Measurements of UV radiation on rotating vertical plane at the ALOMAR Observatory (69° N, 16° E), Norway, June 2007

Erythemaly weighted UV and total UV-A irradiance measured at the ALOMAR (Arctic Lidar Observatory for Middle Atmosphere Research; 69° N, 16° E) in June 2007 by two Kipp & Zonen UV broadband meters type, UV-S-EA-T, are examined. One unit is mounted on rotating vertical plane and the other is permanently fixed horizontally. The UV broadband meters measure simultaneously to compare UV irradiances on vertical and horizontal planes. The entire range of such relative exposure variations during clear-sky and overcast conditions over ALOMAR in the period March–June 2007 is examined using STAR and Radonic1 model (developed at the Meteorological Institute, University of Munich) for various action spectra: erythema, UV-A, and vitamin D3. The model and observations support that the daily means of relative exposures are quite stable, i.e., vary within the range 0.4–0.6 with the mean around 0.5 when the averaged intra-day, day-to-day, and seasonal changes of the relative erythemal exposures are considered. It seems that multiplication of the daily mean dose from a broadband meter placed horizontally by the factor of 0.5 gives reasonable estimation of the daily mean exposure on a vertically oriented receiver randomly oriented towards the Sun. The model studies during clear-sky conditions show that the extreme value and daily variability of relative exposure are the highest for UV-A, next for erythemal UV, then for vitamin D3 weighed UV irradiance. The minima of relative exposure (~0.20–0.30) are almost the same for all weighting functions. The comparison of model simulations and measurements suggests that specific cloud configuration could lead to significant enhancement of UV exposure of rotating receiver