Gear & Craft
Sunglasses, Snow Glare And Damage That Arrives Late
Bright snow and high altitude both raise the amount of ultraviolet reaching the eye, and the resulting injury appears hours after the exposure has ended.
Eye protection outdoors is treated as a comfort item. On snow and at altitude it stops being one, because the exposure is invisible at the time and the effect is delayed.
Snow reflects most of what falls on it
Fresh snow returns a very high proportion of the ultraviolet striking it, which means an eye at ground level receives radiation from below as well as above.
Grass and rock reflect only a small fraction by comparison. Moving from a summer hillside onto a snowfield roughly doubles the total exposure without any change in how bright it feels.
Cloud does not help as much as expected. A thin overcast scatters ultraviolet while cutting visible light, so the scene dims and the exposure continues.
Altitude thins the filter
The atmosphere absorbs ultraviolet, and there is less of it above you as you climb. Ultraviolet intensity rises steadily with height, noticeably so over the span of a large mountain.
Combine the two effects and a snow slope high on a peak delivers a dose no lowland environment produces, which is why the condition has historically been associated with mountaineers.
The visible brightness does not track the ultraviolet, so judging the risk by how much you are squinting underestimates it.
The injury is a surface burn
What is damaged is the cornea, the transparent front surface. The mechanism is closer to sunburn than to anything structural, and the sensation begins some hours after exposure has stopped.
That delay is what makes it dangerous in the field. By the time discomfort appears the party may be somewhere that requires vision to leave safely.
The surface regenerates, and the condition generally resolves on its own, but the recovery period is genuinely disabling and anyone affected needs assessment rather than improvisation.
Darkness and protection are separate properties
A dark lens reduces visible light. Ultraviolet blocking is a coating, and the two are independent, which is why an untreated dark lens can be worse than none at all.
A dark lens opens the pupil. If it is not blocking ultraviolet, more radiation reaches the eye than would have done had you squinted through the glare unprotected.
This is the argument for checking specification rather than tint, and for treating a very cheap pair worn on snow as a genuine hazard.
Coverage matters as much as the lens
Light arriving from below and from the sides bypasses a lens entirely. Wraparound frames or side shields exist because on snow a large share of the dose comes from underneath.
Carrying a spare pair is standard practice among people who work on snow, on the reasoning that a dropped or broken pair turns a comfortable day into an evacuation.
Questions readers ask
Why did my torch come back to life in my pocket?
Cold raised the cell's internal resistance and its voltage sagged under load. Warming it restored the reaction rate, and the charge that appeared to be gone became usable again.
Which batteries are best for winter?
Lithium primary cells generally outperform alkaline in cold. Rechargeable lithium-ion works but loses runtime and should not be charged below freezing.
How should I store batteries between trips?
Cool, dry and out of the device. Lithium-ion keeps better at partial charge than full, and alkaline cells left in equipment can leak and destroy it.
