Jökull - 01.12.1988, Page 63

Precipitation in
Southwestem Iceland
MARKÚS Á. EINARSSON
Icelandic Meteorological Office
Bústaðavegi 9,150 Reykjavík, Iceland
ABSTRACT
This paper describes precipitation in south-
western lceland, which in this case includes the
Reykjanes peninsula, a mountain range lying from
the peninsula to the northeast in direction of
Langjökull and the extensive lowlands to the east of
the mountains.
Precipitation measurements are briejiy discussed.
Monthly and annual precipitation 1931-1960 is
described. Number ofdays with precipitation in lce-
land, both annual averages 1971-1980 and the dis-
tribution for defined weather types are shown, with
emphasis on the southwestern part. Finally the dis-
tribution of precipitation in southwestern Iceland is
shown for weather types in which considerable pre-
cipitation may be expected.
INTRODUCTION
Precipitation in southwestem Iceland shows con-
siderable variations within the region and depends
to a great extent on topography and local conditions.
Southwestem Iceland is in fact the only part of
the country, where the net of precipitation measur-
ing stations is dense enough to give a rather reliable
picture of the distribution, except for the mountains.
In this investigation southwestem Iceland extends
in the first place over the Reykjanes peninsula and a
mountain range lying from the peninsula towards
northeast in direction of Langjökull. There is a
depression in these mountains in the Lake Þingvalla-
vatn area, but Þingvallavatn is Iceland’s largest lake,
84 km2, about 100 m above sea level. The height of
the mountains is in most places below 1000 m, gen-
erally in the range 500-800 m above sea level.
Secondly the region covers the extensive lowland to
the east of the mountains where the height increases
gradually towards the interior.
PRECIPITATION MEASUREMENTS
Fig. 1 shows 46 locations in southwestem Iceland
where regular precipitation measurements have been
performed for a usable period of time, and in addi-
tion 8 places (unshadowed signs), where the dura-
tion of measurements for six of them was only a
year, and two of them were only in operation during
summers. At 40 stations measurements lasted a
decade or more, and at more than half of them 15-20
years or more.
At synoptic weather stations (only 8 in the region)
the precipitation is measured at 09 h and 18h GMT,
but at other stations only at 09h GMT.
In two mountainous areas within the region, one
around Lake Hvalvatn and the other in the Bláfjöll
area, measurements made with totalizators or col-
lecting rain gauges (opening 3.5-5 m above ground
and measurements once a month or even only once a
year) were of some help.
Rain gauges, which in Iceland have openings
about 1.5 m above the ground give too low precipi-
tation values, especially where wind velocities are
high and where considerable precipitation falls as
snow. No systematic investigations have been made
in Iceland to quantify this point, but preliminary
figures indicate that measured values for rain may
JÖKULL, No. 38, 1988 61