Yes i noticied this behaviour time ago.
For example the campaing file has a very different settings that the training or single missions.
i was tired of the same weather when playing the convoy attack mission in the academy i noticied tha the weather was a lot different if i sails during days or weeks .
Convoy Attack Training Mission-------------------------------------------------------------
[Mission]
Fog=0
FogRand=0
Clouds=1
CloudsRand=1
Precip=0
PrecipRand=0
WindHeading=95
WindSpeed=2.000000
WindRand=1
WeatherRndInterval=2
SeaType=0
Actual Campaign SCR-------------------------------------------------------------
[Mission]
Fog=0
FogRand=0
Clouds=0
CloudsRand=0
Precip=0
PrecipRand=0
WindHeading=0
WindSpeed=5.000000
WindRand=0
WeatherRndInterval=5
SeaType=0
in the old forums there were some threads about weather and randomization.
if some one is interested there are program that adds random values in the files.
"Sh3 weather" of Skweetis