A mean pressure- and temperature-weighted photon pathlength in the atmosphere can be inferred from moderate resolution measurements in the O2 A-band. We show a pathlength retrieval method and calibration results for measurements from a Rotating Shadowband Spectroradiometer (RSS), and present the joint statistics of pathlength and cloud optical depth for cloudy skies observed at the Southern Great Plains (SGP) site from Sept. 30 to Dec. 22, 1997. Two different population branches are apparent in the scattergram of the pathlength vs. cloud optical depth; we attribute these to 1) single layer cases exhibiting small variations of pathlength enhancement over large optical depth ranges; 2) multiple layer cases with large variances of enhanced photon pathlengths. ¿ 1999 American Geophysical Union