Selection of cloudless sky conditions by applying solar global ultraviolet irradiance measurements

Taking advantage of UV (295–385 nm) irradiance measurements is one of the objectives of this paper. A new index termed kt″ is established for this band. This new index works as a ‘zenith angle independent clearness index’ for band measurements and has similar applications to those of kt′ for broadband measurements. The new index may be applied to identify cloudless instants from UV band measurements. Both indexes were correlated throughout the period 1998–2004 with a R2 of 0.85. A selection criterion of kt″UV > 1.1 classified cloudless sky conditions with a probability of 95% in comparison with a selection that two criteria-applying broadband measurements would make. This index may be of interest for classifying cloudless sky conditions when only UV band measurements are available. An estimation method from the literature was applied to the period 1998–2004. This method was previously validated for the UV band with a measurement campaign made in Valencia (Spain) in the summer season.