Sunglasses aren't just for summer — as the name suggests they're for the sun !!! In fact, the risk of unprotected eye damage is even greater when the sun's glare is intensified by reflection off snow — giving rise to the term "blinding white".
Snow blindness — also called radiation keratitis, ultraviolet keratitis, photokeratitis or niphablepsia — may not become apparent until several hours after exposure — typically 6 to 12 hours. The cornea (the clear front surface of the eye) and the conjunctiva (the mucous membrane lining the inside of the eyelid) have been burnt and become inflamed.
Outwardly the eyes may appear bloodshot and start to water with the level of tears increasing as time passes. It may feel like there is grit or sand in the eyes and rubbing them will only aggravate the situation. The level of pain will increase without treatment — reaching an excruciating level in severe cases — and the eyes can swell so much they close.
The worst case scenario — usually only after repeated unprotected exposure to UV rays — is solar retinopathy which can result in varying degrees of permanent vision loss — even complete and permanent blindness. You can see how important eye protection is — particularly on the slopes and especially at high altitude — the higher the ground level the lower the level of atmospheric filtering of UV rays giving rise to a 4% rise in UV intensity for every 1000 feet above sea level.
Sunglasses generally don't need to come with a costly designer name — expensive sometimes means better but not necessarily. Key to quality is the CE mark — conforms to European Community Standard — and British Standard BSEN1836. Both of these mean the glasses will provide high levels of protection against damaging UV light.
The best ones will block 100% of UV-A, UV-B and harmful blue light. Don't confuse the shade of the lenses with their ability to filter UV rays and be aware that damage can occur even when the sun is covered — UV rays can filter through the clouds and remain harmful.
For skiing, the additional attributes to look for are polarising lenses which will reduce reflective glare and anti-reflective coatings which will eliminate glare. Also, impact resistance lenses are essential and larger lenses will increase protection while the broad temple arms of a wrap-around style will help keep out peripheral glare and provide protection from indirect light exposure.
Ultimately you will get what you pay for and if you have opted for a pair of sunglasses for skiing that encompass all of the above attributes you might find you've paid a bit more than you had originally intended. The good news is that repairs are generally not expensive and very quickly done so if your glasses do incur any damage you will be able to get them as good as new and your original investment will see you through many ski seasons.