The North Atlantic weather war occurred during World War II. The Allies and Germany tried to gain a monopoly on weather data in the North Atlantic and Arctic oceans. Meteorological intelligence was important as it affected military planning and the routing of ships and convoys. In some circumstances, visibility was necessary and in others concealment. D-day planning was greatly affected by weather forecasting; it was delayed by one day in the expectation that a storm would blow out and sea conditions would be acceptable. British sources of data included ships at sea and the weather stations at Valentia Observatory and Blacksod Point, in neutral Ireland; German use of weather ships also exposed their secret Enigma codes.

Lauenburg was a German weather ship used in the early years of the Second World War to provide weather reports for German shipping, particularly German U-boats. After the German use of such vessels had been identified as a weakness that could be exploited to break the Enigma code, Lauenburg was captured and sunk on 28 June 1941. The Royal Navy acquired important German code books and parts of an Enigma machine.