A study of zenith luminance on Madrid cloudless skies

The zenith luminance (L_z) has been measured for cloudless skies at the IDMP station in Madrid. Mean values obtained at every 15 min have been fitted vs. solar elevation (α). A 5th degree polynomial connecting log (L_z) against (α) gives the best correlation in terms of the correlation coefficient (r=0.9358). A possible explanation of the dependence of the zenith luminance on solar elevation at low values is given.An exponential model obtained by other authors (Vázquez and Bernabeu, 1997) for a nearby temporary station in the same city does not account for the qualitative dependence of L_z vs. α for α<≈15°, and also overestimates experimental values for high solar elevations.The effect of turbidity on L_z is studied categorizing available data in three specified groups of T_v, the visual or luminous/illuminance turbidity. Best fits with 5th degree polynomials connecting log (L_z) against (α) are given for: T_v<3, 3<T_v<5, and T_v>5.