People ask all the time about how waterproofing something is, how to waterproof clothing yourself or what exactly the numbers mean. Here is a short explanation (from what I know):
1. What does that number mean? Well, the number given, like 10,000 mm, is the size of molecule that can get through the material. The actual cloth of the jacket is woven together with a certain 'tightness'. The tighter weave and the stronger the material, the less will be able to get through the 'pores'. The number given is saying that any molecule or compound smaller than that number will be not able to get through, so to speak. So, if somethign is 10,000 mm, a 8,000 mm snow molecule will not get through. Crap, I have to go to class. In short, waterproofing something seals these pores in the jacket, creating no space for water to get through.
I dream of winter.