depth must also be judged. If large quantities of
or other foreign objects have lodged on the ice.
snow are present at some times of the year but is
The rate of ablation will be important to know,
nonexistent at other times, maintenance efforts
but is difficult to determine in a single visit. Com-
may be substantial.
parison of major features seen in old airphotos or
At sites with no snow cover, ablation may be a
Landsat images with current observations may
concern. Differential ice loss due to ablation may
provide a clue.
create unevenness in the runway surface and re-
If evidence of melt features is clearly seen at
quire patching or regrading. Runway support fa-
the site, a snow cover will be required to protect
cilities (buildings, fuel tanks, runway markers)
the ice during some portion of the year (when the
may become "perched" and unstable. The pres-
air temperature and solar angle are highest). The
ence of ablation can often be seen where small
site must have a readily available supply of snow
at that time for this purpose.
snow patches have stuck to the ice or where rocks
a. Photo taken in late January 1965 looking south (U.S. Navy for U.S. Geological Survey).
Figure 12. Airphotos of transition between zones of accumulation and ablation.
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