Weather: An invasion that begins in January or February would have the advantage of frozen ground to support the cross-country movement of a large mechanized force. It would also mean operating in conditions of freezing cold and limited visibility. January is usually the coldest and snowiest month of the year in Ukraine, averaging 8.5 hours of daylight during the month and increasing to 10 hours by February.8 This would put a premium on night fighting capabilities to keep an advance moving forward. Should fighting continue into March, mechanized forces would have to deal with the infamous Rasputitsa, or thaw. In October, Rasputitsa turns firm ground into mud. In March, the frozen steppes thaw, and the land again becomes at best a bog, and at worst a sea of mud. Winter weather is also less than optimal for reliable close air support operations. From: https://www.csis.org/analysis/russias-possible-invasion-ukra...