Fig. 6. Permafrost distribution from north to south: 1 - continuous permafrost;

2 - discontinuous permafrost; 3 - permafrost islands; 4 - area of previously frozen ground; 5 - southern limit of occurrence of permafrost.

spatial distribution of permafrost. A line termed the southern limit of permafrost distribution is used on maps to outline the southern border of the region where permafrost may occur today. The orientation and spherical shape of the Earth's surface results in the minimal amount of radiation arriving at the surface near the poles while the maximum is observed in the equatorial zone. This situation causes the formation of latitudinal zonation in distribution of the equilibrium temperature values of the surface layers of the lithosphere. Consequently the lowest negative surface temperatures (during the periods of permafrost existence) will be noted in high latitudes while the highest ones will be noted in low latitudes. In this connection usually wide and universal distribution of permafrost is observed in subpolar regions, a permafrost island distribution in middle latitudes and complete absence of permafrost in subequatorial regions of the Earth (see Fig. 5). The thickness of the permafrost must decrease with latitude (when moving from poles to equator) in accordance with change of negative temperature values. Frozen formations are represented not only by perennially frozen ground but also by perennial ice covers on the waters of northern seas as well as by ice caps and mountain glaciers on the Earth's continental areas (see Fig. 5).

As with the latitudinal zonation of permafrost (negative) temperatures and the change of permafrost thickness, the altitudinal zonation, associated with the fact that the negative temperature of the surface of the lithosphere decreases with increase of altitude, is clearly followed. Therefore instead of the term southern limit the notion of the lower altitudinal limit of permafrost distribution, above which mainly perennially frozen ground occurs, is used for mountain regions.

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