When looking at items like jackets and ski trousers and other clothing, I see stats on the garments similar to the following:
Waterproofness - 10000mm
Breathability - 15000g/m
Lining - 100% Polyester 40g/m Thermosoft
Waterproofness ranges from a few thousand to 20k, breathability the same.
I get torrentially rained on from time to time and get very hot and sweaty when skiing hard - how much is enough?
As a rough guide to waterproofness - 5000mm is generally rainproof but won't necessarily stand up well to torrential rain. Around 15000 should be fine in that context. If you go higher than that then you're looking at fabric that can be immersed in water and still stay waterproof for a while, but should be ample for any rain shower that might come along!
Breathability works on a similar scale, 5000 being ok-ish, but 15000-20000 is where it makes sense to look if you get very hot and sweaty.
Don't use these figures as gospel though, there's no standard testing practice so results will vary slightly. Some companies will test when new for instance, others after some washes, some will take an average, some will take the best... also bear in mind that these tests are carried out while the fabric is still, not moving, so results might vary slightly because of that. In short, use them as a rough guide but they're definitely not the be all and end all in choosing a product.
