The winter of 1939/1940 was one of the coldest on record, with persistent cold weather from 22nd December through January. Temperatures were the lowest for at least 100 years in many parts of Europe. It is now theorised that the intense military activity in the North Sea was responsible for disturbing the sea temperature and therefore the climate. The movement of ships in convoys, the laying of huge mine fields by both Britain and Germany, the sinking of ships and the widespread use of explosives to sink mines, and in the use of depth charges to seek to destroy submarines were responsible for ‘mixing up’ the warm and cold water layers of the sea. Up to 10,000 depth charges a month were being used to systematically hunt for submarines, using patterns of explosives designed to cover wide areas and different depths. The loss of heat from the sea led to more cold air from the arctic being pulled into the European region, resulting in much colder weather overall. See seaclimate.com.