I only know "g_set_weather" 0/1 .
in some tutorials he also comments on this:
Regardsg_set_weather X f / i
The last one is how fast you want that weather to be applied. If you supply an "f" (forced) that means the weather will switch right away.
If however you supply an "i" (intermediate) then the weather will gradually change to what you selected. (approximately 20 minutes game time)