Effect of the relative optical air mass and the clearness index on solar erythemal UV irradiance

• Correlation of index of clearness and UVER irradiance.
• Correlation of index of air mass relative optical and KtUVER.
• Statistical interrelationship between index of clearness and air mass relative optical.
• Analysis of the data dispersion of the UVER solar radiation.
• Evaluation of the coefficients of the correlation function of the ozone levels.

This paper analyses the effects of the clearness index (Kt) and the relative optical air mass (mr) on erythemal UV irradiance (UVER). The UVER measurements were made in Valencia (Spain) from 6:00 am to 6:00 pm between June 2003 and December 2012 and (140,000 data points). Firstly, two models were used to calculate values for the erythemal ultraviolet irradiance clearness index (KtUVER) as a function of the global irradiance clearness index (Kt). Secondly, a potential regression model to measure the KtUVER as a function of the relative optical air mass was studied. The coefficients of this regression were evaluated for clear and cloudy days, as well as for days with high and low ozone levels. Thirdly, an analysis was made of the relationship between the two effects in the experimental database, with it being found that the highest degree of agreement, or the joint highest frequencies, are located in the optical mass range mr ∈ [1.0, 1.2] and the clearness index range of Kt ∈ [0.8, 1.0]. This is useful for establishing the ranges of parameters where models are more efficient. Simple equations have been tested that can provide additional information for the engineering projects concerning thermal installations. Fourthly, a high dispersion of radiation data was observed for intermediate values of the clearness for UV and UVER.