Oddly playing as a northern faction like England or France, I never had a winter battle, but playing as the Almohads I have had several. I think that the reason that his happens may be real simple.

As was previously stated, winter battles are the fourth battle played in a turn if you have that many. I think I tend to fight more battles in one turn towards the last half of a campaign rather than the first half.

If you start as a Northern faction, you will most likely will be playing the last half of the campaign around the Med, where there will be no snow. (although, I once had snow flurries on one of the Med islands)

Playing as a southern faction, the last half of your campaign with large multiple battle turns, would most likely be fought in the northern climes, and therefore increase the chances of a snow covered winter battle.

Cheers

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Artillery adds dignity to what would otherwise be a vulgar brawl.