In September 1991, the ERS 1 synthetic aperture radar (SAR) collected a set of four colocated images over the western Gulf Stream (GS). The SAR images were supplemented by satellite infrared imagery and measurements of the marine atmospheric boundary layer (MABL) from both a pair of surface buoys and a suite of instruments on the Ukrainian research vessel R/V Vernadsky, which between September 15 and 19 made six transects across the GS northern boundary within the SAR swaths. The influence of wind variations within the MABL was evident in each of the four SAR images. Evidence for thermal wind effects is seen in the changing character of the imagery as the mean wind across the sea surface temperature boundary changed direction from pass to pass. Both in situ data and analytical models suggest that much of the day-to-day SAR image variability was the result of changes in the character of the MABL, especially evident near the northern GS boundary where the sign of the atmospheric stability fluctuated over short spatial scales. Because many of the governing parameters of the MABL rapidly decorrelate, significant changes in the SAR imagery occurred during the 3 days between passes. Nevertheless, at least some manifestation of the GS was evident in the SAR imagery on each of the four passes.¿ 1997 American Geophysical Union