And just for the record, it looks like there is a CIE standard for sky values :
This web app generates sky luminance and radiance distributions using the updated Perez All-Weather Sky model, as defined in ISO 15469:2004(E) and CIE S 011/E:2003. You can dynamically adjust each of the Perez sky coefficients individually, choose any of the 16 CIE Standard General Skies, set your own direct and diffuse illuminance/irradiance values, or use annual hourly weather data for dynamic simulations or cumulative sky distributions. It also provides a range of different sky subdivision techniques and resolutions, including Tregenza’s 145 patch sky and Reinhart’s extensions.
The Commission Internationale de l’Éclairage (CIE) has defined a set of 15 standard sky types for modelling the radiance/luminance distribution under a wide range of weather conditions from overcast to cloudless, and with or without direct sunlight. The 16th varient corresponds to a much simpler mathematical model for overcast skies defined and used by CIE prior to this standard.
The various sky types are generated using different values for each of the 5 sky coefficients shown in the panel above.
Quite cool ! ;-)