Gap, are you familiar with these entries from
CampaignMission.mis (gamesave) file?
Code:
[Mission]
IsVisible=true
MissionType=0
MissionDataType=0
MultiMissionType=0
IsTutorial=false
Year=1939
Month=9
Day=1
Hour=16
Minute=38
CurrentYear=1939
CurrentMonth=9
CurrentDay=2
CurrentHour=15
CurrentMinute=16
GroupMissionStartDelay=0
IconIndex=0
Fog=2
FogRand=2
Clouds=2
CloudsRand=2
Precip=2
PrecipRand=2
WindHeading=0
WindSpeed=5.000000
WindRand=2
WeatherRndInterval=32
SeaType=0
ActivationAchievements=
It appears that this is the reason why entries from your weather cfg files are ignored in the campaign...

Could it be that the key for weather change intervals is somewhere in OHII/campaign files?