Jökull - 01.12.1953, Blaðsíða 28

Fig. 1—3. Average values o£ Temperature, Wind velocity and Vapour pressure in the 200 cm air
layer above ice surface.
Meðaltal hita, vindhraða og rakaþrýstings í 200 cm loftlagi yfir skriðjöklinum.
stage because in many of the tests rnade there
was a marked minimum vapour pressure in the
middle of the 2 metre air layer. Almost all the
profiles of humidity showed that evaporation
was taking place from the surface. In many of
the profiles there was vapour also feeding into
the centre of the two metre layer from above
(Fig. 3). This suggests advection but with only
one mast available to take readings it was not
possible to attempt to measure this advection.
RADIATION.
The radiation reflected from the surface was
measured within 30 secs. of the incoming radia-
tion. Where the amount of incoming radiation.
was small the pile was not sufficiently sensitive
to measure the reflected radiation.
The albedo is the ratio of the reflected to
the incoming radiation and is expressed as a
percentage. The albedo for the clean ice ranged
from 4 to 20 but the majority of the values were
close to the mean of 14%. The rather wide range
of values is due to two main causes: (1) the sensi-
tivity of the pile being insufficient to permit an
accurate reading for very low values of radiation
and reflection; (2) the change of the ice from
a wliite sugary surface, pock-marked with myri-
ads of air holes, to a clean fairly smooth sur-
face as produced by days o£ rain and to a slightly
undulating surface with an irregular sprinkling
of wind blown grit.
Hence the mean albedo of 14% may be a little
low as a representative figure for glacier ice.
Walten used 40%, Ahlmann 50% (1939) but
most other workers have been considering a
snow surface which has a very high albedo. The
observations in Iceland were primarily to con-
sider the effect of the rock debris so that it is
perfectly justifiable to take relatively clear ice
near the glacier snout and moraine as control
for the debris laden areas. No better site of clean
ice presented itself until much higher up the
glacier where the different altitude and expo-
sure would have added greater complexities had
such a site been used as a control for the dirt
covered areas.
The albedo for ice with dirt cones was very
similar to that for the dirt covered ice, studied
only during the first few days, as explained
above. This albedo averaged 4% and ranged
from 2 to 8% the higher figures being obtained
earlier in the summer when small patches of
relatively clean ice could be found between the
dirt cones. The lowest values of albedo were
found later in the summer when some dirt cones
were slumping and all the ice had a film of wet
grit spread over it.
ABLATION — (Micro observations).
Four stakes were maintained in holes on the
glacier snout about 1 km. from the edge. One
on clean ice, one on dirt-coned ice, one on a
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