Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.
Direct and global solar radiation measurements carried out in Kathmandu valley (N, E 1350 masl) have been analyzed in this study. Observations were made using a microprocessor controlled portable aerosol meter Microtops II working in ultraviolet (340, 380 nm) and visible range (440, 500, 675 nm) under cloudless conditions. Aerosol optical depth at 380 nm was found to be 0.33, 0.48, 0.21 and 0.45 in summer, autumn, spring and winter season respectively. Monitoring of global solar radiation was done with a ground based ultraviolet radiometer (GUV). From the aerosol optical depth from Sun Photometer, Angstrom turbidity parameters were estimated by means of the least square fitting technique. The aerosol optical depth is also utilized to make a model fit for UV index and compare the GUV data. A decrease in humidity by about 70% is found to increase the UV index by about 22%. The atmospheric turbidity parameter (β) is found high in the morning and show decreasing trends from morning to late afternoon on average. The wavelength exponent (α) becomes smaller during noon indicating existence of higher proportion of tiny particles in lower atmosphere in day time. Finally an inversion of spectral optical depth measurements technique allowed estimating aerosol size distribution changing between unimodal to bimodal functions yearly.
A weatherproof spectroradiometer has been developed for spectral measurements of direct, global and radiance ultraviolet radiation. The core of the instrument is a Bentham D150 double monochromator with two input entrances with a swing away mirror. Two 4.0 m quarts fibers connect the optical input to the monochromator. Global irradiance was measured with an ideal cosine diffusor. Direct irradiance and radiance measurements were performed with a telescope having a field of view 1.5° mounted on a weatherproof sun tracker. The whole system is operated by an integrated software, running the tracker and the spectroradiometer at the same time. Calibration of the direct unit and the global irradiance were done with a 1000 W FEL lamp. Langley calibration was found more difficult due to changing atmospheric conditions and difficulties having proper clear sky days. Radiance distribution was measured two days in May 2003, showing similar pattern as for other places with a low aerosol content.
The Robertson-Berger sunburn meter, model 500, has no temperature compensation, and the effect of temperature on the instrument response has been investigated and discussed in several reports. It is recommended to control the temperature of the detector or at least measure it. The temperature sensor is recommended to be positioned within the detector unit. We have measured the temperature at three different positions in the detector: At the edge of the green filter where the phosphor layer is placed; at the glass tube covering the cathode; and, finally, the air temperature inside the instrument. These measurements have been performed outdoors since July 1991, with corresponding measurements of the global and direct solar radiation. There was no difference between the temperature of the glasstube covering the cathode and the air inside the instrument, at any radiation level. However, there was a difference between the green filter and the two others. The difference is linearly dependent on the amount of global radiation. The temperature difference, (Delta) T (temperature between the green filter and the air inside the sensor), increased 0.8 degree(s)C when the global irradiation increased by 100 W/m<SUP>2</SUP>. At maximum global radiation in Trondheim (latitude 63.4 degree(s)N) (Delta) T was approximately 5 - 6 K when the global radiation was about 700 W/m<SUP>2</SUP>. This was valid for temperatures between 7 degree(s)C and 30 degree(s)C. Only clear days were evaluated.