Abstract
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 2 [1.0, 1.2] and the clearness index range of Kt 2 [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.
