Gear & Kit
Eye Protection On Snow And Water
Snow and water reflect ultraviolet light onto the eye from below, which is why lens category, coverage and side shielding matter more than tint darkness in bright environments.
Sunglasses in the mountains and on the water are equipment rather than accessories. The reason is reflected light, which arrives from directions the eye is not built to shield.
Reflection is the mechanism
Fresh snow reflects a very high proportion of the ultraviolet reaching it, and open water reflects strongly at low sun angles. Both send light upward into the eye.
Brows, lashes and the natural squint response evolved against light from above. They provide almost no protection against a bright surface underfoot.
Altitude compounds it on snow, because thinner atmosphere absorbs less ultraviolet. A glacier at midday delivers an exposure unlike anything at sea level.
What photokeratitis is
Excessive ultraviolet damages the surface cells of the cornea. The injury is a burn, and like sunburn it produces no sensation while it is happening.
Symptoms appear hours later as intense pain, watering and light sensitivity, often at night after the exposure. Vision can be seriously impaired for a day or more.
The condition usually resolves as the corneal surface regenerates, but temporary blindness in a remote place is a serious operational problem regardless of the eventual recovery.
Tint and protection are separate
Ultraviolet filtering comes from the lens material and coating, not from how dark the lens looks. A dark lens without proper filtering opens the pupil and admits more ultraviolet.
Lens category numbers describe visible light transmission, running from clear to very dark. Snow and glacier travel generally call for the darkest categories, which are unsuitable for driving.
Polarisation reduces glare from horizontal surfaces by filtering light of a particular orientation, which is a comfort and visibility benefit rather than an ultraviolet protection.
Coverage decides the exposure
Light entering around the edge of a lens bypasses the filter entirely. This is why glacier glasses use wrapped frames and side shields rather than a fashionable flat front.
Goggles solve the same problem differently and add protection from wind and spindrift, which matters in moving weather where sunglasses are unusable.
On water, retention is the other half. A leash or strap keeps eyewear attached during a wipeout or a capsize, and eyewear that is lost provides nothing.
The spare pair is not optional
On a multi-day snow trip, losing eyewear is not an inconvenience but a serious problem, because the exposure continues whether or not the protection does.
Expedition parties carry spares as standard, and improvised shields with narrow slits are a documented emergency measure that reduces the light reaching the eye.
Anyone with existing eye conditions, or planning sustained high-altitude travel, should get advice from an optometrist rather than choosing lenses from a general description.
