Experimental and forecasted values of the ultraviolet index in southwestern Spain

The ultraviolet (UV) index is a standard vehicle for informing the public about the magnitude of the UV radiation reaching the Earth's surface and its potential harmful effects over the human health. The first objective of this article is to analyze experimental values of the UV index in three stations located in southwestern Spain during five years (2001-2005). The study reports reliable fundamental statistical parameters, which can serve as relevant values for comparison with other regions and model results. The results show a great influence of the astronomical factors in the seasonal evolution of the daily UV index showing higher values in late spring and summer. Thus high erythemal risk values of the UV index (between 7 and 9) occur in 41.6% of the total cases in Badajoz, 41.4% in Caceres, and 36.6% in Plasencia. These high percentages remark the great interest in reporting values of the UV index in the area of study. Besides the seasonal evolution, cloudiness appears as the main attenuation factor at short-term scales. Thus spring and autumn presented the highest dispersion because of their great variability in sky conditions. Also, changes in skewness and kurtosis are mainly related to the variability in cloudiness and particularly to the difference between the cloudless summer and the rest of the year. The second objective of the present article is to compare the values of daily UV index as calculated by different approaches: 30-min average, 10-min average, and 1-min average daily maximum erythemal irradiance, and value at solar noon. The results show that although the indices obtained by the different approaches are very similar for cloud-free days, they show interesting differences for cloudy days. Finally, this paper aims to compare the UV index measured at the mentioned three locations, with the UV index forecasted using the SBDART radiative transfer code and values of total ozone provided by TOMS. The predictions are always calculated assuming cloud-free conditions. The comparison between experimental and modeled values shows that the forecasted UV index values have an absolute mean error between 7% and 11%, depending on the location and on the delayed daily value of total ozone used as input for the model.