Games Workshop has posted a series of rules to help model winter effects in their games as part of their Winter War series of articles.
From their website:
Whether you’re doing battle in Middle-earth, the far-flung reaches of the galaxy in the 41st Millennium or the blood-soaked fields of the Warhammer world, there’s always a chance to ‘tweak’ your games a little. As part of our series of Winter War articles, we decided it would be fun to concoct some special rules for playing games set in the freezing cold of a fantasy world.
Each environment effect has a description that explains what it is, contextualising it so that you can imagine how it will influence your battle. It is also accompanied by a set of rules, explaining what it does in Warhammer, Warhammer 40,000, The Lord of The Rings and War of The Ring respectively.