Cook Inlet area, Alaska Peninsula, and Aleutian Islands, have a 

 mean annual temperature below freezing (32°F.). Of course, 

 seasonal temperature variation is significant. Plant distribution 

 may be related more closely to summer temperatures than to the 

 mean annual. 



16. Mean July maximum temperature (°F. ). This map 

 of mean July maximum temperature (°F.) in Alaska is from 

 Searby (1968, p. 20), scale about 1:15,000,000. It was published 

 also in "Environmental Atlas of Alaska" (Johnson and Hartman 

 1971, plate 34), scale about 1:10,000,000. As with the other tem- 

 perature maps, data were taken from weather stations primarily 

 at low elevations and do not show the temperatures in the moun- 

 tain ranges. 



Since July is the warmest month in most areas of Alaska, the 

 mean July maximum lines (isotherms) provide a general picture 

 of summer temperatures in Alaska. It can be seen that the warmest 

 summer temperatures are in the interior basin and especially near 

 the Canadian border. Cooler temperatures are found in all coastal 

 areas. The cooler summer temperatures along the west coast of 

 Alaska and through the Aleutian Islands account, in part, for the 

 southward extension of tundra and the corresponding absence 

 of trees. 



17. Mean January minimum temperature (°F. ). This 

 map of mean January minimum temperature (°F.) in Alaska is 

 from Searby (1968, p. 19), scale about 1:15,000,000. It was 

 published also in "Environmental Atlas of Alaska" (Johnson and 

 Hartman 1971, plate 31), scale 1:10,000,000. Since January is 

 the coldest month in most of Alaska, the mean January minimum 

 lines (isotherms) give a fairly representative picture of winter 

 temperatures in Alaska. 



It can be seen that the coldest winter temperatures occur in the 

 Yukon and Tanana Valleys adjacent to the Canadian border. These 

 areas also have the warmest July temperatures, indicating the 

 continental nature of the climate there. 



In contiguous United States and other north temperate areas, 

 January temperatures are often used to delineate hardiness zones 

 for perennial plants. However, in Alaska, because these areas of 

 intense winter cold are also areas of maximum summer tempera- 

 tures, this concept does not seem entirely valid. Whereas the cold 

 winter temperatures may limit the distribution of some of the 

 southern coastal plants into the interior, the cool summer tem- 

 peratures in the western part of Alaska and the Aleutian Islands 

 may be of equal or more significance in limiting the natural range 

 or the introduction of some plants. 



18. Thawing index. The thawing index map was published 

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

 plate 42), scale 1:10,000,000, and was developed from the U.S. 

 Weather Bureau (1965). Thawing index is the total number of 

 degree-days above freezing during the year. It is an indication of 

 both summer temperatures and duration. The thawing index varies 

 from 5,000 degree-days in southeastern Alaska to only 500 degree* 

 days in the extreme north. An area with a thawing index of less 

 than the freezing index (General Map 19) is likely to have some 

 permafrost. 



Much of the State has a thawing index of 2.500 degree-days 

 or more and supports forest vegetation except at high altitudes. 

 The totals indicate that the higher summer temperatures in the 

 interior compensate for the cooler but longer thawing seasons 

 along the southern and southeastern coasts. 



19. Freezing index. The freezing index map was published 



in "Environmental Atlas for Alaska" (Johnson and Hartman 1971. 

 plate 45), scale 1:10,000,000, and was developed from the I 5 

 Weather Bureau (1965). Freezing index is the total number of 

 degree-days below freezing during the year. In Alaska the l 

 ing index varies from zero in the extreme southeast to 8,500 

 degree-days along the Arctic coast. The freezing index can be 

 to obtain an estimate of the depth of ground freezing during the 

 winter, but the presence or absence of permafrost is related to both 

 this and the thawing index (General Map 18). 



20. Plant hardiness zones. A plant hardiness zone map for 

 Alaska in color, scale about 1:13,000,000, has been compiled by 

 Alan C. Epps (1973), of the University of Alaska. It is based on 

 the average annual minimum temperature (i.e., the average of the 

 lowest or extreme minimum temperature in each year) and also 

 on the distribution of tundra vegetation. 



On this map, reproduced here, the treeless areas where plant 

 growth is severely restricted owing to extreme climatic conditions 

 are not classified further. The remainder of Alaska is divided into 

 six zones, based on increments of 10 °F. for the average annual 

 winter minimum temperature, as follows: 



Zone 6, the mildest climate, limited to low elevation areas in 

 southeastern Alaska, where the yearly minimum temperature 

 averages between — 10°F. and 0°F. 



Zone 5, some interior parts of southeastern Alaska, borders of 

 Prince William Sound, parts of Afognak and Kodiak Islands, and 

 a few places in the Alaska Peninsula, where the yearly minimum 

 temperature averages — 10°F. to — 20°F. 



Zone 4, small interior parts of southeastern Alaska, small areas 

 along the coast, parts of the Kenai Peninsula mostly near Homer, 

 sections of the Alaska Peninsula, and the northern portions of 

 Afognak and Kodiak Islands, where the annual minimum tem- 

 perature averages — 30° F. to -20° F. 



Zone 3, primarily the northern Cook Inlet region, the Chitna 

 Valley, and areas around Lake Clark and Dillingham, where the 

 annual minimum temperature averages between — 10 C F. and —3 



Zones 1 and 2, the remainder of Alaska, from the Brooks Range 

 south through most of the interior below the limit of trees, where 

 annual minimum temperatures range from — 70°F. to — 10 F. Pre- 

 sumably this zone could be further subdivided into two or three 

 subzones on the basis of the average annual minimum tem- 

 peratures. 



In the lower 48 States, hardiness /ones based upon minimum 

 winter temperatures indicate the range limits of man) native trees 

 and shrubs and the survival of introduced species. However, in 

 Alaska, other factors, such as low or high summer temperatures, 

 seem equallv significant, as noted under mean minimum tem- 

 perature (General Map 17) and as suggested bj the thawing index 

 (General Map 18). 



21. Fire season elimatic zones. The map of the "Fire 

 Season Climatic Zones" of mainland \la>ka is from that by Triig 

 (1971), of the U.S. Foresl Service, published at a scale of ap- 

 proximately L:12,000,000. It is based on C. W. Thornthwaite's 



Precipitation Effectiveness Index and Temperature Effectiveness 

 Index, both of which integrate precipitation and temperature data 

 from standard weather stations. The main purpose was to designate 

 maximum and minimum fire danger areas in mainland Maska 

 during the wildfire season of \pril through September. This map 

 is also useful in designating areas with similar climates during 

 the growing season. 



