Since 1983 a Swedish network of 12 solar radiation stations, measuring global radiation, G, and direct normal irradiance, E(N), is operated by SMHI. All stations are sited within the BALTEX area. During the fifteen years 1983-1997, for which a homogeneous radiation database has been built up, some clear features in the radiation climate show up. The largest difference in G on annular basis is between the stations Visby, located on the island of Gotland in the Baltic Sea, and Kiruna, the northernmost station. The mean annual total of G in Kiruna (2822 MJm-2) is 25% less than in Visby (3758 MJm-2). This is due to both latitudinal effects and differing cloud conditions. The effect of different cloudiness is clearly seen when comparing the stations Vaxjo and Visby, which are only separated by 0.74°in latitude. At the cloudier site in Vaxjo, G is on the average 12 % less than in Visby. At all stations there is a large year to year variation of 15 %, or more. During the period analysed there is a clear increasing trend in both G, E(N) and duration of bright sunshine at all stations. The trend in G averaged over all stations is + 7.2%/decade. This is mainly caused by decreasing cloudiness, especially during the summer months. Taking the atmospheric water vapour into account, the Angstrom turbidity coefficient, β, have been estimated from the measurements of E(N). Mean values of β during 1983-1997 are 0.082 in Lund (station with highest turbidity) and 0.056 in Kiruna (station with lowest turbidity). These values are strongly affected, approximately to the same extent, by the major volcanic eruptions of El Chichon and Mt. Pinatubo.
Persson, T. (1999). Solar radiation climate in Sweden. Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere, 24(3), 275–279. https://doi.org/10.1016/S1464-1909(98)00050-1