A series of wave-like patterns in ozone mixing ratio, potential temperature, and dew point were observed during aircraft horizontal sampling at a constant elevation of approximately 1700 m over the Gulf of Maine and the western North Atlantic. Several hypotheses about the cause for such phenomena are examined. Both the observational evidence and a simple theoretical analysis suggest that the most likely explanation is the atmospheric perturbation associated with internal gravity waves trapped in a stable boundary layer. ¿ American Geophysical Union 1996