Cloudy sky average photon path lengths were measured using the gamma band of oxygen near 628 nm on many days over Boulder, Colorado, using a zenith-looking spectrograph with a resolution of 0.8 nm. The approach allows accurate measurement of the average photon path length. Days characterized by relatively extensive cloud cover are examined here, which exhibit very large path length variations as the cloud fields overhead evolve. The ability of a plane parallel line-by-line model with a single uniform cloud layer to predict the measured O2 path lengths has been tested by constraining the model to the (independently) observed surface irradiance. Overall, its performance is quite good, which demonstrates that the relationship between the diffuse transmission by clouds and the average photon path length is consistent with plane parallel radiative transfer calculations for the conditions studied. Thus large errors in the cloud radiative transfer of such models, as suggested by some recent cloud absorption measurements, are not supported by this study. Furthermore, direct observations in the blue and red spectral regions do not support even a 1% differential absorption between these spectral regions. ¿ 2001 American Geophysical Union