Jökull - 01.12.1953, Blaðsíða 15

Miðfellstindur, in 1196 m height, was dug and
measured by members of the Nottingham Uni-
versity Expedition to Vatnajökull, and kindly
placed at my disposal by the leader of the ex-
pedition, Jack Ives. The pit was dug between
15 July and 18 July, and was over 6 m deep.
Two distinct horizons marked with dust showed
the autumn surfaces of 1952 and 1951. 238 cm
of firn were left over as surplus accumulation
from the winter of 1951—52. The average speci-
fic gravity of the 1952—53 accumulation layer
was 0.58 and 0.61 for the surplus layer of 1951
—52. The water equivalent of the 1952—53 layer
is 1650 mm, and that of the 1951—52 surplus
1450 mm. It is difficult to tell how great the
corresponding total precipitation in the budget
year 1952—53 has been, but it has hardly been
less than 2500 mm.
When comparing the accumulation values of
1952—53 and 1950—51 for W. Vatnajökull, one
has to bear in mind that the winter of 1950—51
was cold with persisting winds from NE, where-
as during the winter of 1952—53 winds from S
and SE prevailed. We find, however, that in
the central area of western Vatnajökull the
accumulation has been about the same these
two winters.
The average specific gravity in 7 profiles
through the 1950/51 accumulation, situated in
940—1580 m height and measured 28 March—
21 April 1951 by S. Rist, was 0.47 (cf. Rist,
1953). The average specific gravity in 8 profiles
through the 1935—36 accumulation, situated in
1050—1580 m height and measured 20 May—7
June 1936 by the Swedish-Icelandic Expedi-
tion was 0.50 (cf. Ahlmann-Thorarinsson 1939).
The average specific gravity in 7 profiles
through the 1952—53 accumulation, situated in
1080—1988 m height and measured 28 June
— 4 July 1953, was 0.55.
The specific gravity in the profile through
the 1952/53 accumulation, situated in 1196 m
height and measured by J. Ives on 18 July 1953,
was 0.58.
We thus find a regular increase of the speci-
fic gravity from the latest part of the accumu-
lation period towards the middle of the ablation
period. The main cause of this' increase is the
trickling down of meltwater from the surface
into the accumulation layer. From Table I we
find that upon the whole the specific gravity
increases with decreasing height above sea level,
SURFACC I5JULY
SU8FACC II AUC
cus|
□
F1RN WITH THIN
CC PARTINGS
I ICC BA NOS
CZ3DUIT HORIIONS
5 2 2 u. mi
SPCCIF IC CRAVITY
OAII
Fig. 5. Profile VIII, 2.5 km N of Miðfellstindur.
Hjarnsnið meelt i gryfju VIII, 2.5 km norður af
Miðfellstindi.
or from 0.51 in 1988 m height to 0.61 in 1080
m height. As the profiles were all measured
practically at the same time, this increase is
partly due to more precipitation falling in the
form of rain during the period in question at
the lower altitudes, and partly to the earlier
beginning of the ablation period on the lower
regions of the glacier.
The total amount of precipitation within
the Grimsvötn intake area.
One of the profiles registered in Table I
(Profile IV) is situated within the Grímsvötn
depression, and three of these profiles (III, V
and VII) on the borders between the Gríms-
vötn intake area and the intake areas N and
W of it. The average accumulation in these
profiles from autumn 1952 to 1 July 1953, or
during ab. 10 months, was 2050 mm, corres-
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