The present paper analyses the potential vorticity streamer which crossed the Alps during the Special Observing Period of the Mesoscale Alpine Programme (MAP) on 6 November 1999. Insitu aircraft data, data remotely sensed from the aircraft, wind profiler data, model output data from MESO-NH, and ECMWF analyses are used to describe structure and evolution of the streamer. In two cross sections cutting through the streamer simulated and analysed data are compared with measured humidity data. The comparison of the three data fields show good agreement in structures and magnitudes. It will be shown further that MESO-NH simulations indicate that mountain generated gravity waves are transmitted into the stratosphere as soon as the streamer approaches the Alps. The occurring breaking of these waves induces the streamer's break-off above the Alps cutting the streamer into two pieces, the southern part forming a cut-off low.