I hesitated before replying to this tread not because I’m not familiar with snow but because I’m not familiar with T&T rules...
I think I would use the following general rules;
Soft, deep snowInfantry = difficult terrain
Cavalry = difficult terrain or even impassable in some case
Wheeled vehicles = impassable
Tanks/half-track vehicles = normal
Infantry on skis = normal
Hard snow, iceInfantry = normal
Cavalry = normal (or difficult terrain if on ice?)
Wheeled vehicles = normal if on hard snow and difficult terrain with possibility of bugging down, slip off the road if on ice
Tanks/half-track vehicles = normal if on hard snow and difficult terrain with possibility of bugging down, slip off the road if on ice
Infantry on skis = normal (If trained infantry I would probably say that they even can have a bonus movement as people trained for skiing are much faster on "hard" snow than average people walking on foot even when there’s no snow)
Hope that help…
