Every Saturday morning, a group meets at Winslow Green on Bainbridge Island to ride 40 or so miles to Port Gamble and back. The likelihood of rain kept most people home this Saturday. Of the four people who started, two people bailed about a mile into the ride when it actually started raining.
When the rain started, I donned my white garbage bag, which I always carry under the saddle for such occasions. It weighs almost nothing and is quite effective in keeping me warm, if not completely dry in a steady rain.
Twenty odd years ago, I started the Seattle to Portland ride wearing nothing but a short sleeve lycra jersey, even though the forecast promised rain, which indeed started about ten miles down the road. Fifty miles into the ride I was near hypothermic. At the halfway point in Centralia, I emptied the trash from one of the garbage bags at a rest stop, poked holes for my head and arms, put it on and finished the ride in relative comfort.