Operation Fuller, a joint Royal Navy-RAF contingency plan, was devised to counter a sortie by the German ships against Atlantic convoys, a return to German ports by circumnavigating the British Isles or a dash up the English Channel. The concentration of British ships in southern waters was inhibited by a need to keep ships at Scapa Flow in Scotland, in case of a sortie by the German battleship Tirpitz from Norway. The RAF had been required to detach squadrons from Bomber and Coastal commands for overseas service and also kept torpedo-bombers in Scotland ready for Tirpitz, which constrained their ability to assemble large numbers of aircraft against a dash up the Channel, as did the winter weather which reduced visibility and unpredictably blocked airfields with snow.