs. 54 and 55). As a 

 point of interest, the fire season at the other 

 edge of the United States (Arizona and New 

 Mexico) also reaches its peak by mid-July, then 

 begins tapering off. 



Table 13. — Percent of total frequency of burning index by general classes, 



May-August, 1956-58 



Weather Month Annual 



station 







May 



June 



July 





August 



ppt. 







21- 





21- 





21- 





21- 











40 



40 + 



40 



40 + 



40 



40 + 



40 



40 + 



Inches 



Anchorage 



Stick 



48 



2 



























16.23 





Slat 



60 



7 



62 







27 



2 



42 









Fort Yukon 



Stick 



69 



6 



62 



22 



60 



13 



50 







5.61 





Slat 



39 



61 



43 



49 



64 



23 



75 



1 1 





Priest River 



Stick 



58 



3 



46 



1 



53 



30 



33 



41 



39.45 



(Idaho) 



Slat 











47 



26 



33 



58 



27 



55 





Fort Howes 



Stick 



39 



33 



32 



22 



43 



50 



19 



70 



10.47 



(Montana) 



DIURNAL FLUCTUATION OF FIRE-WEATHER 

 FACTORS 



Analysis of fire case histories (ch. 7) revealed 

 some interesting information about involving the 

 diurnal fluctuation of fire-weather factors. The 

 extreme length of daylight in northern latitudes 

 does not cause relative humidity, temperature, or 

 fuel moisture to approach the danger point, as 

 had been supposed previously. Also, the varia- 

 tion of each factor from the most severe observa- 

 tion (usually at 1600) during a 24-hour period is 

 far less than at locations such as Priest River 

 Experimental Forest (fig. 41). 



Perhaps extended periods of moderate 

 weather produce comparable conditions in terms 

 of fire behavior as a short number of hours of 

 severe weather (see ch. 3). The flat curves of 

 figure 42 indicate that the extended daylight 

 period in Alaska does not give fuels much time 

 in which to cool off and absorb moisture. They 

 also indicate that the spread of fire can con- 

 tinue night and day at a relatively uniform rate; 

 fire-weather factors at night do not become mild 

 enough for a fire to "lay down" as it usually 

 does in southerly latitudes. 



44 



