TM 5-852-1/AFR 88-19, Volume 1
occurring during any given freezing season. The index
curve of cumulative degree-days versus time for one
determined for air temperatures at approximately 4.5 feet
thawing season. It is used as a measure of the
above the ground is commonly designated as the air
combined duration and magnitude of above-freezing
freezing index, while that determined for temperatures
temperatures during any given thawing season. The
immediately below a surface is known as the surface
index determined for air temperatures at 4.5 feet above
freezing index.
the ground is commonly called the air that determined for
(10) Freezing season. That period of time
temperatures immediately below a surface is known as
the surface thawing index. A thawing index is shown in
during which the average daily temperature is generally
figure 1-1.
below 32F. Figure 1-2 shows mean dates for beginning
(21) Thawing season. That period of time
of the freezing season in the Northern Hemisphere.
(11) Frost-melting period. An interval of the
(luring which the average daily temperature is generally
above 32F. Figure 1-3 shows mean dates for beginning
year during which ice in the ground is returning to a liquid
of the thawing season in the Northern Hemisphere.
state. It ends when all of the ice in the ground is melted
(22) Thermal regime.
The pattern of
or when freezing starts again.
Although in the
temperature variations found in the ground with time and
generalized case there is only one frost-melting period,
with depth from the surface.
beginning during the general rise of air temperatures in
(23) Wind chill. The excess rate of removal
the spring, one or more significant frost melting intervals
may take place during a winter season.
of body heat from exposed skin by moving air compared
(12) Geothermal gradient. The temperature
to still air at low temperatures. It is often expressed as a
gradient in the ground below the zone of annual
lower equivalent air temperature that is a function of
temperature fluctuations, produced by the continuous
actual air temperature and wind speed.
d. Terrain terms.
flow of heat from the Earth's hot interior toward the
(1) Frost mound. A localized upwarp of
relatively cool Earth's surface.
(13) Mean annual temperature.
The
land surface caused by frost action with or without
average of the average annual temperatures for several
hydrostatic pressure.
(2) Icing. A surface ice mass formed by
years.
(14) Mean daily temperatures.
The
freezing of successive sheets of water.
(3) Muskeg.
average of the average daily temperatures for a given
Poorly drained organic
day for several years.
terrain consisting of a mat of living vegetation overlying
(15) Mean freezing index. The freezing
an extremely compressible mixture of partially
index determined on the basis of mean temperatures. A
decomposed peat, varying in thickness from a few
mean freezing index is illustrated in figure 1-1.
inches to many feet.
(16) Mean monthly temperature.
(4) Patterned ground. A general term
The
average of the average monthly temperatures for a given
describing ground patterns that result from frost action,
month for several years.
such as polygons, circles and nets, stripes, and
(17) Mean thawing index. The thawing
solifluction features.
(5) Pingo (hydrolaccolith). A large ice-
index determined on the basis of mean temperatures. A
mean thawing index is illustrated in figure 1-1.
cored frost mound, often 100 feet high or more.
(18) n-factor. The ratio of surface index to
(6) Thermokarst.
The
irregular
air index for either freezing or thawing. Surface index
topography resulting from the process of differential thaw
equals air index multiplied by the n factor.
settlement or caving of the ground because of the
(19) Period of weakening. An interval of the
melting of excess ice in thaw-unstable permafrost.
(7) Tundra. A treeless region of grasses
year that starts at the beginning of the frost-melting
period and ends when the subgrade strength has
and shrubs characteristic of the Arctic.
returned to normal period values or when freezing starts
again. In seasonal frost areas, the period of weakening
may be substantially longer than the frost-melting period,
but in permafrost areas the periods coincide.
(20) Thawing index.
The number of
degree-days between the lowest and highest points on a
1-4
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