Is this on a new map? If so, you need to use portals to accomplish this then you apply the weather to the portals. IF you need to know about portaling... I'm not really the one to ask, I am not good at it so sorry I can't help you on that much. Perhaps look at some tutorials for portaling though and you should find what you are after.

portals are basically a bunch of planes added vertical (90 degree angle)depending on why your adding them to your map.they also decrease the lag a big map causes.the planes are added lengh ways from left to right on all four sides so that
you end up with.....

| | | | | | |
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| | | | | | |
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| | | | | | |

kind of layout of planes on your map,these planes have to be as close to the exact same distance apart,but it is better to have them exactly the same distance apart.

note: the gap should not be in between the planes the way it looks,that's the best i can do to display it with a keyboard.sorry i can't draw up a diagram my scanner has past it's sell by date.

edit: funny how the lines left to right have turned out in this post lol.
anyway here's a video that should explain a little more about portals.
http://www.youtube.com/watch?v=3SqygQGcPgU&playnext=1&list=PL97A6E1AEB6A1D637&index=19
Edited by ally on Feb 1, 2011 at 01:41 AM

Well there are also exact portals which would be used to make it so that your tain for example would only be on the outside of the building or you could discern outdoor ambient sound from indoor. Those are a b**** and I have tried and failed at those so I decided from now on Hydro will be doing my portals.

Exactportals are the way, the truth, and the life. Use em wisely.

adding things like that are done through the weather palette in the scenario_structure_bsp.