The keys are in scene.dat
Check out Daves work here:
http://www.subsim.com/radioroom/showthread.php?t=124608
/edit:
Try out these values, very varying weather guarenteed...
WindHeadingChangeSpeed = 0,03
WindSpeedChangeSpeed = 0,03
RandomWind.
ChangeTimeMin = 2,0
ChangeTimeMax = 2,0
RandomWindSpeedMax = 0,01
RandomWindSpeedVariationSpeed = 0,009
RandomWindHeadingVariationSpeed = 0,009
RainIntensityChangeSpeed = 0,01