To the OP - wear a cap under your helmet, the brim will protect your glasses a little, and you want to stop rain from coming through the helmet vents. Use an anti-fog solution if fogging up is an issue. Otherwise, don't sweat it. Rain on eyeglasses isn't a big problem. A quick wipe with the finger of your glove should be enough of a "windshield wiper". If the rain is hard enough to interfere with your vision through eyeglasses, it is probably hard enough to interfere with the vision of those without glasses too. I ride in Portland with glasses.