The source region for the continental polar air masses is south of the tundra zone between lat. 50 and 70 N. The climate type here shows very great seasonal range in temperature; winters are severe, and the region's small amounts of annual precipitation are concentrated in the 3 warm months. This cold, snowy forest climate, referred to in this volume as the boreal subarctic type, is classified as E in the Koppen-Trewartha system. This climate is moist all year, with cool, short summers (see Appendix 2, climate diagram for Fort Yukon, Alaska). Only 1 month of the year has an average temperature above 50F (10C).
Winter is the dominant season of the boreal subarctic climate. Because average monthly temperatures are subfreezing for 6 to 7 consecutive months, all moisture in the soil and subsoil freezes solidly to depths of many feet. Summer warmth is insufficient to thaw more than a few surface feet, so permafrost prevails under large areas. Seasonal thaw penetrates from 2 to 14 ft (0.6 to 4 m), depending on latitude, aspect, and kind of ground. Despite low temperatures and long winters, the valleys of interior Alaska were not glaciated during the Pleistocene, probably because of insufficient precipitation.
The subarctic climate zone coincides with a great belt of needleleaf forest, often referred to as boreal forest, and with the open lichen woodland known as tayga. Most trees are small, with less value as lumber than as pulpwood.
The arctic needleleaf forest grows on Inceptisols with pockets of wet, organic Histosols. These light gray soils are wet, strongly leached, and acid; they form a very distinct layer beneath a topsoil layer of humus and forest litter. Agricultural potential is poor, due to the natural infertility of soils and the prevalence of swamps and lakes left by departing ice sheets. In some places, ice scoured rock surfaces bare, entirely stripping off the overburden. Elsewhere rock basins were formed and stream courses dammed, creating countless lakes.