Fróðskaparrit - 01.01.1998, Síða 173

SEINHOLOSENA VEÐURLAGIÐ ELVDI TIL JARÐFORMANDI VIRKSEMI í FØROYUM \ 79
each observational site, while 12 daily
measurements were obtained from May
1996 to June 1998.
The effect of altitude on the mean air
temperatures and the absolute temperature
range is illustrated by Fig. 4, based on data
registered from May 1996 to May 1997.
The oceanic influence is obvious at all alti-
tudes; summers are cool and winters mild.
The typical annual range of daily mean
temperatures is about 15°C. The calculated
annual vertical lapse rate varies from
0.0065°Cm_l to 0.0055°Cm"'. At the sum-
mit of the highest mountains the annual
near-surface mean air temperature is only
slightly above 0°C. Permafrost is conse-
quently presently absent in the Faroe Is-
lands, but would presumably only require a
limited temperature lowering to form in the
highlands, as will be further discussed be-
low.
The altitudinal distribution of the mean
annual cumulative frequency of freeze-
thaws events (FT) and Growing Degree
Days (GDD) is shown in Fig. 5 (1995-97).
Before calculating GDD and FT, all data
were normalised to a daily observation fre-
quency of 24 (Molau and Mølgaard, 1996).
GDD is a substitute for plant growth condi-
tions, and equals the number of degree days
above 5°C (Maxwell, 1992; Molau and
Mølgaard, 1996). Usually GDD shows a
good correlation with plant growth, except
in regions where lack of precipitation limits
growth (Zoltai, 1988). GDDs have rele-
vance within a geomorphic context, since
the presence or absence of a plant cover can
control the development of small-scale pat-
temed ground (Ballantyne, 1996). The FT
parameter equals the number of daily tem-
perature transitions across 0°C. A high FT
presumably indicates conditions suitable
for recurrent needle ice growth and decay at
the ground surface, and associated geomor-
phic effects such as soil creep.
The mean annual number of FTs increas-
es significantly from about 120 to more
than 400 across the altitudinal range from
250 to 450 m a.s.l., parallel with a likewise
marked decrease in the mean annual num-
ber of GDD from more than 2000 to less
than 1100 (Fig. 5). This altitudinal range is
identical to the modem lower limit of ac-
tive, small-scale patterned ground on the
Faroe Islands (Humlum and Christiansen,
in press), and thereby delimits the lower
boundary for the late 20th century peri-
glacial (arctic) zone.
Snow survival in the highlands
In an early description of the Faroe Islands
written about AD 1680 (Descriptio Færoar-
um) the following (translated) general in-
formation on 17th century snow survival in
the Faroes is found (Resen, 1972, p.17):
‘Snow lies without interruption on the high
mountains and never melts, but are forming
overhanging cornices along the highest
summits and headwalls. This frequently
leads to the death of many sheeps walking
on these snow đrifts, when they melt and
sudđenly avalanches down’. Clearly, peren-
nial snow accumulations were widespread
in the Faroes during certain cold intervals
of the Little Ice Age, in contrast to late 20th
century conditions, where snow with sever-
al years of interruption survives the sum-
mer in a few sheltered sites only.