snowpack at Suwon was in 1985, when the first
During its fall to earth, a snow crystal may un-
dergo considerable change. Variation of tempera-
measurable snow depth occurred on 11 Novem-
ture and humidity with altitude leads to changes in
ber. The latest date for the final disappearance
growth rate and form, and there may even be evap-
of the snowpack was in 1976, when the last
oration or melting of the crystal. Particles may be
measurable snow depth occurred on March 22.
"recycled" through some layers by turbulence in
Although the Suwon data cover only point
the air and, during windy conditions at the surface,
information, they do tell us a few simple but
fragmentation of the more delicate crystal types of-
useful things. First, they show that there is a sig-
ten occurs.
nificant amount of snow in those parts of Korea
The character of the surface deposit after a snow-
north of 3500′N latitude. Second, we can see
fall depends on the form of the crystals and on the
that the snowpack is likely to be relatively
weather conditions during deposition. When there
dense, since most of the country experiences a
is no appreciable wind, dry stellar crystals (which
commonly aggregate into large snowflakes) settle
large number of freezethaw cycles. Snow
as a soft, fluffy mass whose density is generally less
depth is in general highly elevation dependent,
than 0.1 g/cm3. Very small crystals of simple pris-
and of course the snow depths in most parts of
matic form, on the other hand, settle to relatively
Korea, especially North Korea, will be much
high initial densities (say 0. 2 g/cm3) for obvious
higher than those at Suwon's relatively low ele-
geometrical reasons. Snow deposited in wind-free
vation.
weather has a smooth surface. When a snowfall is
accompanied by strong winds, crystals are broken
into fragments favorable for close packing, and the
ESTIMATED AVERAGE AND
surface of the deposited snow is mechanically agi-
WORST-CASE VALUES
tated by wind shear and by the impact of bounding
Table 7 shows our recommendations for the
particles. This produces high initial density, com-
monly greater than 0.3 g/cm3, and also leads to the
estimated average and worst-case values for
formation of snow dunes and sastrugi on the sur-
snow depth and density. The worst-case snow
face.
depth represents what one might find at the
After deposition, snow may be dissipated by
height of an unusually severe snow season, and
melting and evaporation or it may persist for long
the density is simply the density that one might
periods. If it persists, it will undergo metamor-
find at the worst point of the same season. These
phism, changing its grain texture and structure and
values correspond to flat, open ground; in the
eventually turning into hard, impermeable ice if it is
field, one will of course typically have to deal
part of a perennial snow deposit.
with sloping ground with a variety of cover
Attempts to estimate snow depth and snow den-
types.
sity in the absence of direct data need to take three
general classes of phenomena into account. Firstly, it
Table 7. Recommendations for aver-
is useful to know the upper air conditions (tempera-
age and worst-case snow density
ture, water content, etc.) that determine how much
and depth values for interior Alaska,
snow is produced in the cloud layer and what type
central Germany and Korea.
of snow crystals fall. Second, one needs to know the
temperature regime at the surface, which deter-
Density
Depth
(g/cm3)
(cm)
mines how fast the snow melts. Third, one needs to
know about the wind regime at the surface, which
Alaska
regulates wind-packing, drifting, etc. Only the sec-
Average
0.230
50
Worst-Case
0.425
125
ond and third classes of phenomena have been di-
rectly addressed in the work by Bilello, Bates and
Central Germany
others that have been summarized above.
Average
0.275
25
Worst-Case
0.400
60
The most difficult snow conditions for vehicle
mobility occur when snow densities are in the range
Korea
around 0.30.4 g/cm3. Newly fallen snow reaches
Average
0.275
18
Worst-Case
0.400
38
these densities only when there is a significant
amount of wind. This is not much consolation for a
Malcolm Mellor's 1964 monograph continues
vehicle mobility modeler, however, since virtually
to be one of the best introductions to the proper-
all areas of the earth, including Korea, Alaska and
ties of snow. One of Mellor's general comments
Germany, do commonly experience significant
about snow is of particular relevance:
winds.
6
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