Jökull - 01.07.2003, Blaðsíða 44

Achim A. Baylich
Whereas the Dalatangi meteorological station
records mean monthly temperatures exclusively
above freezing point, the number of months reveal-
ing mean temperatures below freezing point distinctly
rises with increasing altitude. For instance, at an alti-
tude of 300 m a.s.l. mean monthly temperatures above
0ÆC are found only from May until November, at 600
m a.s.l. from May to October and at 900 m a.s.l. only
from June to September. The general increase of the
air temperature from April-May onwards manifests it-
self in the lower parts of the study area in the begin-
ning of the main snow melt period. In the upper areas,
temperatures, while increasing, remain below freez-
ing. Accordingly, snow cover has a longer duration.
In shaded areas snow patches and fields exist all year
long.
At the Dalatangi meteorological station, frost
events are generally recorded for the first time at
the end of September or in October. The last frost
events normally occur between end of April and be-
ginning of June. At 300 m a.s.l., the frost period
lasts from September-October until June, at 600 m
a.s.l. from September-October to June-July. At 900 m
a.s.l., there is no month which is free of frost (Beylich
1999a; 2000c).
Frost days are mostly and normally recorded from
November to April. From a morphoclimatic point of
view, it has to be emphasised that the lower parts of
the study area have shorter frost phases interrupted
by phases which are free of frost (Figure 7), whereas
the upper areas are characterized by frost phases last-
ing several months from November until April-May
at (600 m a.s.l., and from October to May-June at
900 m a.s.l., (Figure 8) (Beylich 1999a; 2000c). Fur-
thermore, the altitude-related increase of frost days
mainly implies an increase in the number of ice days,
whereas the frequency of freeze-thaw days only in-
creases up to an altitude of 300 m a.s.l. While Febru-
ary, March and April are the months with the highest
number of freeze-thaw days at Dalatangi, these are the
months of January, February, and April at 300 m a.s.l.,
April, May and November at 600 m a.s.l., and May,
June and October at 900 m a.s.l. The minimum air
temperatures on freeze-thaw days are seldom below
-5ÆC, with the daily freeze-thaw events normally be-
ing confined to the near-surface substrates. During the
field research period, minimum air temperatures of -
5.8ÆC on freeze-thaw days did not lead to frost pen-
etration into the ground to depths of 5 cm (Beylich
1999a).
Information on frequencies and intensities of frost
events to be expected in different months at differ-
ent elevations a.s.l. can be gained by the magnitude-
frequency analyses shown in Figure 9. Even at higher
altitudes frost intensities in Austdalur are far from
reaching the levels recorded at Latnjavagge (see Fig-
ure 6).
It can be assumed that frost weathering is en-
hanced during longer and more severe frost spells
(Schunke 1975; Church et al. 1979; Washburn 1979;
Walder and Hallet 1985; 1986; Hallet et al. 1991;
Matsuoka and Sakai 1999). This assumption is sup-
ported by the fact that frost weathering and rockfall
activity from rockwalls in the study area becomes
more intense with increasing altitude a.s.l. (Beylich
1999a). Another indication is the seasonal variabil-
ity of rockfall activity at basalt rockwalls situated at
an altitude of 450 m to 750 m a.s.l. Here the highest
rockfall activity occurs after the winter frost period
that lasts several months (Beylich 1999a).
The duration of frost periods is also important for
the intensity of chemical weathering and for fluvial
channel discharges. Numerous and long lasting frost
spells decrease the intensity of chemical weathering,
which is in any case small due to the subarctic mor-
phoclimate (Beylich 1999). Also geomorphologically
relevant is the lowering of channel discharges during
frost spells.
Daily freeze-thaw events imply the forming of
needle ice at convex, vegetation- and snow- free, soil
covered slope surfaces exposed to the wind in autumn
and winter. Field work in Austdalur showed that even
slight night frosts may cause the formation of nee-
dle ice (Outcalt 1971). The formation of ice crystals
causes the substrate to break up and dry and solids
are lifted. Thus a large number of freeze-thaw days
favours the deflation of fine material and the destruc-
tion of vegetation cover by turf exfoliation. Due to
heaving of particles at vegetation free and inclined
slope areas, needle ice has a direct denudative ef-
42 JÖKULL No. 52, 2003