Jökull - 01.12.1969, Blaðsíða 8

Fig. 4. Development of the Greenlancl Ice
during the winter 1965—66 (British Weather
Bureau).
relationship with the total quantity of the
Greenland Ice as derived from the Danish ice
charts by Lange Koch (1945) for the period
1898 to about 1940. Taking the six years of
minimum ice quantity after Koch, already two
of these were ice years in Iceland, although
minor ones. Of the six years of maximum ice
quantity, none was a major ice year in Iceland.
Of the five greatest Icelandic ice years ín this
century, two were under the average in ice
quantity, two were a little above the average,
and the year of maximum ice quantity was an
intermediate ice year. Tlie very severest ice
year in Iceland was under the average in Koch’s
ice quantity. Koch (1. c., p. 36) points out that
much Eastern ice at Spitsbergen, common in
earlier times, did not cause severe ice condi-
tions at Iceland.
The actual main causes of severe ice condi-
tions in Iceland are special weather conditions.
An example is the year 1965. Nothing unusual
in the far north precedecl the approach of
severe ice at the end of February, as far as the
ice charts show. The ice charts for January and
February indicate that the direct cause of the
ice approach is to be sought near Iceland, and
a comparison with the weather leaves no doubt
that the main cause was a persistent SW-wind
for most of February. Such wind in the area
between Iceland and Greenland thrusts the
drift towards the east north of Iceland. This
condition is precarious because now the ice
margin is very sensitive to northerly storms.
This is due to differential drift of compact ancl
open ice. The velocity of wind-driven open ice
is about three times that of compact ice. Thus,
whereas in February 1965 the wincl pressure
was not sufficient to thrust the compact ice
dangerously close to the northern coast of Ice-
land, its margin was torn up ancl a very open
drift carriecl up to the coast. It was the con-
densation of this open drift at the coast that
caused the blockage of Northern Iceland.
But northerly, even NE-winds may also be a
primary cause of ice approach if they push
marginal ice along the SW-running side of ob-
structing compact ice.
As to the east coast, the conditions are more
special. At the same time as the open drift
was blown up to the northern coast in 1965,
as said above, a tongue of dense ice extended
from the main ice NE of Iceland so far to the
southeast that it was caught by the East Ice-
land Current ancl carried rapidly to the south
at a short distance from the eastern coast.
Inspired by sucli observations, the ice ap-
proach to Iceland is liere divided into three
classes.
Class A (Northwest-ice). Ice comes close to
Cape North (Horn), or Grímsey island, or
4 JÖKULL 19. ÁR