Snow blindness, or solar/ultraviolet keratitis is an excruciatingly painful state that comes from the sun burning the covering of your eye -- the cornea. And it happens, very commonly if you don’t wear sunglasses, or if you don’t wear appropriate sunglasses in any bright light situation – especially easy to encounter at altitude.
A sunny day on fresh snow can be beautiful, but incapacitating if you’re not protected. Keep in mind that the brightness can exceed 10-15 times the amount of light that is safe and comfortable for your eyes to accommodate.
Sometimes, when climbing on oxygen, the warm and moist breathing air will escape your oxygen mask upwards and sometimes clog up your goggles, especially upon climbing down. Your choice will then be to climb "blindfolded" or remove the glasses. You might choose to pull your glasses a bit out from your face, allowing the warm air to pass them. The suns rays will now be able to burn your eyes at the unprotected sides. An anti-fog lens cleaner may help in this situation.
If the weather is overcast you might be tempted to remove the glasses altogether. Yet the rays are just as harmful when cloudy, and the following morning you’ll be sorry. After 8 years of climbing it finally happened to us. It took only a short time without goggles at our summit descent (shooting film), we noticed nothing, and in the morning we were a mess.
Here are some guidelines to use when choosing a good trekking/mountaineering pair of sunglasses: