How does breathable waterproof clothing work?

Breathable waterproofs rely on the fact that water is very sticky. Water has an oxygen atom stuck onto two hydrogens, and looks like a tiny boomerang. The oxygen atom is quite negative and pulls electrons towards it. This means that there are plus and minus bits on the molecules which attract plusses and minuses on other molecules, making the molecules stick together. This attraction is known as hydrogen bonding and is the reason why water has a surface tension. When water forms into drops, they are very large and don't easily break into smaller ones. The fabric in breathable Gore-tex raincoats is called Teflon; the same material used on non-stick frying pans. The holes in the fabric are so tiny that 9 billion can fit into one square inch of the raincoat! When water leaves your body as water vapour, the molecules are small and can fit through the holes. When it reaches the outside of the raincoat, it condenses into big sticky raindrops that can't get back through. But your raincoat will only stay breathable if you keep it well waterproofed. Waterproofing creates a gradient between the inside and outside of the coat: the inside has lots of water vapour while the outside has no water because it has formed drops and run off the fabric. This gradient ensures water is pulled away from your skin and stops you getting sweaty inside your raincoat!