We did not mechanically add any more snow
ice surface from melt features is possible with a
to the runway. We assumed that some additional
less than ideal thickness and composition of snow
snow would accumulate on the runway during
as long as the snow surface is properly main-
spring storms and, by using the methods out-
tained. If kept "fresh" the snow surface can pro-
lined in the section on snow management, we
vide a tremendous degree of protection from
planed to add, decrease or maintain the depth of
radiation damage. In scientific terms, the goal is
snow cover.
to maintain a high albedo surface (high reflec-
To provide adequate protection against solar
tivity).
radiation with a minimal thickness of snow it is
At sites where solar degradation is prevalent,
imperative that the snow be of a uniform density
it may be wise to monitor albedo using radiom-
not much less than 0.45 g/cm3 and be every-
eter measurements. Radiometers are manufac-
where in intimate contact with the ice surface.
tured in a variety of configurations and wave-
This requires that the snow be processed; usually
length ranges. We recommend the type with
compaction with rollers is suitable to accomplish
hemispherical globes and the capability to mea-
both goals. Compacting in thin lifts (<10 cm) is
sure total radiative flux, both short wavelengths
best if possible. The temperature of the snow will
(0.35 m) and long wavelengths (4100 m). Radi-
also dictate the degree to which compaction can
ometer readings should be made with the globe
be accomplished. As temperatures warm, heavier
facing directly up and directly down, at a height
loads and higher contact pressures can be placed
of about 1 m from the surface. No shadows should
be within a 45 cone emanating from the circum-
on the snow resulting in incrementally bringing
the snow density up to the desired level.
ference of the radiometer. Readings taken when
Ideally the protective snow cover over the gla-
pointed upward provide a measure of the incom-
cial ice should consist entirely of fresh, clean snow.
ing radiation, and the downward reading indi-
This may not be possible at some sites. The most
cates how much radiation is being reflected and
common foreign material incorporated in the
emitted from the surface. The magnitude of the
available snow will probably be mineral matter
values read is not as important as the ratio of the
and pieces of ice. Neither is desirable in large
two values. As a percentage, it is important for
concentrations, but mineral content can be very
the protective snow cover to provide as high a
problematic. Almost any foreign material content
reflection ratio as possible, but at least 60% is
will have the effect of increasing the snow cover's
easily attainable. To facilitate taking radiometer
sensitivity to heating up.
readings at a number of representative areas
If snow with a significant concentration of
around and on the runway, we installed a pair of
foreign material must be used, it is best if the
radiometers on a stand (Fig. 24). We placed this
contaminant is well mixed (distributed) with the
tripod at various points of interest, allowed it to
snow. This minimizes everywhere the local con-
stabilize for 5 minutes, took a reading, and then
centration. Frequent processing will also be re-
moved on to another location.
quired to assist in keeping the surface layers as
The key features of the snow surface are its
cool as possible. Planes and drags will accom-
microroughness and permeability. If the snow sur-
plish mixing to a degree; however, typical agri-
face is kept roughened, incoming radiation will
cultural tools like harrows or disks may perform
be scattered to a large degree. This will reduce the
this task better.
amount of longwave energy that is transmitted
The rate of snow loss during the warm period
into the snow and ice. If the processed protective
will be accelerated by the presence of foreign ma-
snow surface is not maintained, it will develop a
terial. Thus, a greater thickness of protective snow
sheen or a glazed appearance resulting in a reflec-
cover should be in place at the beginning of the
tance ratio of less than 50%. This crusty surface
warm period than would be required when using
will impede air flow (permeability) through the
clean snow. Roughly, we estimate that the thick-
snow cover. Thus, it will encourage heat build up
ness of snow should increase by the same per-
in the snow. In addition, the iced surface will trap
centage as the concentration of contaminant in
emitted radiation that is attempting to escape from
the snow.
the earth, reflecting it back into the snow cover
and increasing the heating potential. When this
happens, the snow cover thickness will not ap-
Snow surface characteristics
The vital feature of the protective snow cover
pear to diminish, but the snow beneath the sur-
is its surface. Adequately protecting the glacial
face will deteriorate rapidly and eventually the
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