The 0.55 &mgr;m optical depth of the Saharan dust aerosol plume is determined from C-130 pyranometer data for two different days and the results compared to those from a 2-channel Advanced Very High Resolution Radiometer (AVHRR) retrieval algorithm. When the time difference between the C-130 and AVHRR overpasses is small, the geographic distribution of the aerosol optical depths are similar and differ by less than +/- 0.1 despite the different refractive indices, size distributions and atmospheric profiles implicit in each method. The pyranometer derived optical depth at 0.55 &mgr;m exceeds a value of 1 on one of the days, which exceeds the cloud threshold used in many satellite retrieval algorithms with implications for the accuracy of satellite derived aerosol optical depths. The difficulties in making extensive geographical comparisons between satellite and aircraft measurements owing to the differential speed of the observing platforms are also highlighted. ¿ 2001 American Geophysical Union