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 MIL-HDBK-419A
Permafrost occurs in various degrees throughout much of the arctic and subarctic regions and is defined as that
part of the lithosphere (upper crust of the earth) in which a naturally occurring temperature below OC
has existed continuously for two or more years. The "annual frost zone" is the zone of annual freezing and
thawing. Where permafrost occurs, the thickness of this surface layer varies from less than a foot in the arctic
to depths in excess of 12 feet in the subarctic. The seasonal thaw zone remains unfrozen only during the short
summer months. During this period, it is possible to recognize terrain features which can be located in the
spring and fall if there is little or no snow cover.
Willow groves or aspen generally point to the absence of permafrost and to the presence of groundwater which
freezes only for a short time. River bottoms and lake bottoms are usually frost-free. Generally, slow moving
rivers and streams freeze from the top down (surface ice). Clear, fast moving rivers and streams usually freeze
from the bottom up (anchor ice). Mountains, valleys, lake bottoms, streambeds, tree-covered slopes, tundra
plains, swamplands, ice glaciers, silty estuaries, permafrost areas, and seasonably frozen ground, each will be
found to affect soil resistivity. Consequently, it is easily seen how one area versus another might be more
suitable for good grounding. Basic illustrations of variations, layering and asymmetrical contouring can be
found in Figures 2-28, 2-29, and 2-30.
Resistance to ground and configuration of electrodes are further parameters that must be considered. The
conductivity of cables and overhead wire systems are relatively high in comparison to the earth. Without the
presence of minerals, dissolved salts, and moisture, clean dry soil can be classified as an insulator and possesses
the intermediate characteristics of a poor conductor.
Figure 2-28. Relative Depths of Unconsolidated Materials, Subarctic Alaska
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