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Horft af Vatnajökli yfir óbyggðir
Hofsjökull úr austri.

A new absolute minimum temperature record

for May in Iceland

6.5.2013

On 2 May the automatic weather station at Brúarjökull registered a minimum temperature of -21.7°C. This is the lowest temperature ever measured in Iceland in May. However, it should be kept in mind, that the station has only been in operation since 2005 and it is one of the coldest measuring sites in the country.

This cold spell is probably the most intensive one in May throughout the period of automatic observations in Iceland. The network was established 1993 to 1998 with slightly increasing number of stations since then. New minimum temperature records for May were registered at the majority of the automatic stations during the present cold spell.

The number of synoptic stations measuring temperature has decreased rapidly after 2004. A few of the remaining stations registered new minima for May during the present cold spell. These stations, except one, have only been in operation since 1988 or later. At the other synoptic stations the present minima did not exceed earlier records. Most of these originate from a cold spell in early May 1982. One station that has been in operation since 1924 (not quite continuously, however) registered a new absolute record on 2 May this year.

The present cold spell seems to have been the most intensive in May since at least 1982. It is not known how low the temperature at Brúarjökull was at that time.

On the measurements of minimum temperature in Iceland

At the “manual” stations the minima is measured with a traditional alcohol minimum thermometer. Readings are made two times per day, at 9 UTC and at 18 UTC. The lower of the two readings becomes the minimum of the day. In the middle of winter the daily minimum is quite often recorded at 18 hrs, but during other seasons most often at 9 in the morning.

At the automatic stations the minimum of the day is the lowest 2-minute average temperature during the 24 hour period from 00:01 to 24:00. At most of the automatic stations the temperature is registered every 10-minutes (a 2 minute mean) along with the highest and lowest 2-minute averages during the preceding 10 minutes. The automatic instruments have a somewhat faster response time compared to the traditional minimum thermometers placed in thermometer screens. In calm situations, fluctuations of the minimum temperature are common as the temperature inversion nearest to the ground strengthens and mixes at irregular intervals.

This fluctuating behavior of the minimum temperature has not been examined systematically in Iceland.

The validity of the new record

The old record minimum in Iceland was registered at the manned station Möðrudalur on 1 May 1977, -17.4°C. The new record is more than 3 °C lower than this and must be considered unquestionable even though the new record is made at an automatic station with different attributes than the traditional measurement.

The Brúarjökull station is located at 845 m above sea level at the snout of a very large and flat outlet glacier of the icecap Vatnajökull. This is well within the highlands of Iceland and almost 400 meters above the location of the earlier record station Möðrudalur.

Brúarjökull - Efralón
Brúarjökull 24 September 1993. View to the south across the western margin of Brúarjökull, which is the largest gletscher of Iceland. It is around 1500 square kilometers, flat and even. Many deflected curves can be seen on the surface. They are layers of ash from historical eruptions and the most prominent one is probably from an eruption in Grímsvötn in 1629. The station is located in a place near the left hand margin of the photo. Photo: Oddur Sigurðsson.

The first all-year highland station in Iceland was established in 1965 and a few others were in drift for a few years or seasons until the advent of automatic stations. It has been customary to distinguish between the highland stations on one hand and the stations in the inhabited areas on the other in the listing of weather records. However, regarding the minima, the inhabited stations in the very flat northeastern highlands have been almost on par with the higher level highland stations proper. The earlier May minimum record had thus persisted for 36 years even though many automatic stations have been in operation at higher levels for more than 15 years.

A new absolute minimum in the inhabited areas?

A new absolute minimum for the inhabited areas was also registered at the same hour as the new whole-country record. The minimum at the automatic station at Grímstaðir (another northeastern inland station) plunged down to -17.6°C, 0.2°C lower than the old record.

The automatic station at Grímsstaðir is new, however, established last autumn only. There, measurements at the old manual station are still ongoing. The in-screen minimum thermometer registered “only” -14.5°C on the morning of 2 May. The distance between the two stations is less than 100 meters.

The registrations at the new automatic station at Grímsstaðir have not been compared to the manual registration in a systematic manner and a post-installation calibration has not been made. Until that is done the new record will not be formally confirmed.

 




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