Danish Meteorological Institute Ministry of Transport
Total Page：16
File Type：pdf, Size：1020Kb
DANISH METEOROLOGICAL INSTITUTE MINISTRY OF TRANSPORT ——— TECHNICAL REPORT ——— 98-14 The Climate of The Faroe Islands - with Climatological Standard Normals, 1961-1990 John Cappelen and Ellen Vaarby Laursen COPENHAGEN 1998 Front cover picture Gásadalur located north west of Sørvágur on the western part of the island of Vágar. Heinanøv Fjeld, 813 m high can be seen in the north and Mykinesfjørdur in the west. The heliport is located to the right in the picture - near the river Dalsá. The photo was taken during a helicopter trip in May 1986. Photographer: Helge Faurby ISSN 0906-897X Contents 1. Introduction....................................................................................................3 2. Weather and climate in the Faroe Islands..................................................5 3. Observations and methods............................................................................9 3.1. General methods...................................................................9 3.2. Observations........................................................................9 4. Station history and metadata.......................................................................13 5. Standard Normal Homogeneity Test..........................................................15 5.1. Background.........................................................................15 5.2. Testing for homogeneity.....................................................15 6. Climatological normals.................................................................................17 6.1. Temperature.........................................................................17 6.2. Humidity..............................................................................23 6.3. Fog.......................................................................................24 6.4. Precipitation.........................................................................25 6.5. Snow and snow cover..........................................................28 6.6. Hours of bright sunshine......................................................29 6.7. Cloud cover..........................................................................30 6.8. Wind.....................................................................................31 6.9. Atmospheric pressure..........................................................38 7. Temperature, precipitation and hours of bright sunshine observed at Hoyvík/Tórshavn, 1922-1997.................................................39 7.1. Adjustment of original observations...................................39 7.2. Trends, 1922-1997..............................................................42 8. Conclusions....................................................................................................51 9. References......................................................................................................53 Appendix 1. Station and element catalogue Appendix 2. Element description Appendix 3. Contents of 3.5” floppy disks 1 2 1. Introduction To detect climate trends and changes above the fluctuating background, accurate and systematic ob- servations with calibrated instruments are required from all around the world. These observations form the basis for preparation of data series and calculation of standard normals to be used in climate studies, a increasingly important task both nationally and internationally. This report presents the observed climate of the Faroe Islands, mainly from 1961, based on climatological data series and normals, but a more general description of the weather and climate is also included together with trends in temperature, precipitation and sunshine in Tórshavn from the beginning of the 1920s. The main purpose of this report is to publish the various climatological parameters observed in the Faroes since 1961, including the climatological standard normals from the period 1961-1990, which is the latest standard normal period defined by the World Meteorological Organisation (WMO). WMO Technical Regulations define climatological standard normals as “averages of climatological data computed for the following consecutive periods of 30 years: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc.”. Climatological normals are defined from the same regulations as “period averages computed for a uniform and relatively long period comprising at least three consecutive ten-year periods”. Shorter series and series not sufficiently homogeneous must be referred to as provisional “normal” average values. Thus this report presents: Climatological standard normals from the period 1961-1990 for precipitation based on complete series of observations from six different sites in the Faroes. Climatological standard normals from the period 1961-1990 for temperature, cloud cover and relative humidity based on complete series of observations from two different sites (Tórshavn and Akraberg Fyr). Climatological standard normals from the period 1961-1990 for sunshine, wind, atmospheric pressure, snow and fog based on complete series of observations from one site (Tórshavn). Climatological normals or provisional normals covering other periods within the interval 1961- 1997 for precipitation (5 sites), temperature (5 sites), cloud cover (1 site), wind (2 sites) and relative humidity (1 site). All monthly values on which calculation of the normals is based (144 data series). Three longer data series covering temperature, precipitation and hours of bright sunshine from Tórshavn/Hoyvík in the period 1922-1997. The Faroe Islands are located in the North Atlantic between Scotland, Iceland and Norway and their climate is greatly influenced by the ocean. Section 2 of this report describes this climate in general terms with an overview of the "normal" climate of the Faroe Islands based on observations from the 3 normal period 1961-1990. An similar overview for the period 1931-1960 was published by Lysgaard (1969). Section 3 and 4 describe the various observations, station history, metadata and the methods used to calculate mean values for the period 1961-1990. The problems of studying long series of data and calculating long-term averages in cases where instruments, observation sites etc. have changed are also covered in this report. The rationale and methodology involved are explained in detail in section 5 and 7. Section 7 also present trends in temperature, precipitation and hours of bright sunshine for the period 1922-1997 observed in Tórshavn. The normals for the various parameters are presented in section 6 and conclusions concerning the observed climate and the variations are presented in section 8. Besides tables and graphics, the enclosed floppy disk contains the climatological standard normals, monthly values and information concerning the different measuring sites. 4 2. Weather and climate in the Faroe Islands The Faroe Islands (Føroyar) are situated at approximately latitude 62° N, longitude 7° W and consist of 18 small, hilly islands. The islands have a total area of 1399 km², and extend 113 km from north to south and 75 km from east to west. The highest elevations, reaching nearly 890 m above sea level are found in the northern islands. The climate in the Faroe Islands is greatly influenced by the warm Gulf Stream and by the passage of frequent cyclones, which arrive from the south and west depending on the position of the polar frontal zone. Consequently the climate is humid, unsettled and windy, with mild winters and cool summers. The Azores High is sometimes displaced towards the islands, in which case settled summer weather with fairly high temperatures may prevail for several weeks. During the winter time the course of the lows may be more southerly than normal, in which case cold air from the north dominates the weather. This situation may cause sunny weather with an unusually high frequency of days with frost and also snowfall. The latter occurs in conjunction with the build up of showers in the cold air above the relatively warm sea water. The northern part of the islands particularly almost always experiences wintry weather with snow or frost for a prolonged period during the winter time - occasionally some of the fiords freeze over with a thin layer of ice. The maritime climate is also influenced by the bifurcation of the East Iceland current (polar current), a branch of which is directed from eastern Iceland towards the Faroes. This sea current flows round the Faroe Islands in a clockwise direction. The mixing of the water masses causes a relatively large difference in the sea temperatures to the north and to the south or south west of the Faroe Islands as well as local variations in sea surface temperatures. The cooling of humid air masses by the cold sea water is a contributory cause of frequent fog in June, July and August. The precipitation pattern reflects the topography of the islands, the precipitation being smallest near the coastal areas and rising to a peak at the centre of the hilliest islands. Nearly all coastal areas receive around 1000 mm per year, rising to above 3000 mm in the central parts. Recent investigations (Davidsen et al.) have shown that some places receive more than 4000 mm. This precipitation distribution is attributable to both topographical and meteorological conditions. The topographical orographic precipitation occurs in conjunction with lows moving east and north east. The land lifts the air masses, leading to a discharge of precipitation. The amount and intensity