Jökull - 01.12.1988, Side 66

mountains in east and southeast, an increase which
in special weather situations can be much larger
(Sigurðsson, 1964).
Values of 1000-1200 mm are common in the
extensive lowland areas to the east of the mountain
range.
It is clearly seen that the Lake Þingvallavatn
depression has a minimum annual precipitation
compared with the surrounding higher areas. At the
three weather stations along the coast of the lake the
annual precipitation is of the order 1300-1400 mm.
The interior regions of the eastemmost part on the
chart have annual values lower than 1000 mm. It is
known that farther to the northeast in the watersheds
of the rivers Þjórsá and Tungnaá, the values may be
under 800 mm. This is caused by the glaciers
Eyjafjallajökull and Mýrdalsjökull and mountains
north of them which create a very distinct rain sha-
dow.
In southwestem Iceland only 5-10% of the precip-
itation falls as pure snow. The snow cover in this
part of the country is rather variable and a complete
snow cover seldom persists for weeks or months as
is often the case in northem Iceland. Snow cover,
expressed in per cent of total cover and taken as an
average of the period October-May, is of the order
15-38% with minimum values at the coast.
ANNUAL NUMBER OF DAYS
WITH PRECIPITATIONIN ICELAND
The annual number of days with precipitation of
0.1 mm or more is quite variable in Iceland. A day
with precipitation is defined as the time from 09h
GMT until the same time next day.
Fig. 5 shows average annual number of days with
precipitation in Iceland 1971-1980, expressed in per
cent of all days. It is seen that in southem and
southwestem Iceland 50-65% of all days have
measurable precipitation. Inland areas in northem
and eastem Iceland have lower values, generally
40-50% and in places even under 40%. In
southwestem Iceland the annual number of days is
200-237 with numbers lower than 200 days only at
four locations.
It is worth noting that the distribution of the
Fig. 5 Average annual number of days with precipi-
tation in Iceland 1971-1980 in % of all days.
Mynd 5. Arlegur meðalfjöldi úrkomudaga á Islandi
1971-1980 í % allra daga.
number of days with precipitation shows a rather lit-
tle variation in southwestem Iceland compared with
that of precipitation amount as shown in Fig. 4. In
other words, there may be a considerable difference
in precipitation amount between locations/areas,
although the number of days with measurable pre-
cipitation is the same.
Influences of mountains and glaciers on the
number of days with precipitation are clearly distin-
guishable. The northeastem interior of the country is
in the very effective rain shadow created by
Vatnajökull. Inland regions of northwestem Iceland
are shadowed by the glaciers Langjökull and
Hofsjökull in southeasterly winds and by the moun-
tains between Eyjafjörður and Skagafjörður and to
the west of the latter in northeasterly winds. Finally,
Barðaströnd, the southemmost part of the Vestfirðir
headland, is shielded in wind directions between
east and north, which are the most frequent ones in
that region. The same applies to the bottom of
Isafjarðardjúp, the biggest fjord cutting into the
Vestfirðir mountains.
64 JÖKULL, No. 38, 1988