The air flow over hills and mountains leads quite often to flow separation on the lee side of such obstacles. Two mechanisms are known which can cause this phenomena: either the steepness of the mountain itself or the occurrence of gravity waves. In order to investigate the dynamics and the flow structure an experiment was set up in 1999/2000 on the Isle of Arran in the west of Scotland. During a three month period absolute pressure, wind direction, wind speed, temperature and humidity were measured across the island. Flow separation could be detected on several occasions on an isolated hill called Tighvein in the south of the island. The separation can be linked to a synoptic flow from northwestly direction and a trapped gravity wave field generated by mountains to the north and propagating southwards. It is believed the separation on Tighvein is caused by this gravity wave field. Several case studies of this phenomenon will be represented.