copied from levelforge's forum (
http://levelforge.teamhuh.com/forgelink/viewtopic.php?t=142)
If you have a large indoor map with a few outside areas/ledges... use the outside brushes on areas you want to be affected by weather.
If you have a outdoor map with a few buildings that you enter, use inside brushes.
You can only use one or the other.
Once you have these brushes placed... cover them with another brush (with the handy weather zone texture made by Raven (Ford) on it.)
Right click, goto misc>misc_weather_zone.
This designates the brush area as a zone to be affected by weather (snow rain etc)
The only reason weather zones are used is so the engine calculates weather for these areas ONLY. That way it doesnt calculate weather for the entire map