The northern continental fringes of North America, from the Arctic Circle northward to about the 75th parallel, lie within the outer zone of control of arctic air masses. This produces the tundra climate that Trewartha (1968) designates as Ft. Average temperature of the warmest month lies between 50F (10C) and 32F (0C).
The tundra climate has a very short, cool summer and a long, severe winter. No more than 188 days per year, and sometimes as few as 55, have a mean temperature higher than 32F (0C). Annual precipitation is light, often less than 8 in (200 mm), but because potential evaporation is also very low, the climate is humid.
Vegetation on the tundra consists of grasses, sedges, lichens, and willow shrubs. As one moves south, the vegetation changes into birch-lichen woodland, and then into needleleaf forest. In some places, a distinct tree line separates forest from tundra. Koppen (1931) uses this line, which coincides approximately with the 50F (10C) isotherm of the warmest month, as a boundary between subarctic and tundra climates.
Soil particles in the tundra derive almost entirely from mechanical breakup of rock, with little or no chemical alteration. Inceptisols with weakly differentiated horizons are dominant. Continual freezing and thawing of the soil have disintegrated its particles. Like the northern continental interior, the tundra has a permanently frozen sublayer of soil known as permafrost. The permafrost layer is more than 1,000 ft (300 m) thick throughout most of the region; seasonal thaw reaches only 4-24 in (10-60 cm) below the surface.
Geomorphic processes are distinctive in the tundra, resulting in a variety of curious landforms. Under a protective layer of sod, water in the soil melts in summer to produce a thick mud that sometimes flows downslope to create bulges, terraces, and lobes on hillsides. The freeze and thaw of water in the soil sorts out coarse particles, giving rise to such patterns in the ground as rings, polygons, and stripes made of stones. The coastal plains have numerous lakes of thermokarst origin, formed by melting groundwater.
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