A study of zenith luminance on Madrid cloudless skies

The zenith luminance (L-z) has been measured for cloudless skies at the IDM P station in Madrid. Mean Values obtained at every 15 min have been fitted vs. solar elevation (a). A 5th degree polynomial connecting log (L-z) again st (alpha) gives the best correlation in terms of the correlation coefficie nt (r = 0.9358). A possible explanation of the dependence of the zenith lum inance on solar elevation at low values is given. An exponential model obtained by other authors (Vazquez and Bemabeu, 1997) for a nearby temporary station in the same city does not account for the qu alitative dependence of L-z vs. alpha for alpha < <approximate to> 15 degre es, and also overestimates experimental values for high solar elevations. The effect of turbidity on L-z is studied categorizing available data in th ree specified groups of T-v, the visual or luminous/illuminance turbidity. Best fits with 5th degree polynomials connecting log (L-z) against (alpha) are given for: T-v < 3, 3 < T-v < 5, and T-v > 5. (C) 2000 Elsevier Science Ltd. All rights reserved.