annual temperature below 32 °F., and an extreme range of tem- 

 peratures between summer and winter. The Arctic Climate has 

 low precipitation but does not have the wide range of temper- 

 atures, the summers being much cooler than those of the Continen- 

 tal Zone. The Transitional Zone has conditions primarily inter- 

 mediate between Continental and Maritime Zones. 



It should be noted that these broad zones cannot take into 

 account local differences caused by variations in .altitude. Within 

 the Continental Zone, for instance, there are extensive alpine areas 

 with a climate nearer that of the Arctic Zone than the Continental. 



Of all environmental factors, climate has the greatest influence 

 on tree and shrub distribution. Many species are limited to one 

 climatic zone, and most have an optimum development in one 

 zone, although found occasionally in another. Thus, one group of 

 tree and shrub species is confined to the mild, humid coastal areas, 

 whereas another group is limited to interior Alaska with its hot, 

 dry summers. Trees and tall shrubs in the Arctic Zone are restricted 

 to the warmest, most favorable sites. 



Precipitation and mean annual temperatures of the climatic 

 regions are given in the following table from "Major Ecosystems 

 of Alaska" and credited there to Watson (1959). 



CLIMATOLOGICAL DATA FOR ALASKA 



Climatic Zone 



Arctic 

 Continental 

 Transitional 

 Maritime 



inual precipitation 



Annua 



1 temperature 



(inches] 







(°F.) 





Max. Min. 



Avg. 



Max. 



Min. 



Avg. 



21 4 



7 



90 



-61 



10-20 



24 10 



15 



100 



-75 



15-25 



30 12 



17 



90 



-70 



22-35 



300 11 



65 



99 



-42 



35-45 



12. Hydrology. For many purposes it is useful to divide 

 Alaska into regions based on the major drainage basins or water- 

 sheds. This hydrology map is modified slightly from a colored 

 map, scale about 1:12,000,000, on reverse side of "Major Ecosys- 

 tems of Alaska" (Joint Federal-State Land Use Planning Commis- 

 sion 1973). 



Alaska has 6 major watersheds and 18 subregions. These 

 regions, except the Southeast, are based on major drainage pat- 

 terns and include: 



1. Southeast, with many steep, short watersheds plus the lower 

 sections of several large rivers entering the Pacific Ocean from 

 Canada. 



2. South-central, which includes those rivers entering the Gulf 

 of Alaska, of which the Copper River is the largest, the Cook Inlet 

 subregion, dominated by the Susitna River, and the Kodiak- 

 Shelikof subregion, consisting of Kodiak and Afognak Islands 

 and the eastern drainages of the Alaska Peninsula. 



3. Southwest region, consisting of the western end of the 

 Aleutian chain and the two main southwestern Alaska rivers, the 

 Nushagak entering Bristol Bay at Dillingham, and the Kuskokwim, 

 the largest watershed in southwestern Alaska. 



4. The Yukon region, containing 130 million acres, is by far 

 the largest watershed in Alaska. It is divided into five subregions, 

 the Lower, Central, and Upper Yukon, and its two major tribu- 

 taries, the Koyukuk and Tanana Rivers. 



5. The Northwest region is divided into: (a) all of the drainage 

 into Norton Sound and the south section of the Seward Peninsula; 

 and (b) the drainages into Kotzebue Sound, which includes both 

 the Noatak and Kobuk watersheds. 



6. The Arctic region is separated into three subregions: (a) 

 the Colville River watershed, (b) all of the rivers to the east of 

 the Colville River, and (c) all of the drainages to the west and 

 north of the Colville River watershed. 



13. Precipitation. This revised map shows the mean annual 

 precipitation in inches for Alaska. It is reduced from the map pre- 

 pared by the National Weather Service in cooperation with the 

 U.S. Geological Survey, scale 1 :5,000,000, and based on all avail- 

 able data through 1972. This map has been published also in 

 "Alaska's Agricultural Potential" (Alaska Rural Development 

 Council 1974, p. 38), scale about 1:11,500,000. 



Precipitation includes both rain and snow (melted), but mean 

 annual snowfall is presented in General Map 14. The lines (iso- 

 hyets) are drawn through points of approximately equal value but 

 are estimated for many mountainous areas. The map takes into 

 account the increased precipitation in higher elevations and thus 

 differs from precipitation maps compiled only from National 

 Weather Service stations, most of which are at low elevations. 



Mean annual precipitation varies from more than 300 inches 

 in the southeastern mountains to less than 5 inches in the ex- 

 treme northern portions of the Arctic slope. Areas of low precipita- 

 tion also occur in the interior basin between the Alaska Range 

 and the Brooks Range. However, adequate moisture is available for 

 tree growth because of low rates of evaporation and transpiration 

 coupled with an impervious permafrost layer. In spite of this low 

 rainfall, there are large numbers of lakes and bogs and the rivers 

 have a significant annual runoff. 



In Arctic Alaska, where precipitation is low, soil moisture is 

 abundant, except in porous rocky soils, because of the nearly con- 

 tinuous permafrost layer close to the surface. 



In coastal Alaska the mountains intercept moisture-laden winds 

 blowing landward from the Pacific Ocean, causing very high 

 precipitation rates. In the high altitudes much of this falls as snow, 

 accounting in part for the extensive glaciers and ice fields in this 

 region. At lower elevations the high rainfall and cool summers 

 provide near optimum conditions for tree growth. 



14. Mean annual snowfall. The revised map of mean annual 

 snowfall is reduced from that prepared by the National Weather 

 Service in cooperation with the U.S. Geological Survey scale 

 1:5,000,000, and based on all data available through 1972. This 

 map has been published also in "Alaska's Agricultural Potential" 

 (Alaska Rural Development Council 1974, p. 39), scale about 

 1:11,500,000. It takes into account the increased snowfall at 

 higher elevations. 



Snowfall in Alaska is extremely high in the southern coastal 

 mountains, up to 800 inches per year, but in general drops off to 

 the north and in certain areas of the interior basin to less than 40 

 inches. South of the Alaska Range, the snow is often melted by 

 winter warm periods, but north of the range the snow usually 

 persists from the time of the first snowfall in September or October 

 until spring melt in April, May, or June. 



15. Mean annual temperature (°F. ). This map of mean 

 annual temperature (°F.) in Alaska by Harold W. Searby is from 

 "Environmental Atlas of Alaska" (Johnson and Hartman 1971, 

 plate 35), scale approximately 1:10,000,000. The data were ob- 

 tained primarily from low-lying weather stations and therefore do 

 not record the lower mean annual temperatures that occur in all 

 mountain ranges. 



Mean annual temperatures decrease generally from south to 

 north in Alaska. Most parts of the State, except the southeast, 



