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The dynamic climate of Iceland

"If you don’t like the weather right now, just wait five minutes," people sometimes say in Iceland. This is an indication of the strong variability of the Icelandic climate, where one may occasionally experience the four seasons over a day: sunshine and mild temperatures; windy, cool temperatures and rain; snow and temperatures below zero degrees C. When this happens (rarely), it is an expression of the location of Iceland at the border between Arctic and temperate seas, and between cold air masses of the Arctic and warm air masses of lower latitudes.

Ocean currents and sea temperatures - Iceland, located at 63-67°N and 18-23°W, has considerably milder climate than its location just south of the Arctic Circle would imply. A branch of the Gulf Stream, the Irminger Current, flows along the southern and the western coast greatly moderating the climate (Figure 1). The cold East Greenland Current flows west of Iceland, but a branch of that current, the East Icelandic Current, approaches Iceland’s northeast- and east coasts. This is reflected in the coastal sea surface temperatures around Iceland. They are generally close to +2°C during the coldest months (January-March). Sea temperatures rise to over +10°C at the south- and west coasts of Iceland during the summer, slightly over +8°C at the north coast, but are coolest at the east coast where summer sea temperatures remain below +8°C. During years with heavy sea ice off northern Iceland, sea temperatures during summer can remain close to winter temperatures.


Figure 1: Present oceanographic surface currents around Iceland. Image source: http://www.hi.is/~jeir/panis_currents.html

The maritime climate of Iceland – A simple classification of Icelandic climate puts it as cool temperate maritime, reflecting that it is very influenced by the cool ocean waters around Iceland. A map of the annual mean temperature (Figure 2) shows that only along the coasts of southern and southwestern Iceland do temperatures reach 4-6°C, but are lower in other parts of the island.


Figure 2: Annual mean temperatures in Iceland. Image source: Veđurstofa Íslands at http://www.vedur.is/vedurfar/vedurfarsmyndir/EV_DTO/ann.html

The average temperature of the warmest month, July, exceeds 10°C in the lowlands of southern and western Iceland, but is below that in other parts of the country (Figure 3). This makes the larger part of Iceland belonging to the arctic climate zone. The warmest summer days around Iceland can reach 20-25°C, with the absolutely highest temperatures recorded at around +30°C.  


Figure 3. Mean July temperature in Iceland. Image source: Veđurstofa Íslands at http://www.vedur.is/vedurfar/vedurfarsmyndir/EV_DTO/jul.html

Winters in Iceland, on the other hand, are generally very mild for this northerly latitude (Figure 4). The coastal lowlands have mean January temperatures close to 0°C, and only in the highlands of central Iceland do the temperatures stay below -10°C. The lowest winter temperatures in northern Iceland and the highlands are generally in the range -25 to -30°C, with -39.7°C the lowest temperature ever recorded.


Figure 4. Mean January temperature in Iceland. Image source: Veđurstofa Íslands at http://www.vedur.is/vedurfar/vedurfarsmyndir/EV_DTO/jan.html

The pattern of precipitation in Iceland reflects the passage of atmospheric low pressure cyclones across the North Atlantic Ocean from south-westerly directions, exposing the south coast to heavy precipitation (Figure 5).


 

Figure 5: Mean annual precipitation in Iceland for the period 1931-1960: 1. < 600 mm; 2. 600 – 1199 mm; 3. 1200 – 1999 mm; 4. 2000 – 3999 mm and 5. > 4000 mm. Figure source: http://www.nnv.is/skrar/DFHM03%20p167-178.pdf

The average climate in Reykjavík, in the table below, illustrates what can be expected in terms of temperatures and precipitation (Figure 6).

Month

Mean Temperature oC

Mean Total Precipitation (mm)

Mean Number of Precipitation Days

Daily
Minimum

Daily
Maximum

Jan

-3.0

1.9

75.6

13.3

Feb

-2.1

2.8

71.8

12.5

Mar

-2.0

3.2

81.8

14.4

Apr

0.4

5.7

58.3

12.2

May

3.6

9.4

43.8

9.8

Jun

6.7

11.7

50.0

10.7

Jul

8.3

13.3

51.8

10.0

Aug

7.9

13.0

61.8

11.7

Sep

5.0

10.1

66.5

12.4

Oct

2.2

6.8

85.6

14.5

Nov

-1.3

3.4

72.5

12.5

Dec

-2.8

2.2

78.7

13.9

Figure 6: Weather Information for Reykjavik. Climatological information is based on WMO Climatological Normals(CLINO) for the 30-year period 1961- 1990. Source: The World Meteorological Organization, http://www.worldweather.org/097/c00189.htm

The dominant wind directions in Iceland are from easterly directions, E, NE-SE, and reflect the passage of cyclones on paths just south of Iceland (Figure 7). Westerly and north-westerly winds are infrequent.

Figure 7. Frequency of wind directions on Iceland, data from 1930-1960. Image source: Einarsson (1976).

Regionally and locally both wind directions and wind speeds are highly influenced by local topography and altitude. Generally, wind speeds are higher in the highlands than the coastal lowlands, but local topography can canalize winds and cause very high winds in some lowland valleys. The highest wind speeds measured in gusts only very rarely exceed 50 m/second. The frequency of storms is highest in the fall and during the winter months. Storm days per year, with average wind speeds exceeding 18 m/second, are generally 10-20 in the lowlands, but at places >50 in the highlands and at exposed outer coastal areas. The large ice caps, Vatnajökull in particular, can generate strong catabatic winds. 

Dust storms –  Strong (>15-20 m/second), dry winds coming off the interior or the large ice caps onto the large proglacial sandur areas and the arid highlands can generate heavy dust storms (Figure 8).

Figure 8. A major dust storm coming off the dry highlands and sandur fields of Southern Iceland, 5th of October 2004. Image courtesy of MODIS Rapid Response Project at NASA/GSFC; http://rapidfire.sci.gsfc.nasa.gov/gallery/

The dust storms are very effective in eroding and transporting soil materials, and it has been calculated that more than 10 tons of material can be in motion across every transect meter per hour. In the arid highlands north of the Vatnajökull ice cap, where vegetation cover is sparse and the soils heavily affected by volcanic tephra fallouts, dust storms are very frequent in the summer and early fall (Figure 9).

Figure 9. Dust storm in the highlands north of Vatnajökull, August 2004. Photo: Ólafur Ingólfsson. 

Thunderstorms – Thunderstorms are very rare in Iceland, but do occasionally (less than 5 days per year) occur in southern Iceland in late summertime when warm air is deflected to northern latitudes from warm air masses over Europe. Thunderstorms can also occur in connection with deep lows approaching from the southwest in winter. Cold air drawn off Canada warms rapidly over the ocean, forming thunderclouds. Lightning during a thunderstorm in March 1865 killed three people in southwestern Iceland. Usually, lightnings are observed in connection with ash plumes from volcanic eruptions.

Figure 10. Satellite photo and meteorological chart over Iceland and the North Atlantic, 16th of September 2003. Image source: http://www.fsu.is/~ornosk/vedur/laegd.html

The Icelandic Low and the North Atlantic Oscillation – The Icelandic Low is a term adopted by the meteorological community for describing the semi-permanent low pressure centre near Iceland (mainly between Iceland and southern Greenland) on mean charts of sea-level pressure. It is a principal centre of action in the atmosphere circulation of the Northern Hemisphere (Figures 10 and 11).

Figure 11. Deep low pressure area off Iceland, September 2003. Image courtesy of MODIS Rapid Response Project at NASA/GSFC; http://rapidfire.sci.gsfc.nasa.gov/gallery/

As this semi-permanent low-pressure system intensifies and weakens, it affects the amount of air (generally warm) being brought into the Arctic to the east of the low and the amount of air (generally cold) being swept out of the Arctic to the west. The Icelandic Low is part of a larger weather pattern called the North Atlantic Oscillation (NAO). NAO is the name for changes in the difference of air pressure between the Icelandic Low and a semi-permanent high-pressure system centered near the Azores Islands. Both of these systems are present all year; however, both are strongest in winter. When both the high and the low intensify and fluctuate in pressure relative to one another, they change the circulation of cold and warm air in the region.

Finally... When visiting Iceland, expect fairly good weather,  but prepare for spells of bad weather. Summers in Iceland are generally fairly mild and calm, but occasionally a deep low pressure cyclone can come charging with rain and storm. Weather in the highlands can be very unstable and unpredictable. Be sure to pack sunglasses and T-shirts, as well as raingear, warm clothing and windbreakers...

References, web links, further reading

Einarsson, M.Á. 1976: Veđurfar á Íslandi. Reykjavík, Iđunn, 150 p.

Einarsson, M.Á. 1989: Hvernig viđrar? Reykjavík, Iđunn, 152 p.

http://www.fsu.is/~ornosk/vedur/laegd.html

http://www.vedur.is

http://www.ldeo.columbia.edu/NAO/

http://www.hafro.is/Bokasafn/Timarit/steingr.pdf

http://www.worldweather.org/097/c00189.htm

http://www.hi.is/~jeir/panis_currents.html