Just in case there is any doubt... The big reason it doesn't rain much in Sunnydale is because rain is an absolute pain in the arse to film. It causes a lot more bother than you might think...
For one thing, you can't heat a wet set or it steams. Wet + Cold = Cranky actors. The water makes make up, set paint and occasionally hair dye, run, making continuity nearly impossible (See how many phlubs you can spot in the last minutes of 5x5 and the start of Sanctuary) Water damages interior sets irreperably (The US version of The Ring has water stains all over Noahs apartment long before his visitor arrives) To make falling water properly visible, you either have to have lots of it, or you add milk. Lots of water is really, really noisy (Check out any BVTS or Angel scene with falling water and you'll notice some bad overdubbing.) and milky water gets gross. You also need rain rigs, and it's unsanitary to just dump the water over the set and then dump it back onto your actors, so you have to keep supplying the rigs with clean water. And all that water has to go somewhere, meaning you need to construct a drain system and make the set completely water proof. All that expensive equipment (Boom mikes, light rigs, cameras...) has to be waterproofed so it doesn't get damaged, but you also have to make sure it doesn't get into the groundwater and kill your actors (Sigourney Weaver very nearly electrocuted herself by grabbing equipment while she was filming the underwater scenes in A:R)
So... lots of money, possibly dead actors. You really want to be sure about a rain scene before you include it. 😀 Or, alternatively, you can make your actress wet and add CGI rain, like I'm nearly a hundred percent sure they did in This Years Girl...
