Jökull - 01.12.1991, Page 4

Fig.l: Weather stations used in this
investigation.
Mynd 1 : Veðurstöðvar sem einkum er
byggt á í þessari könnun.
weather stations were at 08, 14 and 21h Icelandic
Mean Time (IMT), or at 09, 15, 22h GMT. After that
observations at 21h IMT (22h GMT) were step by
step moved to 20h IMT (21h GMT). The GMT is
now used as a mean time in Iceland. For synoptic sta-
tions the hours of observation were mostly at 06, 13
and 16h IMT (07, 14, 17h GMT) in the first 2-3
decades and sometimes also including some of the cli-
matological hours, especially at 08h IMT (09 GMT).
Since 1924 monthly mean temperatures have been
published in Veðráttan, a publication giving monthly
and annual summaries of weather observations in
Iceland. Before that time the summaries were pub-
lished in íslenzk veðurfarsbók 1920-1923 and in the
Danish Meteorologisk Aarbog, II.Del 1873-1919.
For one weather station, Stykkishólmur, con-
formed monthly mean temperatures were already
available (Veðráttan, annual report 1975). For the
other 31 selected stations values are in this paper
revised in order to give an approximation to monthly
mean temperature obtained from eight equidistant
temperature measurements per day.
Various formulae were used to compute monthly
mean temperatures (for details see Einarsson, 1989).
The most frequently used formula for Icelandic
weather stations until 1955 was:
tm - 1/9 (2ts + 2t,4 + 5t2,)
where tm is the mean temperature and t8, t14, t2, mea-
sured temperature at 08, 14 and 21 h IMT.
The formula was in use for 15 of the 32 selected
stations. Hovmöller (1960) determined corrections
for each month of the year for 9 weather stations and
in addition for 11 stations in an indirect way. An
analysis of this data was sufficient to revise the val-
ues for the 15 selected stations.
It should be noted that special summer corrections
for the months May-August were in use for this for-
mula while Icelandic weather observations were pub-
lished in Meteorologisk Aarbog, i.e. 1901-1919.
These corrections were generally -0.1 °C, but -0.2° to
-0.3°C for some interior stations. They had to be
removed before using the ordinary corrections for the
formula.
For three of the weather stations starting before
1930 the following formula was used:
lm = 1/6 (5tg +117)
This formula was meant for stations not perform-
ing observations later than at 17h IMT.
New formulae were applied for synoptic stations.
For Reykjavík the following formula was in use
1911-1920 and for Akureyri 1920-March 1922:
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JOKULL, No. 41, 1991