Jökull - 01.12.1988, Page 69

convective precipitation. Southern with warm air
mass results in drizzle and orographic precipitation.
In weather type eastern (Fig. 10) the number of
days with precipitation reaches 80% in the south
part of the Reykjanes peninsula, but is lower in
other parts of southwestem Iceland, down to 50-
60% in the inland areas.
In weather types northeastern and northern
(Figs. 11 and 12) the precipitation days in
southwestem Iceland are always fewer than 20%
and as a matter of fact in most places under 10%.
According to the above description it is sufficient
for an investigation of precipitation amount in
southwestem Iceland for different weather types to
concentrate on the four types: southeastern,
southwestern or western, southern with warm air
mass and eastern.
PRECIPITATIONIN SOUTHWESTERN
ICELAND IN WET WEATHER TYPES
Figs. 13-16 show the distribution of the precipita-
tion in southwestem Iceland for each of the four
weather types in which considerable precipitation
may be expected. Number of days used was 30 for
weather type southeastern, 30 for southwestern or
western, 18 for southern with warm air mass and
35 for eastern. As the number of selected days is
limited and not the same for all types, the amount of
precipitation is presented as a ratio to the precipita-
tion at Reykjavík. It should be stressed that values
for the main mountains, where no daily measure-
ments take place, are rough estimates.
Fig. 13 shows the precipitation in weather type
southeastern. The chart resembles in many ways
the annual one (see Fig. 4). A very distinct minimum
area or a rain shadow caused by Mýrdalsjökull and
mountains to the north of the glacier is seen in the
eastemmost interior, where the amount is less than
half of the Reykjavík value. Reykjavík itself and the
coast of Faxaflói are shielded by the Reykjanes
mountain range. A slight minimum is also found in
the Lake Þingvallavatn depression.
In weather type southwestern or western (Fig.
14) the picture changes. The lowest values are found
along the coast and increase inland. The orographic
effect of the Reykjanes mountains is still distinct and
the same effect is seen in the eastem part where the
land rises. Mountains to the west and southwest of
Lake Þingvallavatn create a slight rain shadow from
the lake towards east.
In weather type southern with warm air mass
(Fig. 15) warm and humid air is coming from south.
In this case orographic influence is very strong. Pre-
cipitation in the Reykjanes mountains may be up to
three times the values at the coast. Even a slight ris-
ing of land in the eastem part seems to increase the
amount. Precipitation is generally larger in the
westem part than in the eastem one. This could at
least partly be explained by the fact that in this
situation surface pressure rises towards east, and on
the other hand a cold front may in many cases be a
short distance to the west of Reykjanes.
Fig. 16 shows at last the weather type eastern. In
this situation the highest amounts of precipitation
are usually found in eastem and southeastem Ice-
land. In southwestem Iceland considerable precipi-
tation falls along the coast, especially in the south-
em part of the Reykjanes peninsula and in the
eastemmost part. Inland areas have comparatively
small amounts because of rain shadowing effects.
This applies both to the Faxaflói area, i.e. the west
coast, and the extensive lowlands to the east of the
Reykjanes mountain range.
REFERENCES
Einarsson, M.A. 1976: Veðurfar á íslandi. Iðunn,
Reykjavík, 150 pp.
Einarsson, M.A. 1983: Breytileiki og einkenni
nokkurra veðurþátta eftir veðurlagi á íslandi.
Veðurstofa íslands, Reykjavík, 51 pp.
Einarsson, M.A. 1984: Climate of Iceland. In: H.
Van Loon (Ed.).Climates of the Oceans, World
Survey of Climatology, Vol.15. Elsevier Science
Publishers, Amsterdam, p. 673-697.
Sigurðsson, F.H. 1964: Urkomumælingar í nágrenni
Reykjavíkur. Veðrið, 9: 31-34.
Veðráttan 1924-1985: Monthly and annual climato-
logical summaries. Veðurstofa Islands, Reykjavík.
JÖKULL, No. 38, 1988 67