Waterproofness is important to prevent snow, rain and water from seeping into your ski clothes. Breathability, on the other hand, allows sweat to evaporate and is crucial to keep you dry from the inside out. So while they might seem opposites of each other, waterproofness and breathability actually work together to keep you dry. Waterproofing and breathability often scale together as well. This has everything to do with the quality of the materials that are used. For instance, most jackets with a waterproof rate of 10K also have a breathability rate of 10K.