I have a pair of thin goretex rain pants designed for running and cycling, with velcro bands to snug in the lower leg. I use them if it's really pouring, but the goretex is flimsy and they still get clammy inside, so anything less than a full downpour I wear homemade rain shorts over tights. I cut them off so they hit right below the knee when my leg is bent. They keep my butt dry, and ventilate very well. The front of my lower leg still gets a bit wet but I can live with that.