Table 8. — 9:00 a.m. and 3:00 p.m. wind velocity classes (in miles per hour) by 



number of days per month 

 (Av. 1950-58) 



Wind velocity classes, miles per hour 



Weather 



0- 7 





S 



• 1 2 



1 o 



1 J - 



1 8 



1 o 



24 



1 C 1 



25 + 





station 



9 AM 



3 PM 



9 AM 



3 PM 



9 AM 



3 PM 



9 AM 



3 PM 



9 AM 



3 PM 













May 













Anchorage 



19.2 



7.0 



8.1 



13.6 



3.1 



7.7 



0.6 



1 .6 



0.1 



0.1 



Bethel 



9.5 



4.5 



12.0 



13.2 



7.8 



10.8 



1 .6 



2.3 



.1 



.2 



Fairbanks 



20.3 



13.9 



7.1 



10.0 



3.3 



6.1 



.3 



.9 







.1 



Ft. Yukon 



9.8 



7.8 



1 1 .7 



1 3.4 



7.9 



8.1 



1 .6 



1.3 







.4 



Galena 



13.4 



10.5 



1 1.1 



1 1 .4 



5.8 



7.3 



.7 



1 .6 







.2 



McGrath 



20.0 



1 3.1 



9.2 



12.9 



1.8 



4.6 







.2 







.2 



Northway 



15.4 



11.1 



1 1 .8 



12.3 



3.7 

 J u ne 



7.2 



.1 



.4 











Anchorage 



20.3 



1 1 .6 



8.0 



12.4 



1.6 



4.6 



.1 



1.4 











Bethel 



8.1 



7.2 



14.5 



13.4 



6.6 



8.6 



.7 



.8 



.1 







Fairbanks 



19.7 



13.3 



6.7 



10.5 



3.1 



5.1 



.4 



1.0 



.1 



.1 



Ft. Yukon 



1 2.6 



7.8 



9.2 



13.7 



6.7 



6.3 



1.4 



1.8 



.1 



.4 



Galena 



15.2 



1 1.6 



8.8 



1 0.8 



4.9 



5.7 



.8 



1 .6 



.3 



.3 



McGrath 



20.2 



15.7 



7.2 



9.2 



2.6 



4.8 







.3 











Northway 



15.3 



10.0 



10.2 



13.1 



3.9 

 July 



6.3 



.6 



.7 











Anchorage 



21.9 



15.6 



7.7 



1 1 .0 



1.3 



3.7 



.1 



.7 











Bethel 



12.0 



8.5 



1 1.8 



12.6 



6.2 



8.2 



.9 



1 .6 



.1 



.1 



Fairbanks 



23.3 



15.7 



6.3 



10.5 



1.3 



4.6 



.1 



.2 











Ft. Yukon 



14.8 



9.6 



9.3 



12.2 



5.4 



6.8 



1.4 



2.1 



.1 



.3 



Galena 



18.2 



14.0 



6.6 



9.5 



4.9 



4.8 



.9 



2.2 



.4 



.5 



McGrath 



22.8 



1 6.8 



6.3 



10.8 



1 .9 



3.2 







.2 











Northway 



17.9 



14.8 



9.4 



1 1.2 



3.6 



4.6 



.1 



.4 











canyons. Taku winds, Knik winds, Delta River 

 winds, and Summit winds are well-known ex- 

 amples of this phenomenon. Occurrence of such 

 winds can usually be predicted by alert fore- 

 casters. Table 9 shows the variations between 

 reporting stations on the frequency of changes 

 in wind direction during the month. Of interest 

 is the shifting from month to month of predomi- 

 nant wind direction at the same location. These 

 observations can be valuable in long-range fire 

 control planning. The extremely small number 

 of samples recorded below presents the proba- 

 bility that even though two reporting stations 

 have similar characteristics the intervening area 

 may vary greatly from them. 



SKY CONDITIONS 



Sky conditions have a multiple influence on 

 behavior and control of forest fires. Some gen- 

 eral knowledge of what to expect in various 

 places and at different times of the season is im- 



portant to a fire control officer. Appendix tables 

 29 through 32 summarize in detail the available 

 information on the amount of cloud cover, types 

 of weather (predominant moisture forms), visi- 

 bility distances, and ceiling heights. 



The amount or extent of cloud cover and the 

 prevalent weather type greatly affect fire be- 

 havior and the flammability of fuels. Increased 

 density of clouds and smoke reduces the pene- 

 tration of sun rays, and allows only a portion of 

 their heat concentration to reach the earth's sur- 

 face. It also reduces the radiational heat escap- 

 ing from the earth's surface. The combined ef- 

 fect reduces the diurnal temperature fluctuation. 

 Rapid changes of surface temperature resulting 

 from intermittent shading by clouds may cause 

 troublesome changes in wind direction and ve- 

 locity. On one-half to two-thirds of the days 

 during the fire season, three-fourths of the sky 

 is covered by some type of clouds. This is equal- 



30 



