The map_climates.tga is the map tga that controls both the type of texture used for each ground_type displayed on the campaign map and also controls various elements of the battle map.
Map_climates.tga will be found in the data/world/maps/base folder of a vanilla installation, it may also be placed in the data/world/maps/campaign/campaign_name folder if the other map files are copied there as well.
Certain climates are hardcoded to have a snow effect in winter, whilst others do not. This can be changed visually by altering descr_aerial_map_ground_types.txt references, but the game will still associate 'snow' movement and unit penalties/bonuses with that climate in winter.
The climate used also determines the type of tree displayed on both the campaign and battle maps.
The textures displayed on the campaign map can be varied by changing the textures referenced in descr_aerial_map_ground_types.txt. In the vanilla game many combinations of climate and ground_type reference the same texture .tga, if additional tga's are available they can be added for selected combinations to enhance variety.
Effect of Climates on Battle Map
The climate defined by this file for a particular location on the campaign map controls a variety of effects on the battle map this includes the weather and cloud effects. In RTW these are controlled from descr_daytypes.txt, and are set by climate and by season. The weather effects change both the appearance of the battle-field and lighting and the effectiveness of battle play. Visibility can be severely affected by weather. Typically desert areas are prone to dust storms and northern areas prone to blizzards.
The textures used for the battle map ground surface are also determined by climate.
In RTW the weather conditions in the descr_daytypes.txt file also reference sound files, so ambient sounds can also be varied by climate.