Abstract.A system to record the ultraviolet (UV) spectra of atmospheric global irradiance with the miniature fiber optic spectrometer AvaSpec-256 was developed for continuous computer-aided spectrometry at Tartu Observatory in 2005. As a result, the database of spectra recorded with 15-mininterval round 24 h over 300-400 nm, has been developed. The quantities retrieved from the spectra have been compared with those measured by the Scintec erythemal UV-SET sensor and the Kipp & Zonen narrowband 306 nm sensor. Almost clear and overcast days were selected for comparison. Reliable results on the spectral distribution of the UV global irradiance as well as the integrated daily spectral doses could be obtained at least during the bright half-year. The results were compared with the calculations performed by means of the LibRadtran package. The biases in irradiance were significant at SZA above 70-75 • . At dominating larger SZA the recorded values need sophisticated corrections and remain less reliable. At lower latitudes than that of the study site (58.3 • ), the reliability of the spectrometer is expected to increase due to a smaller contribution of data measured at large SZA.The variations of the ratio of UV-A/UV-B irradiance, retrieved from the spectra, were investigated. Also the covariation of the narrowband 306 nm irradiance and the irradiance integrated over the whole UV-B range was studied. The biases between the UV-A/UV-B irradiances calculated by means of the LibRadtran package and measured with the AvaSpec were small at SZA below 70 • . At larger SZA the values of the ratio as well as the biases increased, significantly depending on total ozone.

Abstract.A proxy-based reconstruction of the erythemallyweighted UV doses for has been performed for the 26 • 28 E, 70 m a.s.l.) site. The pyrheliometer-measured daily sum of direct irradiance on partly cloudy and clear days, and the pyranometer-measured daily sum of global irradiance on overcast days were used as the cloudiness influence related proxies. The TOMS ozone data have been used for detecting the daily deviations from the climatic value (averaged annual cycle). In 1998-2004, the biases between the measured and reconstructed daily doses in 55.5% of the cases were within ±10% and in 83.5% of the cases within ±20%, on average. In the summer half-year these amounts were 62% and 88%, respectively. In most years the results for longer intervals did not differ significantly, if no correction was made for the daily deviations of total ozone from its climatic value. The annual and summer half-yearly erythemal doses (contributing, on average, 89% of the annual value) agreed within ±2%, except for the years after major volcanic eruptions and one extremely fine weather year (2002). Using the daily relative sunshine duration as a proxy without detailed correction for atmospheric turbidity results in biases of 2-4% in the summer half-yearly dose in the years after major volcanic eruptions and a few other years of high atmospheric turbidity. The year-to-year variations of the summer half-yearly erythemal dose in 1955-2004 were found to be within 92-111% relative to their average value. Exclusion of eight extreme years reduces this range for the remaining to 95-105.5%. Due to the quasi-periodic alternation of wet and dry periods, the interval of cloudy summers 1976-1993 regularly manifests summer half-yearly erythemal dose values lower than the 1955-2004 average. Since 1996/1997 midwinters have been darker than on average.

The variations of erythemally weighted ultraviolet (UV) irradiance and its relationships with atmospheric characteristics during 3 annual cycles (1998)(1999)(2000) at 1 site (Tõravere, Estonia; 58.3°N, 26.5°E; 70 m above sea level) were studied, with the further aim to estimate the erythemal doses for past years. In clear sky conditions the daily erythemal dose and noon irradiance exhibit total-ozone-related asymmetry in the annual cycle, with 27% lower values at spring equinox than at autumnal equinox. The clear sky ratio of erythemally weighted to broadband (300 to 3000 nm) irradiance increases from (0.5 ± 0.25) × 10 -4 to (2 ± 0.1) × 10 -4 with an increase in solar elevation from 10°t o 55°. In overcast low cloudiness conditions this ratio is higher. A simple empirical method to estimate the summer half-year erythemal dose for past years is proposed, and the doses were calculated for 1967-2000. A sharp drop in summer half-year sunshine duration and erythemal dose was found around 1975, with a following quasi-linear recovery to the values met in [1967][1968][1969][1970][1971][1972][1973][1974][1975]. A comparison with the measured and Total Ozone Mapping Spectrometer (TOMS)-derived data is provided for 1998-2000. In the fine summer of 1999 and the 'normal' one of 2000, the measured, estimated and TOMS-derived half-year doses agreed to within 0.7%. In the extremely cloudy summer of 1998, the difference between the measured and estimated values was 1.7% and between the measured and TOMS-derived values 6%.KEY WORDS: Erythemal ultraviolet radiation · UV dose · UV index · Atmospheric turbidity · Total ozone · Cloud factor · Relative sunshine duration Resale or republication not permitted without written consent of the publisherClim Res 22: 245-253, 2002, Josefsson & Landelius 2000, Sabburg & Wong 2000. The local UV radiation data records often cover time intervals too short to enable the construction of statistically confident climatic mean annual cycles or to find the trends or periodic changes. This difficulty to some extent is expected to be overcome by using statistical relationships between the UV radiation and major governing factors as the proxies.In the present paper we study the climatologically important relationships between the erythemally weighted UV and the potential proxies, with the final aim of estimating the erythemally weighted UV doses for past years. Summer half-year doses are calculated for 1967-2000 using the relationships found. INSTRUMENTS AND DATA SETSThe UV data were collected in 1998-2000 using the Scintec UV SET sensor at the Tartu/Tõravere Baseline Surface Radiation Network (BSRN) station (58.3°N, 26.5°E; 70 m above sea level). Simultaneous data from routine pyranometric and sunshine-duration measurements, using the Campbell-Stokes heliograph, as well as hourly cloud detection data were used.The Scintec UV SET sensor has an erythemally weighted spectral response (CIE 1987, DIN 5050) and is thermostated to avoid temperature dependence. The spectral response was checked at th...

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