The effect of sun elevation, cloudiness, and altitude on the ultraviolet index in Czechia

Ultraviolet (UV) radiation has recently become an important topic in relation to the loss of stratospheric ozone. High doses of UV radiation have a negative effect on many organisms. This paper focuses on the UV index (UVI), which expresses the risk of UV radiation on human health. The aim of the paper is to describe the definition of UVI, and its measurement, and to summarize geographical parameters and meteorological conditions affecting the values of UVI. The effect of sun elevation, cloudiness, and altitude is demonstrated using observed data from the Hradec Králové, Košetice and Labská bouda stations during the period 2011–2017. The results show a strong effect of both sun elevation and cloudiness. The highest values of UVI (up to 8) are generally observed on sunny days around midday from May to July. The reduction of the UVI caused by clouds, fog, and rain is, on average, 85% of values typical for sunny days. The effect of altitude is distinctly weaker; a rise of UVI with increasing altitude is 0.4 per 1 km for clear sky and the surface without snow cover.