After a run of mild winters, December 1939 came as a shock. The month started pleasantly enough, then the thermometer started to drop, surprising meteorologists. “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,” remarked AJ Drummond of the Royal Meteorological Society.
December was 2C below average; then it got colder. Eight miles of the Thames were frozen, along with the entire length of the Grand Union Canal. Ice covered stretches of the Mersey, Humber and Severn. The sea froze at Bognor Regis; Folkestone and Southampton harbours were iced over. Although later eclipsed in memory by the epic winter of 1947, this was the coldest winter for forty-five years, the first of a series of bad winters.
Some people assumed that this worsening of the weather was related to the war, which had started a few months earlier. The blame was put particularly on exhausts from the vastly increased naval activity in the North Sea. Recent studies suggest that the massed bomber raids of 1944 did really influence local conditions, as their collective contrails reflected sunlight and brought cooler temperatures. However, any impact by the war on the winter of 1939-40 remains speculative.