This article presents observational and numerical results on gravity waves in the Arctic polar vortex during the SAGE III—Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO) campaign. Long-duration balloons that were launched from Kiruna, Sweden, on 18 February 2000 provided in situ meteorological measurements for several weeks in the lower stratosphere. A strong gravity wave event was observed above southern Scandinavia on 2 March 2000. The main characteristics (amplitude of disturbances, frequency, and wavelengths) are reported, and it is shown that the wave induced mesoscale temperature fluctuations were large (18 K peak to peak). Furthermore, it is found that the gravity wave was most likely generated by flow across the Norwegian mountains. The observations are compared with results of numerical simulations. In particular, the mesoscale and ray-tracing simulations reproduced some features of the observed wave packet. However, the fluctuations induced by the wave were significantly underestimated in the general circulation model of the European Center for Medium-Range Weather Forecasting. Finally, the overall gravity wave activity during the flight is analyzed and is found to be relatively small.