Waterproof breathable materials form the outer layer. The term waterproof-breathable refers to any fabric with a PTFE coating laminated to the inside, that both repels water and moves moisture and vapour from the inside to the outside. Because a drop of water is very large in comparison to a molecule of water vapour, the small pores within these fabrics have the ability to block out rain, snow and fog while allowing vapour from body moisture to evaporate through the membrane. You stay dry and warm.