Some important factors need to be mentioned first. The land war started immediately in 1914, the naval war commenced fully since the autumn of 1916. On land, there was already the famous icy winter battle in Masuria (north-eastern Poland), in February 1915, between the German Army and the Russian Tenth Army, which determined the German Field Marshall Hindenburg to wonder: “Have earthy beings really done this things or is all but a fable or a phantom?” (citation from NYT, the 7th of January 1942)
If rainmaking along the Maginot Line/Westwall, in autumn 1939, is the comparison element, then the devastating battle of Verdun is much more significant. The German attack on Verdun started on the 21st of February 1916, with one million troops; the battle became the longest of WWI and ended on the 18th of December 1916. French and German Army lost several hundred thousand men each. From a climatic perspective, it is to note that close battle field regions had been wetter than usually, e.g. Baden had 30% more precipitation, in the Black Forest rain level was even 50-80% higher than normal.
The battle of Verdun followed one of the top ranking cold winters during last century. The winter 1916/17 matched closely the record winter 1939/40. To keep in mind! The naval war started its devastating war phase only in the autumn of 1916. Submarines only went into action in 1915, sinking about 100,000 ship tonnages per month, which accelerated to about 300,000 tons per month in the second half of 1916. In addition, in 1916, a flotilla of more than 500 vessels was permanently navigating the seas around the British Isles, sweeping a daily average of 1.000 square miles. Together with the very increased use of sea mines, mine sweeping operations, and depth charges, the result was particularly significant on the weather all over Great Britain. The result can be read from weather records. In Britain, June 1916 was very cold and dull. Rain was persistent in the east and north, e.g. with about 150 hours of rain in Aberdeen and up to 200mm. The next extreme month was October 1916, which was wet, and stormy, with record daily rainfall of 200mm. Up to this point, it was the highest daily rainfall ever recorded for the British Isles, and an extremely cold December 1916 followed.
With single events or statistical months, it is difficult to establish evidential circumstances. In addition, more factual data may provide the required proof. Great Britain surrounded by naval war may do it. For this purpose, we refer again to the time witness, A. J. Drummond from Kew Observatory at Richmond (London), who expressed his astonishment in 1943: “The present century has been marked by such a wide-spread tendency towards mild winters that the “old-fashioned winters”, of which one has heard so much, seemed to have disappeared for ever. The sudden arrival at the end of 1939 of what was considered to be the beginning of a series of cold winters was therefore all the more surprising.” He continued: “Since comparable records began in 1871, the only three successive winters as snowy as the recent ones (1939/40 to 1941/42) were those during the last war, namely 1915/16, 1916/17, 1917/18.
Not to miss what naval war may have done on snow conditions in Great Britain, the comparable situation of the war years 1915-1918 shall be explained with the war winters 1939-1942, which were investigated by Lilian F. Lewis who concluded that snow coverage in the British Isles during January and February over the three war winters 1940, 1941, and 1942 were unusual severe; the snow was considerable, and the number of days of snow-laying numerous and without precedent in the British Isles for at least 60 years. According to Drummond, during the first three WWII winters, snow fell on 23%, 48% and 23% of the days which was about 100% to 400% more snowfall than the average. The reasons for such a deviation are easy to explain: snow is likely to fall when humid air cools down. The more naval warfare has decreased sea water temperatures in the sea areas around Britain to below average level, the greater the chance of extensive snowfall due to lower air temperatures.
For the cooling down of the seas around Britain, it is also possible to find hard evidence. In 1935, J. K. Lumby published a seawater temperature series taken in the English Channel from 1903 until 1927. From 1901 until 1914, the temperature varied on a narrow band, from 11.5ºC to 12.2ºC. During the war years 1914-1917, the temperature dropped to its lowest point of the series, viz. to 10.9º C. By all means that should not come as a surprise when realising what actually happened during World War One for many times:
“In September 1916, the U-boat flotilla from Zeebrugge alone sank nearly 50,000 tons of shipping in the Channel, without any hindrance from patrol vessels. It was soon clear that the existing methods of combating submarines were simply not working. For example, in one week of September 1916, three U-boats operated in the Channel between Beachy Head and Eddystone Light, an area patrolled by forty-nine destroyers (49), forty-eight torpedo boats (48), seven Q-ships (7), and 468 armed auxiliaries – some 572 anti-submarine vessels in all, not counting the aircraft. Shipping in the Channel was held up or diverted. The U-boats were hunted. They sank thirty ships, and were entirely unscathed themselves.” 
Another investigation of the situation in the Irish Sea over the period 1900 – 1950 made by D.C. Giles in 1949 also shows a deep decline from 1914 to 1919. Sea chilling is inevitable when naval warfare occurs during autumn and wintertime, when thousands of ships movements churn the sea day and night, when thousands of explosions under and above the sea surface turn sea levels up-side-down. Consequences are obvious: in autumn, the sea cools out quicker and colder air establishes subsequently, followed by more snow which leads to harsher wintertime, and so on. Sometimes, physical conclusions are very simple. The cooling down of Britain and the unusual temperature decline on the Isles, from 1915 until 1918, has its cause in naval warfare and in nothing else.
In conclusion, it can be said that weather anomalies in Britain during WWI and WWII have so many similarities that they can be taken as proof of the impact that war at sea had on weather conditions.
 Drummond, A.J.; ‚Cold winters at Kew Observatory, 1783-1942’; Quarterly Journal of Royal Met. Soc., No. 69, 1943, pp 17-32, and: Drummond, A.J.; Discussion of the paper: ‚Cold winters at Kew Observatory, 1783-1942’; Quarterly Journal of Royal Met. Soc., 1943, p. 147ff.
 Lewis, Lilian, F.; ‘Snow-cover in the British Isles in January and February of the severe winters 1940, 1941 and 1942’, in: Quarterly Journal of Royal Met. Soc., 1943, pp. 215-219.
 Lumby, J.K..; ‘Seasonal changes of deepwater temperatures’; Quarterly Journal Royal Meteorological Society, Vol.67, July 1941, pp.234-238.
 Winton, John; ‘Convoy – The defense of sea trade 1890-1990’, London 1983.
 Gilles, D.C.; ‘The Temperature and Salinity of the Surface Waters of the Irish Sea for the Period 1935-46’, in: Monthly Notices of the Royal Astronom. Society, Geophys. Suppl., Vol.5, Nr.9, London 1949, pp. 374-397.