It might not be exactly rain proof but I use cross-country ski pants personally, you can get them with full zippers up the side. The fronts are usually very water resistant with vents in the knees and usually breathable fabric on the back, that way you don't get swampy underneath (previously had full water proof rubber ones a friend gave away to me very unpleasant to use). Long as your bike has full fenders then the backs being breathable become a big plus. I just keep a pair in the bottom of my bag with me and a rain poncho and Im ready for whenever it decides to pour.