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The distribution of permafrost in the northern hemisphere is well marked. Three zones are recognized. Continuous permafrost, which extends without gaps or interruptions under all topographic features, coincides largely with the tundra climate (ET), but also includes a large part of the continental sub arctic climate (Dfc, Dfd, Dwd) in Siberia. Discontinuous permafrost, which occurs in patches separated by frost-free zones under lakes and rivers, occupies much of the continental sub arctic climate (Dfc) zone of North America and Eurasia. Sporadic occurrence of permafrost in small patches extends into the southern limits of the continental sub arctic climate.

Depth of permafrost reaches 1000 to 1500 ft (300 to 450 m) in the continuous zone near latitude 70°. Much of this permanent frost is an inheritance from more severe conditions of the last ice age, but some permafrost bodies may be growing under existing climate conditions. Permafrost presents problems of great concern in engineering and building construction in these cold regions. Another serious problem is in the behavior of streams in winter. As the surface of streams or springs freezes over, the water beneath bursts out from place to place, freezing into huge accumulations of ice. Highways are thus made impassable. Scraping of insulating peat, forest litter, and vegetative cover from the frozen ground to make roads and airfields may result in dire consequences. The summer sun thaws the bare ground, which turns into a liquid mud, often growing into sizable lake basins by melting and disappearing around the edges of the exposed areas.

Most of the source areas of the continental polar air masses are in regions of less than 20 in (50 cm) of precipitation annually, whereas the northerly portions have less than 10 in (25 cm).

Precipitation is largely cyclonic in type and shows a very definite maximum in the summer months. Snowfall, although conspicuous in winter because it remains upon the ground, accounts for only a fraction of an inch of precipitation per month in the coldest months. Cyclonic storms crossing these areas bring little precipitation at this season. In summer, cyclonic rains are frequent, although thunderstorms are few.

The soil-moisture budget of the continental sub arctic climate resembles the arctic regime. Evapotranspiration is effectively zero over a seven or eight month period when soil moisture is solidly frozen. However, evapotranspiration peaks sharply in summer and exceeds precipitation by substantial amounts in June, July, and August. Thus a summer moisture deficiency may develop.

The sub arctic climate zone coincides with a great belt of needleleaf forest, often referred to as boreal forest, and open lichen woodland, the taiga. Trees tend to be small, so that they are economically of less value for lumber than for pulpwood.

Soils of the podzol group are associated with the arctic needleleaf forest. As explained before, these soils are strongly leached and of acid type. They are light gray and have a very distinct leached layer beneath the uppermost layer of humus and forest litter. These soils are extremely poor from the standpoint of agriculture. Added to the natural inadequacy of soils of this region is a great prevalence of swamps and lakes left by the departed ice sheets. Some rock surfaces were scoured by ice, which stripped off the soil entirely. Elsewhere rock basins were formed, or previous stream courses dammed, making countless lakes. Insufficient geologic time has elapsed for good drainage even to begin to be reestablished over large areas.

Next: Polar, Arctic, and Highland Climates