SNfactory observations of the standard star GD71 under various level of cloud extinction during non photometric nights. The top panel shows the flux-calibrated spectra from both spectroscopy channels, without grey extinction correction, compared with the standard star reference template (green). The middle panel shows the additional extinction δT(λ,ẑ,t). The bottom panel shows the variation with wavelength of the ratios spectra/reference. It is notable that even with strong extinction by clouds the transmission is compatible with grey extinction (δT(λ,ẑ,t) = δT(ẑ,t)) at the  ~1% level (grey band).