A new low pressure record for July in Iceland

A remarkably deep low developed to the southwest of Iceland on 19 and 20 July 2012. According to the analysis of the ECMWF the central pressure in the low dropped to 966 hPa. This is amongst the lowest values ever observed in the North Atlantic in July.

After the low reached its maximum intensity it moved towards Iceland and filled gradually. In the evening of Sunday 22 July the pressure was at minimum near the south coast of Iceland. The lowest mercury barometer reading was 972.8 hPa, first at Vestmannaeyjar at 21 pm and then again at Kirkjubćjarklaustur 6 hours later. The lowest value at an automatic station was reached in Vestmannaeyjar during an extended period of one hour, 21:30 to 22:30, 972.4 hPa.

The pressure in Iceland has only on three occasions dropped down to 975 hPa or lower in the entire record extending back to the 1820s. These cases were 974.1 hPa in Stykkishólmur on 18 July 1901, 974.3 hPa in Stykkishólmur on 19 July 1923 and 975.0 hPa in Reykjavík 11 July 1912. It should be kept in mind that even the pressure was recorded at a few stations all over the country during the period from 1874 onwards, longer intervals passed between readings than now. Three readings per day were typical, but eight readings have been made at most of the barometric stations since the mid-1940s.

The low has brought rain to most of the country. Some parts have been experiencing an unusual drought since May. If the rain will alleviate the low water levels in the non-groundwater-fed rivers remains to be seen.

The country more or less escaped the high winds associated with the low as the centre passed just south of the country. The wind was strongest to the south of the low, as well as in the Greenland Strait (Denmark Strait) to the northwest of Iceland. The strongest gust associated with the low was measured 39.5 m/s at the station Steinar at the southern coast.


Temperatures in Iceland - as originally published

Due to the recent confusion regarding the Icelandic temperature series I have decided to post a list of monthly temperature averages as they were originally publised in the Icelandic and Danish yearbooks prior to 1960. Later original published data are available online at the website of the Icelandic Met. Office (IMO) (see the supplementary information attachment).

This post consists of three files, the data list (original_pub_temperature_values.txt), a station list (original_pub_temperature_stat_info.txt) and supplementary information (original_pub_temperature_values_suppl_info.txt) on the data format etc. Please feel free to download the attachments (viđhengi) at the end of this post.

The IMO has calculated series with some appropriate adjustments for each individual station. The IMO generally uses the version with adjustments made on pre-1961 data rather than the present dataset. These data are available on request (trausti@vedur.is) for individual stations but the whole dataset will be made available later.

It must, however, be emphasized that the adjustments will change in the future as more information on station history and inter-station comparison will be added. One should therefore expect changes in the adjusted dataset through time.  

The present dataset (original_pub_temperature_values.txt) will remain static, except for the addition of values from individual stations during the pre-1920 period which have not yet been digitized. Readers should note that these values are available in the Danish yearbooks (see a link in the supplementary information attachment).

Please feel free to comment and if you find any errors a note will be appreciated either by filling out the comment form (see below) or by sending your questions or notes to: trj@simnet.is

The comment form [The "athugasemdir" link near the bottom of the text] is in Icelandic, but most of it should be transparent to English readers. When you submit the comment you will be prompted for your name, e-mail (netfang - essential), vefslóđ (web link - not essential) and a question: What is the sum of two numbers? Just add the two numbers (within a short period of time) write the sum in the box and resubmit. This is for the spam-filter (ruslpóstsvörn).

[Dec. 4, 2012 The comment option has a timeout - please direct queries to trj@simnet.is}


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June 2012 in Iceland

June was very sunny in both western and northern Iceland. The total number of bright sunshine hours at Akureyri and Reykjavík were close to the all-time records. The total number of sunshine hours at these two stations in May and June as a whole is unprecedented.

It was a very dry month in most of the country. Parts of the west and northwest experienced the driest June (or even any month) ever. Stykkishólmur has the longest precipitation series in the country, the precipitation observations began in October 1856. June 2012 is the driest month on record, with the total being only 0.6 mm of rain. In some places in the northwest the earth is very dry and the vegetation suffers. A few farms are almost without clean running water and depend on tanking for water consumption. However, the main part of the winter had much precipitation in the area so there is some snow still surviving in the mountains feeding rivers and groundwater.

June was warm in the south and west, amongst the ten warmest on record in the Reykjavík area, but in the north and east it the temperature was slightly below average as northeasterly winds were prevalent.

The highest temperature in June was registered at the stations Hella and Árnes in the southern lowland on the 2nd, 22.8 degrees C. The lowest temperature of the month was registered at Gagnheiđi - a mountain in the east on the 5th, -5.7 deg C. The lowest temperature in inhabited areas was registered at Brú in the eastern inland, -4.4 deg C on the 13th. There were only ten nights entirely frost-free in the inhabited areas.

The full official report is available (in Icelandic) on the Icelandic Met. Office web.


May 2012 in Iceland

On the whole the temperature was close to the long-term average of the season. Actually the month was divided into two long spells, the first a very cold one lasting from the 6th to the 19th and a warm and sunny one for the rest of the month.

The weather was unusually sunny. In Reykjavík it was the 5th sunniest May since the beginning of observations in 1911 and in Akureyri (in the north) it was the second sunniest one, the observations starting there in 1928.

During the cold spell three new daily minimum records were set for the whole country. This was on the the 7th at Setur in the central highland, -13,7°C (older record, -13,4°C), on the 17th when the temperature at Brúarjökull in the eastern inland dropped down to -16,6°C (older one -11,0°C), and on the 18th also at Brúarjökull -13,1°C (older one -11,9°C). No new daily records for the country were exceeded in the inhabited areas where the observation series are much longer than in the highland. Many daily records where, however, exceeded at the individual stations during the cold spell, and also during the following warm spell.

The highest temperature measured in Iceland in May was 22,0°C at Kollaleira in the east on the 26th. The 20deg mark was exceeded somewhere in the country on six days in a row during the warm spell. This is unusual in May.

The full report for May (in Icelandic) is available at the web of the Icelandic Met. Office.


April colder than March in almost half of Iceland in 2012

April was colder than March in many parts of Scandinavia (met.no; smhi.se). This was also the case in the Eastern part of Iceland. On a whole this happened at 44% of the automatic stations. (Results are not in from all of the manned ones). 

That April happens to be colder in March is not that unusual, however. If all of the stations in all years are taken into the batch March is warmer than April about 17% of the time. On about 36% of the years at least one station is colder in April than in March.

One April out of 10 is colder than March at almost all (or all) of the stations. A majority of the stations had a colder April than March in 2010, but one has to go back to 1977 to find an all out April colder than March.

The likelyhood of a colder April after March is of course largest when March has been unusually warm. This was the case now.

But as a whole April was a rather warm month in Iceland, most of the stations had average temperatures above the 1961 to 1990 normal. Only a few stations in the East had below average temperatures.

The lowest temperature in the country was -25.7°C on the 2. (Brúarjökull, in the Eastern highland), but the highest 18.2°C on the 30. (Hallormsstađur, in a deep eastern valley).

April was rather dry in most areas.  

Official IMO April overview (in Icelandic only).


A new March maximum temperature record in Iceland

The last week has been warm in Iceland, especially in the north and east. Daily station records have been exceeded by the hundreds and also daily countrywide maxima on a few days. On Thursday 29 March the temperature at the farm Kvísker on the Southeast coast the maximum temperature reached 20.5 degrees C. This is a new March record for the whole of Iceland exceeding the old one by the large margin of 1.7 deg C.

The old record, 18.8 deg C, was set on 28 March 2000 in the town of Eskifjörđur in the Eastern Fjords.

The individual monthly temperature maxima in Iceland are:

MonYearMonthDaymaximum °CStation
January200011519,6Dalatangi East
February199821718,1Dalatangi  East
March201232920,5KvískerSoutheast
April200742923,0Ásbyrgi Northeast
May199252625,6VopnafjörđurNortheast
June193962230,5TeigarhornEast
July194671730,0HallormsstađurEast
August200481129,2EgilsstađaflugvöllurEast
September194991226,0DalatangiEast
October197310123,5DalatangiEast
November1999111123,2Dalatangi East
December2001121418,4SauđanesvitiNorth 

The official statement of the Icelandic Meteorological Office.

Dalatangi is a lighthouse on an peninsula at the East coast. It often experiences short „heat waves“ caused by decending warm air aloft along the very steep mountainside on the peninsula. The Dalatangi winter maxima have no diurnal preferences and often occur in the middle of the night. The station is however one of the most oceanic in Iceland the annual range of monthly temperatures is only 8.2 degrees C. At a few oceanic stations the heat waves of winter occasionally offer the highest maxima of the year. Later, I will give a list of these at this blog.

The station Sauđanesviti (Sauđanes lighthouse) is on the tip of a peninsula in the North, the topographical conditions are similar to the ones at Dalatangi.

The topography at Teigarhorn in the East is also similar to Dalatangi and Sauđanesviti. The warm descending winds are not as common as in the other locations, but the aspect to the noon sun in summer is more favourable.

Kvísker is located near the highest mountain in Iceland, Örćfajökull (a dormant volcano). The maxima at Kvísker typically occur in westerly winds blowing along the side and downwards the slope near the station.

The remaining stations are all located in the valleys of the northeast and east. Vopnafjörđur at the seaside in a broad fjord, Ásbyrgi in a very broad valley, but not far from the coast and Hallormsstađur and Egilsstađir airport well inland in the largest valley of the East.

 


Reykjavík - monthly means as originally published

Measurements in the modern style started in Reykjavík in May 1880. There are earlier observations available but made with unsheltered wall instruments. These can be added to the series back to 1866 and observations were also made in Reykjavík during 1820 to 1854. We will hopefully look at these early measurements later.

The history of the Reykjavík station is quite complicated. The Reykjavík station was operated by the Danish Met. Institute (DMI) from 1880 to 1910 and monthly mean temperatures were published in the yearbooks of the Institute (Meteorologisk Aarbog) which are available online (see an earlier post). During this period there were a few observers and the station was relocated within the town three or four times. A wall-mounted instrument screen was used during the first part of the early period but it was later replaced by a free standing one - the first one in Iceland.

The thermal characteristics of the early screens are better known than elsewhere in Iceland at this time as it was for a considerable period equipped with a thermograph which was read every hour. So we know the diurnal cycle quite well. It conforms well with later-time diurnal cycle observations. A few months of thermograph sheets are missing.

The main thermometer was read three times per day at fixed hours. The DMI always used the average of these + a separate fixed constant for each month (to account for the seasonal variation of the diurnal range). In comparison with modern methods these early calculations tend to have a daytime bias - resulting in slightly elevated temperatures. This is one of the main reasons for the adjustments that have been used later on the means. We will look at this problem later.

The climatic station in Reykjavík stopped making observations in December 1910, temperature measurements continued at a telegraph station in the town but data from this station were never published - and are not used in the attached dataset, except during 1920 and 1921 - which were published.

When the station in Reykjavík was discontinued in 1910 a new one was established just out of town at the newly built Vífilsstađir sanitarium. This location is slightly warmer in summer but colder in winter than the downtown stations. The published monthly means from this station are included in the attached dataset.

During the early period from 1880 to 1920 a few months are missing from the DMI publication. The Icelandic Meteorological Office (IMO) was established in 1920 and took over the DMI station net. Proper observations began again in Reykjavík (in a wall screen) in May 1920. Observations were made at fixed hours. In 1924 a thermograph was introduced again at the station and during 1924 to 1948 all the published temperature means in Reykjavík were calculated as the average of thermograph readings every two hours. There were daily comparisons of the thermograph and the fixed hour observations.

Since 1949 the Reykjavík monthly mean is calculated as the average of observations made every three hours.

The DMI also published the average monthly minimum temperature during the first few years. A minimum thermometer was, however, in use in Reykjavík during most of the early period, but the means were not published. The minimum thermometer was used for the calibration of the thermograph mentioned above during most of the 1884 to 1907 period.

The IMO has published monthly means of maximum and minimum thermometer readings since 1920. These means are included in the attached dataset.

wi-blog210312_s001t_original

The figure shows the annual mean temperature in Reykjavík 1881 to 2011 (as published). No external adjustments have been used.

Relocations are marked with vertical lines. In late 1931 the station was relocated to a rooftop in the town centre and remained there until the end of 1945. The data (above and in the attachment) have not been adjusted for this change nor others. Later versions of the dataset (e.g. the one available at the IMO website) do include adjustments for the relocations. All adjustments are subject to revisions at a later date.

Some internal adjustments are needed during the early part of the series due to later changes in calculation methods. The fixed-hour means that form the basis of both the DMI average method and recent adjustments by the IMO will be made available at this website later.

The data in the attachment are semicolon delimited with a period as decimal seperator.


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Snowcover in Reykjavík and Akureyri

Most people abroad think that there is a lot of snow in Iceland. This is true for some parts of the country. As a rule there is much more snow in the North and East than in the South and West and the snowfall is more intense and longer lasting in the mountains and the central highlands than in lowland areas. Outside the towns blowing snow is often a traffic hazard even though the snow cover is light.

In Reykjavík in the Southwest, and in most of the surrounding lowlands, the snowfall is surprisingly light and many winter days are entirely snow-free. The snow is a bit more persistent at Akureyri in the north. Until recently tourism in Iceland has been more or less concentrated in the summer, but in recent years the influx of tourists has been increasing in the winter as well. I receive more and more enquiries on the snow cover and below you can find two standard tables summarizing the situation in Reykjavík and Akureyri during the period 1981 to 2010. It must be noted however that there is considerable year-to-year variability in the snow conditions. The table is also found in a text-file appendix for easier viewing (see the bottom of the page).

Snow cover and snowfall in Reykjavík and Akureyri 1981 to 2010
table headings are explained below):

Reykjavík (Southwest part of the country)  

STOD    MON     ALHV    ALAU    SNB     SNFJ    SNDG    SLALLS  SNALLS  URKALLS TEL
1       1       13.6    12.7    51.7    78.7    13.8    42.1    12.4    83.0    30
1       2       12.8    10.5    54.0    80.6    14.0    49.1    12.4    85.9    30
1       3       12.1    12.4    49.6    81.6    15.1    55.3    11.1    81.4    30
1       4       3.2     23.1    16.9    62.9    8.0     27.1    3.9     56.0    30
1       5       0.2     30.5    1.2     38.0    2.1     11.1    0.8     52.8    30
1       6       0.0     30.0    0.0     13.0    0.2     0.8     0.0     43.8    30
1       7       0.0     31.0    0.0     0.7     0.0     0.0     0.0     52.3    30
1       8       0.0     31.0    0.0     0.1     0.0     0.1     0.0     67.3    30
1       9       0.0     30.0    0.0     7.3     0.9     3.3     0.0     73.5    30
1       10      1.0     28.6    5.5     33.6    3.9     18.8    0.9     74.4    30
1       11      6.2     20.6    25.9    61.5    8.7     37.2    5.2     78.8    30
1       12      12.2    15.0    45.4    73.8    13.5    54.3    10.0    94.1    30 


The earliest case of snow covering the ground in Reykjavík is on 9 September 1926. There are cases of snow covering the ground in Reykjavík in early June, but it has always melted
before the offical morning observation (09 hrs). 


Akureyri (Northern part of the country) 


STOD    MAN     ALHV    ALAU    SNB     SNFJ    SNDG    SLALLS  SNALLS  URKALLS TEL
422     1       21.2    5.2     75.7    90.3    15.5    25.0    30.0    60.7    30
422     2       17.2    5.1     71.5    88.5    13.5    14.8    32.1    50.6    30
422     3       16.5    6.6     66.0    88.5    15.6    19.1    25.3    49.7    30
422     4       8.8     13.5    42.2    82.2    10.4    12.2    12.4    28.5    30
422     5       0.5     27.5    6.5     59.6    4.1     8.0     0.8     22.2    30
422     6       0.0     29.9    0.3     30.6    0.5     1.8     0.1     20.7    30
422     7       0.0     31.0    0.0     7.2     0.1     0.2     0.0     32.1    30
422     8       0.0     31.0    0.1     1.1     0.1     0.4     0.0     41.5    30
422     9       0.1     29.5    1.0     19.7    1.9     7.0     0.1     46.8    30
422     10      4.7     22.1    22.3    60.5    8.9     25.6    9.9     72.2    30
422     11      13.5    9.9     56.1    80.9    12.3    26.6    18.7    58.8    30
422     12      17.8    8.6     64.7    86.6    14.2    19.6    29.3    57.0    30 


The earliest case of snow covering the ground in Akureyri is on 7 September 1940, the official snowcover observations started in 1924. The snow probably reached sea level in Akureyri during the first days of August 1912. There are a few cases of snow covering the ground in Akureyri in the middle of June.   Table headings:

STOD: station id
MAN : month (1-12)
ALHV : number of days with 100% snow cover
ALAU : number of days with no (0%) snow cover 
SNB : average snowcover during the whole month (%)
SNFJ : average snowcover at 500-700 meters in nearby mountains (%)
SNDG : Number of days with snowfall or (snow and rain mixed) as precipitation type
SLALLS : Average total precipitation (mm) as snow and rain mixed
SNALLS : Average total precipitation (mm) as snow
URKALLS : Average total precipitation (mm) (all types)
TEL : number of years (here: 1981 to 2010)end


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Links to published temperature series

Those interested in the original and adjusted monthly means of temperatures in Iceland are advised to check on these links:

There is no mystery surrounding the data from the Icelandic Meteorological Office.

The current version (2003) of the Reykjavík data is available at the website of the IMO (and has been for many years): http://www.vedur.is/Medaltalstoflur-txt/Reykjavik.txt 

and the Stykkishólmur series (has not been adjusted since 1975 – only updated: http://www.vedur.is/Medaltalstoflur-txt/Stykkisholmur.txt

and Akureyri (2003 version): http://www.vedur.is/Medaltalstoflur-txt/Akureyri.txt 

and Vestmannaeyjar (2003 version) http://www.vedur.is/Medaltalstoflur-txt/Storhofdi.txt 

and Teigarhorn (1981 version – only updated) http://www.vedur.is/Medaltalstoflur-txt/Teigarhorn.txt

Monthly values at all stations since 1949 (1961 for secondary stations) at (click on station name): http://www.vedur.is/Medaltalstoflur-txt/Manadargildi.html 

Eariler published values are available at the timarit.is website For 1924 to 1997 at: http://timarit.is/view_page_init.jsp?pubId=278&lang=is 

The text is in Icelandic but in 1925 to 1956 monthly values are found at the second page of every monthly report. After 1956 the table is on pages 4 and 6. Earlier original values are available at the same site.1920 to 1923 at: 


http://timarit.is/view_page_init.jsp?pubId=240&lang=is 

and 1873 to 1919 at:

http://timarit.is/view_page_init.jsp?pubId=241&lang=is 

Everyone can look up the numbers and compare both adjusted and earlier published values. Most of the later adjustments that have been made by the IMO are concerned with changes in the observing hours that neccessiate slight adjustments. The methodology of the current calculation of monthly means is described in detail in:


 

http://www.vedur.is/media/vedurstofan/utgafa/greinargerdir/1995/Climatological1960.pdf 

Exactly the same method has been in use since 1956.


Calculating monthly average temperatures in Iceland – part 2

The monthly diurnal temperature range can be defined in at least two senses; (1) as the difference between the average daily maxima and minima, and (2) as the difference between the highest average fixed hour mean (FHM) and the lowest such mean.

In low and middle latitudes the sun more or less fixes the diurnal temperature cycle, the 24-hr maximum usually occurs in the early afternoon, 1 to 3 hours after local noon and the minimum at sunrise. Sea breeze, state of ground etc. can modify this general rule.

At high latitudes, as in Iceland, the noon sun remains below 15° from about 20 October to 20 February. In the southern part of the country the feeble influence of the sun can be seen in long-term averages during the greater part of the low sun period, but in the north the influence is entirely absent in December and most of January. So - the diurnal range in the second sense above (2) is very small during winter in Iceland.

Synoptic disturbances, however, cause large fluctuations in day-to-day temperatures so the diurnal range in the first sense (1) remains considerable. The synoptic scale variability does not know about the time of day, the daily maxima and minima can occur at any time during a 24-hr period. They are not confined to the hours after local noon.

The first figure (below) shows the fixed hour diurnal variation (2) at the automatic station at Ţingvellir in the Southwestern interior of Iceland during January and June (15 years of data).

w-icewe-blogg150212a

Fig. 1

The red trace shows the range in June, the blue is January. These are relative means, the 24-hr average is set to 0°C. We can see that in June the maximum occurs about 1-2 hours after local noon (13:30z) [utc is used in Iceland during the whole year] and the minimum around sunrise (03:00utc - midnight at 01:30) in June. The daily range in sense (2) is 7.1°C in June.

In January the highest fixed hour mean occurs at 14 utc, so the sun clearly has some influence on the thermometer (probably more than on the air because of the setup of the sensor). The diurnal range (2) is just 0.9°C in January.

The diurnal range in sense (1) is, however, 7.5°C in January and 10.2°C in June at Ţingvellir (see figure 3 for the annual cycle of (1)).

We will also take a look at another station, for contrast. This is the station Seley, situated on a small and low island off the East coast of Iceland. This station has the smallest diurnal temperature range (2) in the country and also the smallest annual range (expressed as the average difference between the warmest and coldest month). The graph is plotted using the same range as the Ţingvellir plot for comparison.

w-icewe-blogg150212b

Fig. 2 

In January the daily range (2) is 0.1°C and in June it is only 1.0°C, January entirely flat and June nearly so. The diurnal range (1) at this station is 3.8°C in January and 3.2°C in June.

w-icewe-blogg150212c

Fig 3

The seasonal variation of the two senses of the diurnal cycle at Ţingvellir (W-Iceland inland). The difference between average daily maximum (tx) and minimum (tn) remains at 8 to 10°C the whole year (1), but the difference between the highest and lowest fixed hour means shows a much larger seasonal variation.

w-icewe-blogg150212d

Fig. 4 

The ratio of the two diurnal senses (2)/(1) shows the "persistence" of the diurnal cycle throughout the year for Ţingvellir (blue) and Seley (red). At Ţingvellir the daily solar cycle (2) dominates in summer but the synoptic daily variability (1) dominates in winter. At Seley the solar (2) diurnal cycle is small throughout the year so the synoptic disturbances dominate also in the summer.

As mentioned in an earlier post in this series, there is no universal agreement on the definition of the monthly mean temperature. In Iceland, combinations of fixed hour means that are supposed to approximate the full 24-hour mean have been used since the DMI took over the observations in 1873 and were continued after the establishment of the IMO in 1920. During the early and middle periods there was no universal (that is, for Iceland) formula in use. The Hovmöller reform (see an earlier post) that has been in effect since 1956 amended this state of affairs.

The next post will include a comparison of a few formulas that have been used in the calculation of monthly temperature means. These two stations will be used as examples.

Note: Icelandic mean time is = utc = z = GMT during the whole year.


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Iceland Weather blog

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Trausti Jónsson
Trausti Jónsson
Senior meteorologist at the Icelandic Met. Office. Speciality: Climatology
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