The midnight sun removed the darkness that could shield a convoy from visual patrols. German aircraft and surface ships could search or engage throughout the day because visual detection remained central to their methods. Steadier summer weather also gave submarines more time to operate. Winter was far harsher for crews, but its long darkness made the route tactically safer.
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How the Arctic summer exposed convoys
Constant summer daylight made Arctic convoys easier for German aircraft, surface ships, and submarines to find and attack.