An AIRMET is a message containing information issued by a meteorological watch office concerning the occurrence or expected occurrence of specified en-route weather phenomena which may affect the safety of low-level aircraft operations and which was not already included in the forecast issued for low-level flights in the flight information region concerned. In this context, low-level operations means flight below FL100 (or below FL150 or higher in mountainous areas). https://skybrary.aero/articles/airmet | 