Jökull - 01.12.1959, Side 7

FIG. 3. ’CLEAN' I CE
sun m
DIRT CONES
* *** ^****>t*XXX
#8882 «é 76741657 O 5 8 325
8 0 0 18 8 77736435 0 17 2 25
SUN VISIBLE
SUN THROUGH
THIN CLOUD
RADIATION
CONVECTIVE HEAT
CONDENSATION AND
EVAPORATION
MELTING
UNEXPLAINED
DATE I AUC 2 12 IOSEP II SAUCb 7auG 6 II 22 AUC
tal heat available at the surface. The warm,
moisture-laden air, chilled by contact with the
ice surface seems to be common over Icelandic
glaciers and only when the sun disperses the
cloud and reverses the vapour pressure gradient
does evaporation occur. Hence most of the ab-
lation is by melting.
DIRT CONED ICE.
The radiation absorption by the dull black
surface is all important in the lieat supply,
but during the night hours convective heat,
though small in amount, is dominant. Cori-
densation is small because this same dirt cover
precludes contact of the moisture-laden air with
the melting ice; there is frequently a steep tem-
perature gradient through the dirt cover. Eva-
poration is similarly small but is greater than
that of „clean“ ice and is probably slightly
assisted by the capillary attraction of meltwater
through the dirt particles.
It will be seen from a comparison of figures
2 and B how the rapid increase of ablation,
particularly at the foot of the cones where the
dirt cover is thinner, is generally coincident
with a high rate of incoming radiation supply-
ing heat to the surface. The highest columns in
figure 3, the greatest heat supply, cause a more
uniform melting at both top and bottom of the
cones. Tlris indicates how a dirt layer which
offers protection in overcast weather is inade-
quate for protection in clear weather. Radiation
is the principal source of heat supply for pro-
ducing large dirt cones and, once the dirt has
slumped to leave a thinner cover, radiation
again produces the bulk of the heat for direct
melting of the covered ice. The steepest part
of the cones had slightly less than 1 cm. dirt
cover (Lister 1953, p. 28) and since no marked
change in shape occurred it seems that this pro-
vides adequate ice protection save when in-
coming radiation is great. Tlien the dirt thick-
ness must be 2 to 4 cm., to continue the pro-
tection.
5