Jökull - 01.12.1959, Blaðsíða 6

loe e-----log e-----
ö z0 6 Zl
TABLE I. Constants for each of the three areas.
Constant I II III
B for E in gm/hr. C for H in cal/lir. 4.52 6.8 4.83 (x 10-3) 1.66 2.49 1.77 (x 10-2)
Using these expression the available heat was
calculated for each test and hence che expected
ablation, assuming all the available heat used.
The fair agreement between measured and cal-
culated ablation as shown in figure 2 lends
some validity to the heat balance calculations
shown in figure 3 and summarised as totals in
Table I:
A simple comparison of the causes of ablation
is difficult since the observation over each area
had to be made at separate times and in dif-
ferent weather conditions. Figure 3 shows how
very important is the presence of direct sun and
how, a small amount of cloud, so long as it
does not obscuré the sun, increases the ra-
diation available, probably by multiple reflec-
tion. The proportions in each column indicate
the relative importance of the different heat
mechanisms.
CLEAN ICE.
The net radiation is negative in half the tests
over clean ice but unobscured sunshine can pro-
duce up to 90% of the heat available at the
surface. Hacl there been less cloud, radiation
would have been more important yet even with
such overcast days, which is quite typical of
temperate glacier weather, radiation supplied
half of the heat available at the surface. This
proportion would have been near one tenth if
the ice had been really clean, with albedo nearer
the customary 40—50%. Ahlmann (1953, p. 6)
gives 8% as the proportion of heat supplied by
radiation on Hoffellsjökull and Sverdrup ana-
lysing the same data found the importance of
radiation decreased from 50% to zero as the
summer progress so that radiation contributed
little more than 10%, this amount increasing
with altitude (Ahlmann & Thorarinsson 1940
p. 226). The present study indicates that where
a dirt cover reduces the reflectivity of the ice,
wlrich is commonly found, the importance of
radiation increases much beyond this 10% mean.
The spread of dirt over the ice increases as the
summer progresses and partly compenstates for
the decline in radiation importance. The am-
ount of dirt cover and amount of cloudiness
govern this importance og radiation.
Condensation at the ice surface supplies
rather less than a quarter of the heat available
while the eddying air supplies convective heat
to form a little more tlian a quarter of the to-
TABLE II.
Source and Sink as Percentage of Total ILeal Available at Surface.
A r e a S u r f a c e I Clean Ice II Medium Dirt Cones III Small Dirt Cones
Total valid tests 13 16 11
Tests with sun obscured 11 9 1
Heat Net Radiation Condensation 50.22% 22-17% 78.65% 7.18% 80.75% 3.33%
Available Convective heat 27.61% 14.17% 15.92%
Heat Melting 98.25% 97.36% 97.22%
Used Evaporation 1-75% 2.64% 2.78%
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