Quote Originally Posted by Furious Mental
Moisture should affect things like bows, firearms etcetera. It should also make ground more difficult to pass. Ideally, very heavy rains should flood rivers and make fords impassable, while very cold winters should freeze them and allow easy crossing.
Yes, good points FM.

I think also heavy rains should cause mud which would quickly fatigue troops. Snow should have a similar effect on fatigue, but not as bad as for mud.