A comparison of ultraviolet radiation measured at an arctic and an alpine site

Ultraviolet radiation contributes relatively little energy to the solar spectrum; however, it is very important because it is biologically very active. Measurements were carried out at a high altitude station in Switzerland (jungfraujoch 3576 m), and at a high altitude station in Alaska (Fairbanks 64.82°N) with identical instrumentation. For all season the UV flux for Jungfraujoch was larger than for Fairbanks. In summer the differences between the stations were less pronounced because the lower solar elevation is compensated by a longer day length. In winter the differences are more severe. For both stations the authors find an increased relative intensity of the UV (UV/Global) with increasing cloudiness, while the absolute values decreased with increasing cloudiness. This shows that the clouds absorb more in the near IR than in the UV region of the solar spectrum. For Fairbanks, the UV values in spring were substantially higher (mean value 18%) than for identical solar elevations after summer solstice. Cloudiness could not account for this, because the authors also observed differences for clear sky conditions. A simple model was developed, which took multiple reflections of the highly reflecting snow cover in spring into account, which correctly explained 83% of the observed differences. (author)