The GORE-TEX® membrane has 9 billion pores per square inch, with each pore 20,000 times smaller than a water droplet. These tiny holes are too small for water and wind to pass through from the outside, which is how we can guarantee you'll stay dry and warm in a garment made with GORE-TEX® fabric. But these same pores are large enough for moisture vapor to pass through, so your body's perspiration is able to escape and you don't get clammy and uncomfortable.