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The northern continental fringes of North America and Eurasia from the Arctic Circle to about the 75th parallel lie within the outer zone of control of arctic-type air masses, whose source region covers the Arctic Ocean and Greenland. To the south lie the continental polar (cP) and maritime polar (mP) air mass source regions. The land fringes are thus in a frontal zone, which has been known as the arctic front, but which may be more simply identified as belonging to a widely shifting polar front, well developed over the northern Pacific and Atlantic oceans. Here many intense east-moving cyclonic storms are developed, and much bad weather may be expected.
These conditions produce a tundra climate, described by Koppen with the symbol ET. This is a polar climate in which the average temperature of the warmest month is below 50°F (10°C), but above 32°F (0°C).
2. Main article
The tundra climate is illustrated by the temperature-precipitation graph of Upernivik at 73° N lat. on the west Greenland coast. Note that
(a) temperature range is large, but not nearly as great as in the continental sub arctic climate;
(b) the. warmest month averages are just over 40°F (4°C) and the coldest month average is well below 0°F (-18°C); and
(c) precipitation total is less than 10in(25cm),with an increased amount falling during and after the warm season. Nearness to the Arctic Ocean explains the somewhat more moderate temperature range and minimum, compared to the continental centers. Coolness of the summer is explained by the nearness to the large ocean body, keeping air temperatures down despite large receipts of solar energy at this altitude near summer solstice. Possibly a more persistent cloud cover is also a factor.
3. References
Evapotranspiration is effectively zero for an eight to nine month period when soil moisture is solidly frozen. However, evapotranspiration rises sharply in summer, exceeding precipitation and giving rise to a moisture deficiency in June, July, and August.
Vegetation of the treeless tundra consists of grasses, sedges and lichens, along with shrubs of willow. Traced southward the vegetation changes into birch-lichen woodland, then into the needleleaf forest. In some places a distinct tree line separates the forest and tundra. Coinciding approximately with the SO' isotherm of the warmest month, this line has been used by Koppen as a boundary between Df and ET climates.
Tundra soils are noteworthy in that the soil particles are produced almost entirely by mechanical breakup of the parent rock and have suffered little or no chemical alteration. Grayish loam and blue-gray clay layers are present with much peat Continual freezing and thawing of soil moisture has been responsible for disintegration of the soil particles. Like the soils of the northern continental interiors, soils of the tundra are affected by the permanently frozen, or permafrost, condition. The permafrost layer is more than 1000 ft (300 m) thick over most of this region; seasonal thaw reaches only 4 to 24 in (10-60 cm) below the surface.
Geomorphic processes have a somewhat distinctive pattern in the tundra regions, and a variety of curious landforms results. Under a protective sod of small plants, the soil water melts in summer, producing a thick mud, which may flow downslope to create bulges and terraces without breaking through the surface. This process is known s solifluction, or sludging, and forms solifluction terraces and lobes on slopes. In the desert tundra, solifluction occurs without the confining cover and may be described as a layer of thick mud creeping down the slopes. Observers who have studies this flowage find that it is most rapid at midday, the mud flowing at a rate of several feet per hour and carrying along large blocks of rock.
The freeze and thaw of water in the soil gives rise to a curious system of polygonal cracks in flat ground. They may result from the shrinkage of the clay as water is withdrawn to form ice crystals in lenses or layers within the soil. The resulting pattern is termed polygonal ground. On hill and mountain summits the cracks are filled with stones, which seem to be gradually sorted and pushed to the sides of each polygon during alternate freeze and thaw of soil water. These forms are called stone rings, or stone polygons. On slopes, the stone rings are drawn downslope to produce stone stripes, appearing from the air like giant hachures on the ground.
