Fixed! (hopefully), at least I found an error (does not mean there is no other

)
There is a data called WeatherHierarchy, which is there to see if a worse weather can propagate from a low terrain to a high terrain (example, in the same weather zone, if it snows in plain, then it snows in mountain). In WIA, swamp is a terrain used, but the hierarchy was not defined in the code, so it used 999, meaning swamp was supposed to be higher than mountains.
Thus, as it could snow in mountains in the weather area of Ocmulgee (south upland), the code gave a snow weather too to our swamps (remember the swamps were supposed to reach 15.000 m altitude

).
I have added support for the Swamp terrain in the code. It is hard coded, because this is the old system... Now (ROP & VGN and beyond) the Weather Hierarchy is a data specified in the terrains files, so there is no problem in the new games.
Lodilefty, I would need you to check all the terrains used in WIA so I can verify they are all handled by the hard coded data in the code.
Alternatively WIA can switch to the new model, but this is a bit of work for not much of a gain now that the game is stable.