The Russian winter closed in during October 1941 and was one of the most severe on record. The Germans were unprepared for such severity, leading to thousands freezing to death, Luftwaffe grounded, tanks bogged down in the mud.
They halted their attack on Moscow in December 1941 just 20km from the capital. 113,000 Germans had died of frostbite, total of 830,000 German casualties to this point (of 3.8 million men)