Meterological conditions are described during NASA's Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) that was conducted over the North Atlantic Flight Corridor (NAFC) during October and November 1997 to study the impact of aircraft emissions on atmospheric concentrations of NOx and ozone. The SONEX period exhibited frequent closed cyclones and anticyclones, as well as high-amplitude troughs and ridges. These flow patterns often caused aircraft exhaust from the NAFC to follow broad looping north-south trajectories, instead of more easterly routes that would have occurred if the flow had been more zonal. Mean meterological conditions during SONEX include a pronounced long wave trough over the East Coast of the United States, as well as weaker low pressure over middle-latitude portions of the Atlantic Ocean. Conversely, a well-developed ridge was apparent over the North Atlantic near Iceland. Cloudiness exceeded climatology off the East Coast and the subtropical North Atlantic, with abundant lighting in these regions. There was less than average cloud cover over the middle latitudes between Newfoundland and central Europe. The tropopause was higher than climatology over much of the SONEX region, and the jet stream was located north of its typical position. These circulation features during SONEX are consistent with typical year-to-year variations. Meterological conditions during individual SONEX flights also are described. Upper tropospheric flow patterns, 5-day backward trajectories from the flight tracks, tropopause heights, lighting data, and differential absorption lidar ozone imagery are employed. Effects of aircraft were observed on numerous flights. Stratospheric conditions were encountered during many flights, sometimes because the DC-8 passed through a tropopause fold. SONEX flight tracks frequently were downwind of regions of lightning, especially during flights from Bangor and the Azores. Finally, trajectories indicated that continental pollution signatures observed during some flights had originated over the United States. ¿ 2000 American Geophysical Union