Table 6. — 3:00 p.m. relative humidity classes (in percent) by number of days 



per month 

 (Av. 1950-58) 



Weather 











Month 











station 





A A 



May 







June 







July 









30- 







on 

 JU- 







JU- 







<-30 



49 



50 + 



<30 



49 



50 + 



<30 



49 



50 + 



Anchoraae 



1.1 



16.9 



13.0 



0.6 



10.0 



19.4 



0.2 



6.2 



24.6 



Bethel 



.3 



6.6 



24.1 



.4 



6.0 



23.6 







4.4 



26.6 



Fairbanks 



6.7 



17.6 



6.6 



5.1 



16.0 



8.9 



3.5 



12.3 



15.2 



Fort Yukon 



1.3 



16.9 



12.8 



1.0 



18.7 



10.3 



.7 



15.6 



14.7 



Galena 



3.3 



13.9 



13.8 



3.2 



12.9 



13.8 



1.2 



12.3 



17.5 



McGrath 



2.6 



16.9 



1 1.5 



2.9 



13.8 



13.3 



1.0 



11.1 



18.9 



Northway 



5.2 



15.0 



10.8 



5.2 



15.9 



1 1.7 



2.2 



15.1 



13.7 



Table 7 compares the number of hours of 

 daylight for stations at three latitudes: Fort 

 Yukon (lot. 66°35'N.), Anchorage (lat. 61 °10'N.), 

 and Missoula, Montana (lat. 46°55'N.). 



Table 7. — Duration of daylight 



Location 



Date Fort Yukon Anchorage Missoula 





Hrs. 



Min. 



Hrs. 



Min. 



Hrs. 



Min. 



May 1 



17 



30 



16 



1 1 



14 



25 



1 1 



18 



52 



17 



06 



14 



53 



21 



20 



22 



17 



57 



15 



18 



June 1 



22 



19 



18 



43 



15 



38 



1 1 



24 



00 



19 



13 



15 



50 



21 



24 



00 



19 



25 



15 



53 



July 1 



24 



00 



19 



15 



15 



51 



1 1 



22 



18 



18 



47 



15 



38 



21 



20 



31 



18 



06 



15 



19 



The length of day or duration of possible 

 sunshine is much greater at higher latitudes — a 

 maximum of 5 hours greater at Fort Yukon than 

 at M issoula, Montana. Missoula, however, re- 

 ceives more intense heating because the sun's 

 rays are more nearly perpendicular to the earth's 

 surface when the sun is at its zenith. This in turn 

 often dries out fuels more than does the longer 

 period of lower maximum temperatures farther 

 north. 



WIND 



Wind influences the behavior of a fire. 

 High windspeed may cause a fire to jump bar- 

 riers and travel in the crowns of trees, or to spot 

 ahead of the main fire front. Wind combined 

 with topography can cause erratic and violent 

 fire behavior. 



As should be expected, afternoon winds 

 usually are stronger than morning winds. Weath- 

 er records indicate that Bethel is windier than 

 most places, as the to 7 miles-per-hour speed 

 appears on very few days, but the 8 to 1 2 and 

 1 3 to 18 miles-per-hour range is high for morn- 

 ing readings and at least average for afternoon 

 readings. Fort Yukon follows the same general 

 trend. In May, many stations record the 13 to 18 

 miles-per-hour range on more days than in June 

 or July (table 8); this indicates that winds in- 

 fluence fire behavior more in May than in other 

 months. 



Many factors influence the direction of air- 

 flow at any specific place. Geographic location 

 determines whether maritime or continental air- 

 flow affects a given area. Topography can cur- 

 tail, accentuate, or change the surface direction 

 of a prevailing wind. Winds of unusually high 

 velocity that blow out of mountain canyons are 

 generally associated with glaciers lying in these 



29 



