'Bellwether' was the British movement of RAF squadrons from Old Sarum in the south of England to airfields in the Allies' Normandy lodgement after the start of 'Overlord' (June 1944).

The object of this and other operations of the same type was to build up the Allied air strength in Normandy as and when land suitable for rapid adaptation into airstrips became available. From these forward airstrips, Allied tactical aircraft were then able to provide more timely support for land forces without the delay which would otherwise have resulted from take-off and approach from bases in the UK, and also without the danger inherent in a long over-water return flight by damaged aircraft.