We describe a family of inversion methods to infer the optical depth, &tgr;, of warm clouds from surface measurements of spectral irradiance. Our most complex retrieval also uses the total liquid water path measured by a microwave radiometer to obtain the effective radius, re, of the cloud droplets. We apply these retrievals to data from the Atmospheric Radiation Measurement (ARM) Program, and compare our results to those produced by the GOES satellite for episodes where total overcast was observed. Our surface-based estimates of &tgr; agree with those from GOES when the optical depths are <10, but are consistently larger by as much as a factor of 2 when optical depths are greater. We show that the uncertainties associated with the surface-based retrievals are less than those done from a satellite, and argue from the time series of the observations and the statistics of the measurements that the disagreement is not merely a consequence of the larger spatial average sampled by the satellite. ¿ American Geophysical Union 1996