TrailHeads Performance Mens Running Hat - Lightweight, Quick Dry Workout Hats for Men - Athletic Sports Cap for Tennis, Gym

Q: Would it keep your hair fairly dry if running in the rain?

Unfortunately that is the triad off, it is ideal for running in warm weather because of the ventilation qualities of the mesh material, but if you use it to run in the rain your hair is going to get wet. So for wet weather it is not the cap I use. I wouldn't let this put you off as it is one the best piece of running gear I ever purchased.