Right now we have a useless "gets less tired in snow/desert" statuses. Since I saw you can artificially add or retract attack or defense points, why not add +x/y amount of Att or Def point to a unit that is used to those extreme conditions?
But also make them -2x/y effective at the other extreme.
I think this would bring much needed buffs and diversion to using different units.
Also, add heavy unit deficit to att or def at deserts, because if anyone did any basic training with heavy gear at +35C, he knows you get much more tired and weaker when in full gear.