Comment on the clear-sky index and on the variogram

Abstract

The work presented here has a major interest in the measurements of the downwelling solar irradiance at surface. The authors put emphasis on the production of electricity of PV panels but their work will reach a broader community as solar irradiance has much broader applications. The amount of work is impressive and I may highlight several points. However, I am not a reviewer and I will keep to a few points and questions.