Each map has its own "Cloud" file. Thus they can be customized for each map, ie cloud layers over strats as opposed to other areas. They can be used to encourage bombers and fighters to lower altitudes. Weather fronts are also on a one hour timer, much like the day time and night time in the game. Then you have the type of clouds, the saturation level, lol it gets complicated real fast. Each type of cloud has its altitude range, get that wrong and you have the Icon problem we had in etoma, learned that one the hard way.