Jökull - 01.12.2006, Síða 4

Haraldsdóttir et al.
Most of the avalanche mapping, as well as most
studies of weather situations causing avalanches, have
only been presented in internal reports, of which many
are in Icelandic. In this paper, an overview of the
weather related to avalanche hazard, in 15 towns
(Figure 2) is presented, together with maps in an
Appendix showing the outlines of avalanches in the
vicinity of 13 towns. The purpose of this paper is
to bring a synthesis of this extensive data collection
and research to the international avalanche commu-
nity. Furthermore, it can be considered as a basis for
research e.g. on snowdrift, aiding future selection of
sites for monitoring and studies of the phenomena. In
the next section of this paper, a brief review of pub-
lished studies of weather prior to avalanches is pre-
sented. This is followed by a short review of work on
the geographic settings of avalanches in Iceland, in-
cluding the most severe events in recent years. The
registration of avalanches in Iceland is reviewed in
the following section, which includes an overview of
avalanches in the towns in question. Plans for evacu-
ation are explained and the relevant weather prior to
avalanches in each town is described.
STUDIES OF WEATHER PRIOR TO
AVALANCHES
The climate of Iceland
Iceland’s winter climate is characterised by snowfall
during strong winds and frequent thaw periods. Con-
siderable snowdrift is frequent and it has a strong ef-
fect on avalanche hazard.
An overview of averages and extremes of winter
climate from 1961 to 1990 at several weather obser-
vation stations (Figure 2) is presented in Table 1. The
table reveals that the average winter temperature at
sea level is close to 0◦C (giving lower mean temper-
atures at the starting zones of avalanches). The av-
erage 7-month precipitation ranges from 335 to more
than 950 mm. At 400-800 m a.s.l. above the towns
at risk, most of this precipitation falls as snow. These
numbers may seem low, but here it should be kept in
mind that during strong winds and solid precipitation,
the conventional observations give a large underesti-
mation of the true ground precipitation (Friðriksson
and Ólafsson, 2005). Furthermore, precipitation in
the mountains can be expected to be greater than in
the lowland where the weather stations in Table 1 are
located (Rögnvaldsson et al., 2004). The mean wind
speed is remarkably high at most of these lowland sta-
tions and the extreme wind values are far above what
is needed for significant snowdrift.
Studies of weather and avalanches
Björnsson (1980) presented statistics on deaths and
types of avalanches in Iceland based partly on Jóns-
son (1957). The Björnsson paper also describes cli-
matic conditions and terrain features for the main ar-
eas endangered by avalanches as well as weather prior
to both wet and dry avalanches (see also Björnsson,
1979).
In recently published avalanche history reports
(Veðurstofa Íslands, 1997–2004), some information
about weather related to avalanches is included, and
these reports have been the background for several
studies of avalanche weather.
Two catastrophic avalanches fell in Neskaup-
staður on 20 December 1974, causing 12 fatali-
ties. Guttormsson (1975a,b) registered information
about the avalanches and the preceding weather,
and avalanche maps were published in Haraldsdóttir
(1997). Egilsson (1995a,b, 1996) described the
avalanches that struck the towns Súðavík and Flat-
eyri, in January and October 1995, killing 14 and
20 people, respectively. More detailed studies of
the Flateyri avalanche, including an analysis of the
preceding weather conditions, is given in Haralds-
dóttir (1998a,b). A weather analysis of the Súða-
vík avalanche is found in Ólafsdóttir (1996). All
the above studies underline the importance of heavy
snowfall and transport of snow in strong winds.
Jóhannesson and Jónsson (1996) studied weather con-
ditions prior to avalanches in Vestfirðir (Figure 2).
Jónsson (1998) found that dry avalanches in Vest-
firðir and Austfirðir were predominantly associated
with northerly and mainly north-easterly winds, and
that registered dry avalanches are twice as common in
Vestfirðir than in Austfirðir. Jónsson attributed this
difference in frequency to the winter climate being
warmer in Austfirðir than in Vestfirðir.
2 JÖKULL No. 56