Considering how often fantasy fiction likes to portray battles fought in less than optimal conditions, it does seem like an important missing piece. (Sort of like my Queen in my last chess game.....)

For your consideration:

Cold Winter - rivers, ponds, marshes are frozen. No impediment to movement. Movement along river allowed as on a road. (Mongols Ride Free!)

Snowy Winter - ALL ground movement is at bad going rate.

Rain - All Shooters and Artillery receive a -1 CF for shooting combat.

Unpaved roads are treated as bad going. (Mud! My cloak is dirtied!) Aerial movement halved.

Fog - All movement as through Woods. Shooting distances halved. All command distances halved. On a PIP die roll of 1, the fog lifts. (The PIPping side is so amazed, they momentarily freeze.....)

and so on.

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