I've often heard it repeated that the the Russian winters of 1812-13 and 1941-42 were unusually cold and harsh. Is this supported by climate records and other evidence?
For Napoleon, the winter of 1812 wasn't particularly cold. According to Dominic Lieven in Russia Against Napoleon, the months of Napoleon's retreat weren't unusually cold and "all the Russian sources say... that November 1812 was cold but seldom exceptionally so for this time of the year." He further continues to explain that the first half of that November was warmer than usual, allowing the Berezina and causing a problem for the retreat as everyone had to use bridges rather than march across ice.