The Aleutian low pressure cell is usually 

 weak in early spring,- hence, April has the least 

 precipitation of any month of the year at prac- 

 tically all points over the zone except the Copper 

 River portion. Precipitation increases markedly 

 over the mainland beginning in late June. The 

 low tends to move northward across the Bering 

 Sea and brings a rather persistent southwesterly 

 flow into the Interior. During August cloudy, 

 rainy weather predominates and the interior 

 points of the West Central portion receive meas- 

 urable precipitation on 4 days out of 5. The 

 westward drift of the low becomes pronounced 

 in late November or early December, and pre- 

 cipitation declines rather sharply over most of 

 the Transition Zone. 



The permafrost area varies with summer 

 warmth and winter cold, but it extends south- 

 ward well into the northern portions of this zone. 

 It is present from the northern slopes of the 

 Wrangell Mountains through the Glennallen and 

 Holy Cross areas, along the inland borders of 

 Cook Inlet, Bristol Bay, and West Central por- 

 tions. The amount of continuity is shown in 

 figure 28. 



Over the Copper River and Cook Inlet por- 

 tions, winds are usually light, chiefly because of 

 the sheltering by nearby mountain ridges. Strong, 

 localized winds develop in some areas as the 

 result of downslope drainage. Most frequent ob- 

 servations of these winds have been in the lower 

 Matanuska and Knik River Valleys, mostly dur- 

 ing the winter. These strong winds may persist 

 for days when even slightly reinforced by flow 

 patterns usually associated with low pressure 

 systems centered near Kodiak Island or the Gulf 

 of Alaska. Certain areas of the Bristol Bay and 

 West Central portions are relatively unsheltered 

 and are frequented by strong winds that often 

 extend their effectiveness well into the interior. 



DOMINANT CONTINENTAL ZONE 



Two major factors contribute to the typical 

 continental climate: (1) the area's remoteness 

 from the open sea, and (2) mountain barriers 

 that prevent inland movement of marine air. 



The Interior Basin experiences great sea- 

 sonal temperature extremes.- Maximum tempera- 



tures reach or exceed 90° F. almost every sum- 

 mer. Fort Yukon and Eagle have daily maximum 

 readings averaging 70° to 75° F. during July 

 and August. Prolonged daylight in early June 

 through late July contributes strongly in main- 

 taining high temperatures. The sun remains 

 above the horizon continuously for about 1 

 month at Fort Yukon beginning about June 5. 

 During this season, the average diurnal tempera- 

 ture change is about 30° F. ; however, ranges of 

 only 10 degrees have been recorded. 



The Interior Basin has recorded the highest 

 and lowest readings for all of Alaska. Tempera- 

 tures at Fort Yukon have ranged from a high of 

 100° F. to a low of — 75° F. Combined with its 

 counterpart in Canada's Northwest Territory, the 

 interior Basin records provide a classic example 

 of the northern hemisphere continental climate. 



Terminal dates of the freeze-free season 

 (mid-May to late August) can be depended on 

 as a result of the sharp rise in spring tempera- 

 tures and an equally sharp decline in the fall. 



Permafrost underlies the soil in most of the 

 Interior Basin in spite of the warm summertime 

 temperatures. Ground temperatures remain 

 rather cool except for a shallow surface layer. 

 Gradual thawing of the permafrost during the 

 summer allows ice-cold water to permeate the 

 soil layers immediately above it. The cooling 

 effect, when extended to the soil mantle utilized 

 in vegetal growth, slows seasonal production of 

 vegetation. 



The Interior Basin is almost surrounded by a 

 high ridge of mountains; their sheltering effect 

 is a primary cause for the light precipitation (6 

 to 14 inches) in this area. Most of it falls in June 

 and July, but occasionally some occurs in Aug- 

 ust. Average monthly rainfall during these 

 months totals close to 2 inches — slightly less 

 than averages for the growing season over the 

 central and western parts of the Dakotas. Total 

 summer precipitation may vary widely within 

 relatively short distances chiefly because shower- 

 type precipitation predominates. In local areas 

 thunderstorms may occur on several consecutive 

 days. 



24 



