How summery summer?

Once the temperature climbs above 80 or 85 degrees I don't worry about it. I just finish the ride with whatever I happen to have on. Anything that will keep the rain off will also keep the sweat in and you'll be just as wet and even more uncomfortable.

Temperatures below 70 are another story. The wet isn't the issue so much as the cold. I used to mountain bike with a group of guys every Saturday of the year. No excuses. Below zero might be cold but, for pure misery, temperatures in the 30's with rain are much worse. Years ago I bought a Craft rain jacket that is made from some miracle breathable fabric and has underarm zippers for ventilation. It's a favorite of mine for a variety of weather conditions. I also have a couple of long sleeve "wind shirts" that are made from some coated fabric. They are great for 50's and dry or 60's into the 70's with rain. I always keep one in the bike gear tube that I keep in my car.