New precipitation code checked into HEAD

Have fun. If anyone has the chance, go beat the hell out of it and see if you can spot any issues. If you find any, let me know so that I can fix them asap. I can tell you that any mission that has precipitation already in it WILL be somewhat broken and using default values. I'd suggest conforming all of your missions first thing before doing anything else. You can find some somewhat decent docs in the source code.