Table 5 



■ — 3:00 p.m. temperature classes (degrees F.) by number of days 



per month 

 (Av. 1950-58) 



Weather Month 



station 





June 







July 







30-49 



50-69 



70-89 



30-49 



50-69 



70-89 



Anchorage 



1.0 



25.7 



3.3 







24.7 



6.3 



Fairbanks 







Vl 



13.9 







15.0 



16.0 



PERMAFROST 



Permafrost consists of organic and soil ma- 

 terial that remains frozen year round. Regional 

 climatic differences result in variation of perma- 

 frost thickness from more than 1 ,000 feet in 

 northern Alaska to permafrost-free terrain in 

 southen Alaska (fig. 28). Precipitation (through 

 ground water), temperature, and insulation ma- 

 terial affect the presence and depth of perma- 

 frost. Permafrost, in return, somewhat influences 

 local temperature and considerably influences 

 the supply of usable ground water. 



Because of their active water movement, 

 streams generally are underlain by deeper and 

 wider unfrozen areas than are lakes,- coarse, 

 permeable sand or gravel is more likely to be 

 free of permafrost than is impermeable silt. 

 Abundant unfrozen zones at shallow depth can 

 be expected in mountainous areas, especially on 

 south slopes. The most favorable sites for for- 

 mation or preservation of permafrost in moun- 

 tain areas are on north slopes and beneath poor- 

 ly drained surfaces on broad interfluves and 

 valley bottoms (Hopkins, et al. 1955). Table 33 

 shows the time of season by which the ground 

 is thawed to various depths. 



Permafrost affects vegetation in several 

 ways that bear on fire behavior and conse- 

 quences. The cold soil above the permafrost 

 layer inhibits growth and delays the "greening- 

 up" of plants in the spring to the extent that 

 much dry material is available for burning early 

 in the fire season. Roots tend to grow laterally 

 and above the frozen layer. When fire passes 

 through a stand of timber and consumes the 

 organic mantle, tree roots have nothing left to 

 cling to,- thereafter, even light winds can blow 

 down large areas of trees that otherwise would 

 have survived the fire. 



The presence of permafrost often misleads 

 firefighters. Frozen organic matter thaws and 

 dries out when a fireline trench exposes it to 

 open air ; this permits a smoldering fire to escape 

 across the once safe zone. 



RELATIVE HUMIDITY 



Air moisture is generally thought of in terms 

 of relative humidity. In Interior Alaska, humidi- 

 ties in May and June are lower than in July, 

 and considerably lower than in August (tables 

 6 and 25). This situation is the reverse of what 

 is usual in most of the western United States. 



Air moisture affects burning conditions 

 mainly by varying the fuel moisture content. 

 Most fine fuels are sensitive to changes in air 

 moisture and follow the humidity pattern rather 

 closely. In heavier fuels, moisture content 

 changes more slowly since a much smaller per- 

 centage of the total volume is exposed for rapid 

 transfer of moisture. 



LENGTH OF DAYLIGHT 



Both air and fuels receive heat by solar 

 radiation. The prolonged hours of daylight and 

 sunshine contribute to maintaining fairly high 

 temperatures. Lengthening or shortening of day- 

 light at a given latitude follows the change in 

 the meridian angle of the sun. Surface tempera- 

 tures are higher in the summer than in the winter 

 not only because the sun shines longer, but be- 

 cause it shines more directly, and therefore, more 

 intensely on the earth's surface. This potential 

 worsening of fire-weather conditions is some- 

 what balanced by the fact that the amount of 

 radiant energy received on any surface area de- 

 creases as we move from tropical to northern 

 latitudes because of the lowering cngle of inci- 

 dence of solar radiation. 



28 



