This is the greatest material ever invented. Gore-Tex keeps you dry, period. I've stood under a
waterfall in a Gore-Tex
parka and Gore-tex
pants and not gotten more than a few splashes on me.
How it works:
Gore-Tex consists of a two or three-layered membrane. This membrane has holes several thousand times the size of a water molecule (to let humidity out for comfort and breathability) but much smaller than the smallest possible droplet of water (to keep water out for dryness). Gore-Tex kicks ass. I just wish it wasn't so frickin' expensive