In reply to &y_Gee:
Cat 4 sunlglasses are made cat 4 by having a coating on both sides of the lens, this wont work if you've got another pair behind.
It may seem over the top but on a bright sunny day a few hours on a glacier or snow field will creat so much glare behind you glasses that you will probably end up with a splitting headache.
Contacts and a pair of jublos (cheap light and do the trick) will be the best option.
You could get some googles but you will end you looking like a negative panda, but then many skiiers do..