banks-Fort Yukon area, causing fuels to become 

 progressively drier. Humidities are lower in May 

 and June than in July and August. The length 

 of day increases with latitude; Fort Yukon has 

 nearly a month of continuous daylight. As ex- 

 pected, winds in the afternoon are stronger than 

 those in the morning; winds in May are stronger 

 than those in July. Proximity to glaciers lying in 

 long, deep canyons tends to increase the force 

 and irregularity of windspeed and direction. 

 Cloud ceilings are generally above 1,000 feet 

 during May to early July, but become lower 

 more frequently during the rest of the summer. 

 Smoke and haze become increasingly detriment- 

 al to firefighting activities after the end of June. 

 Permafrost is more than 1,000 feet thick in the 

 extreme north of the State but is nonexistent in 

 the southern portion; the thick layer of mosses 

 and lichens insulates the soil and retards its 

 thawing; roots remain cold late into the spring 

 and tend to delay the start of vegetative growth; 

 the resultant late dormancy may cause fuels to 

 remain dry much later into the early summer 

 than one might normally expect. 



Since Alaska has no fuel type classification 

 system, fire behavior is described according to 

 its relative violence in various general cover 

 types. As in continental United States, fire be- 

 comes more active when it travels through finely 

 divided fuels. Mosses, lichens, and spruce 

 branches extending to the ground provide a 

 nearly unlimited path of fine fuels through which 

 fires may advance. Each of several major cover 

 types presents various fire behavior possibilities. 

 Birch, aspen, and cottonwood stands do not 

 normally carry fire rapidly. Increase in the 

 spread rate of fire is closely associated with in- 

 creased ratio of spruce to hardwoods. Rate of 

 spread is most rapid in black spruce because of 

 the combined horizontal and vertical continuity 

 of fuel in this cover type. Light burns do not 

 often cause severe type retrogression, but severe 

 single burns or repeated mild burns do. An 

 empirical table groups expected rates of fire 

 spread in major cover types into fire classes. 



Fire-danger rating was not used in Interior 

 Alaska prior to 1956, nor were there any data 

 available from which to develop a suitable sys- 

 tem. The need for a guide to help fire control 

 officers do a more competent fire management 

 job led to establishment of the Intermountain 



System in Interior Alaska. Use of this system 

 has accomplished two objectives: (1) to serve as 

 a fire management guide, and (2) to obtain re- 

 search data to be used in improving fire-danger 

 rating techniques and in making local modifica- 

 tions to a national uniform system. Fire-weather 

 factors are not as severe as those in continental 

 United States, but rates of spread in Interior 

 Alaska may approach those known to occur in 

 many of the more southerly States. The diurnal 

 fluctuation of fire-danger rating factors is less in 

 Interior Alaska than in northern Idaho; this indi- 

 cates that perhaps extended periods of moder- 

 ately severe weather produce the same condi- 

 tions in terms of fire behavior as a short number 

 of hours of very severe weather. Establishing 

 fire-weather stations and using information from 

 them was a long step forward, but the 14 sta- 

 tions in operation by 1960 were still grossly in- 

 adequate for intensive fire control management 

 purposes. Measurements from these stations in- 

 dicate that burning indexes are highest in May 

 and June; in Montana and Idaho they are high- 

 est in July and August. These burning indexes, 

 along with climatological information, show why 

 the greatest fire load is in May and June. 



Forest fires have burned in Interior Alaska 

 from time immemorial. Until recent years, nearly 

 all fires in the State were thought to have been 

 man caused. Analysis of all fires reported dur- 

 ing the 9-year period 1950-58 revealed much 

 valuable information. Individual fire reports 

 show that lightning causes about one-fourth of 

 all fires and that these lightning fires account 

 for three-fourths of the acreage burned. Sixty- 

 two percent of all public domain land protected 

 by the Bureau of Land Management is in Alaska, 

 or 27 percent of all land under organized protec- 

 tion in the entire United States. Reports show 

 that on the average 253 fires burn 1.1 million 

 acres annually, with an average area per fire of 

 4,400 acres. This is compared to 99,848 fires 

 burning 3 million acres, or 30 acres burned per 

 fire, on all other land under protection in the 

 rest of the United States. The number of reported 

 fires per million acres protected is 1.1 in Interior 

 Alaska compared to 168 in the rest of the United 

 States. 



Records indicate that if a fire in Interior 

 Alaska is not controlled while its area is less 

 than 300 acres, it may and often does spread 



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