The relevance of naval war on weather had been revealed to science in a neatly tied parcel for a long time. A number of scientists merely expressed their astonishment, but did not look for the physical causes, like Drummond:

· The present century has been marked by such a widespread tendency towards mild winters that the ‘old-fashioned winters’, of which one had heard so much, seemed to have gone for ever. (Drummond, 1943)

Rodewald (1948) emphasized that the winter of 1939/40 had come suddenly, and in contrast to the principle of conversion of the circulation and temperature deviation. Rodewald points to the air pressure aspects in the Atlantic during the months preceding these winters (1939-42) as follows:

· “From October to November a huge area of low depression covers most of Europe . The center with –11mb (from mean value) is stationed between Norway and Shetland, which is usually south of Iceland …..December shows an inverse picture. Europe is dominated by a pressure increase of +12mb (from mean value) with the center west of the Hebrides .”

Liljequist (1943) identified this group of winters as the coldest ever observed in central Sweden , and presumably for the eastern Baltic as well.

Which further observation does atmospheric science need to become active? Over the last 100 years every weather expert familiar with Europe would have told you that three cold winters in a row are highly unlikely. The Russian campaign of the German Army in winter 1941/42 was severely punished by General Frost. It was the fault of the forecasters, because they had been absolutely convinced that after the two cold winters 1939/40 and 1940/41 a third cold winter was unlikely if not impossible. But they were completely wrong. The 3rd WWII winter topped the first two war winters at a number of locations.

The first three war winters did not only very suddenly end a long period of an increasing trend of milder winters since the middle of the 19th century, but even exceed conditions of the Little Ice Age, by coming up with three winters in succession that matched the lowest winter condition during the Little Ice Age (LIA). If there are strong indications that the war in question came along with the lowest temperature over a period of 300 years from about 1550 to 1850, it seems time to ask some serious questions.

It is worth noting that only Europe experienced the three arctic winters during 1939/40, 1940/41 and 1941/42. North America and Asia did not go through the same experience.

b. Warm & warmer – The situation prior to WWII

The last decade before WWII was the warmest period since meteorology had started recording in the 18th century. In this respect, temperatures had been steadily increasing since the Little Ice Age. Here is how Rodewald (1948) summarizes it:

· “a ‘secular heat wave’ made itself felt over most of the Earth, we noticed this especially in the increasing mildness of the winters, which became more and more striking between 1900 and 1939.”

The eruption of the volcano Krakatoa, in Indonesia, on August 27 1883, that spread dust clouds around the globe and blocked solar radiation by up to 20% for three years (Wexler, 1951), had no significant effect on weather and climate. At least, no extraordinary weather conditions were reported.

Unprecedented was the increased warming of the Northern Hemisphere, which started in winter of 1918/19 in the region where the North Atlantic and the Arctic Ocean meet (see: Fig. A3-1; Chapter I). Temperatures of the Northern Hemisphere rose steeply and reached their peak around 1938/39 (Fig. A3-6 and A3-9, p.18 & 22).
It should be noted that the previous warming had already come to a stop in the USA around 1933, while the European record moved higher and higher until the beginning of WWII.

Actually, northern Europe was more affected, and the North Cape more than the south of Scandinavia (See TM14, p. 191, second row). In Scandinavia , 1867 was the last very cold year. It had a polar character, with a mean of –2,7°C, in contrast to that of 4,3°C, recorded in 1938 (Groissmayr, 1948). This primarily had something to do with the warming recorded at high latitudes since 1919. Whether this has been the only reason for the difference in temperature trends between the USA and Europe since 1933, is not to be answered here. Whoever is interested in this aspect, should consider that ship’s traffic increased steadily in the North Sea and Baltic during 1919 and 1939. But, whatever the cause, it is certain that for 20 years (until the winter of 1939/40), it had been getting warmer and warmer in Europe . The global situation is well demonstrated by the Temperature Map 1 (TM1, page 5), which shows the winter temperature anomalies (DJF) during the winters of 1935-1939. North America was colder, while Europe was warmer in comparison to the mean temperatures from 1900-1939.

The rising trend stopped abruptly in the winter of 1939/40. From one day to another, the scenario changed dramatically and with it, so did winter weather. There was a change from warm to extreme cold. And this did not happen only once or twice, but for three winters in a row. The contrast is impressively illustrated, if one compares the winter season (DJF) of 1938/39 (TM1; p.5) with the corresponding winter period 1939/40 (ditto), or for the first three war years – 1939/40 to 1941/42- (TM3, p. 25). That is quite a contrast. The scenario in Europe is shown for the annual conditions and for all seasons. Interesting details become clearly visible. The Baltic region was most severely affected by General Frost. Another interesting aspect is that in Great Britain there were colder conditions than in some parts of the North Sea (TM1 and TM3).

c. Contemporary witnesses see a lot, but understand little

The hat-tip for this section goes to the Swedish scientist Gösta H. Liljequist. His paper ‘The severity of the winters at Stockholm 1757–1942’, already published in 1943, made it clear that the three war winters of 1939/40, 1940/41 and 1941/42 had been unprecedented. Such three cold winters in succession had never been observed before. This was not a new record by a small margin; it was so different from the previous ones, that one could get the impression that the three war winters played in another league. Few time witnesses realised that exceptional conditions occurred but could not see the force that stood behind the event: The naval war from the Atlantic to the Baltic made the winter weather colder.

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