Why do your eyes hurt at night if you skip sunglasses on a snowy mountain?
― Snow blindness is a "sunburn" on the surface of the eye
You feel fine all day, then at night your eyes suddenly start to hurt. This is snow blindness, a sunburn on the surface of the eye. Snow bounces ultraviolet light up into your eyes from below. And the pain arrives about half a day after the exposure, so you can't tell it's happening while it happens.
It's a clear winter day, and you spend it skiing or snowboarding on a snowy mountain. Your goggles keep fogging up, so you take them off partway through. It's dazzling, but it doesn't hurt.
Then, around the time you finish dinner at the lodge, your eyes begin to feel gritty, as if sand had got in. Tears won't stop, and even looking at a lamp becomes painful.
Nothing got into your eyes. During the day, ultraviolet light bounced off the snow and slowly damaged the surface of your eyes.
Only two things make snow blindness likely
Grass and soil reflect almost no ultraviolet light. Fresh snow is said to reflect up to about 80% of it. Your eyes get ultraviolet light from both the sky and the ground at your feet.
The surface of your eye can be damaged without hurting while you are exposed. Symptoms are said to appear 6 to 12 hours later. "It doesn't hurt, so I'm fine" doesn't work here.
Let's take them one at a time. In both cases, the protections that normally guard your eyes stop working only on snow.
Eyes are well protected from light from above, but not from below
Human eyes are naturally shielded from light from above. The brow bone sticks out slightly and acts as an eave. The eyelids also cover the eye from above. We don't normally look straight at the midday sun.
But light coming from the ground has no eave to stop it. Usually the ground is dark and reflects almost no ultraviolet light, so this isn't a problem. On snow, things change. The pure white surface sends ultraviolet light up into your eyes.
On top of that, the air is thinner up a mountain, so ultraviolet light is stronger. It is said to rise by about 10% for every 1,000 metres of height. Look at Figure 1. Move the slider to change the altitude and see how much more ultraviolet light reaches your eyes on a snowy mountain than on flat grassland.
On a snowy mountain at 2,000 metres, you get about twice as much as on flat grassland. And even "twice" is a conservative estimate. For your eyes, what matters most is the light from below, which the brow and eyelids can't block. That is why you can get snow blindness even when wearing a cap.
Why does the pain come only at night?
The outermost layer of the eye is a clear membrane called the cornea. Most of the short-wavelength ultraviolet light is absorbed at the surface of the cornea. This protects the inside of the eye, but the cells on the cornea's surface take the damage.
Damaged cells don't break down right away. It's like sunburned skin that turns red and stings only in the evening. Over several hours the surface cells peel away, and then the exposed nerves start to send pain signals. Look at Figure 2.
The surface of the cornea is one of the fastest-renewing parts of the body. So in most cases it is said to heal on its own in one to three days. However, it has also been pointed out that repeated exposure may lead to other eye diseases. Just because it heals doesn't mean you can ignore it.
The Inuit of the Arctic have long used snow goggles carved from wood or bone. They have no glass, only a narrow horizontal slit. The slit is so thin that it blocks most of the light coming from above and below. It is a clever way to cut the light reaching the eye while keeping a view straight ahead.
The blue-white light from metal welding also contains strong ultraviolet light. If you look at it without a protective mask, symptoms very like snow blindness appear at night. In welding workplaces it is called "arc eye" (electric ophthalmia), a long-known occupational hazard.
So what should you do?
- Wear goggles or sunglasses that block ultraviolet lightChoose a pair that follows the shape of your face, with few gaps at the sides and bottom.
- Wear them on cloudy days just as on sunny daysPlenty of ultraviolet light passes through clouds, and it bounces back and forth between the snow and the clouds.
- If your eyes hurt at night, don't rub them; see an eye doctorTake out contact lenses, then close your eyes and cool them.
Tinted lenses that don't block ultraviolet light just make things darker. In the dark your pupils open wider, so more ultraviolet light may actually get in. Check the lens label for UV protection. If you have strong pain that lasts, or your vision seems wrong, don't wait and see on your own; go to an eye doctor right away. If you can't see well on a snowy mountain, even getting down becomes dangerous.
Summary
Snow blindness is a state in which the cornea, the surface of the eye, has been sunburned by ultraviolet light. Snow bounces ultraviolet light up from below, and higher up the mountain the ultraviolet light itself is stronger. The pain comes half a day late, so you can't notice it while you are being exposed.
On snow, there are two suns: one in the sky and one at your feet.
By the time it hurts, the exposure is already over.
For skin sunburn, see "Why can you get sunburned even on a cloudy day?". For why snow looks white, see "Why does snow look white when ice is clear?".
- On a sunny day, put a white sheet of paper and a black one on the ground, and stand next to them without tilting your face down.
- Compare the brightness under your chin and on your neck when you stand over the white paper and over the black paper, using a hand mirror.
- Over the white paper, even the underside of your chin should be lit up brightly. On snow, this is spread over the whole ground.
This experiment uses visible light, but snow reflects ultraviolet light well too. Never look directly at the sun itself.
For those who want more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics or biology
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity-level topics not taught in high school (photobiology, ophthalmology)
- ResearchNot yet settled even at university; researchers are still studying it
Middle schoolTerms: this phenomenon has names
- Snow blindness (photokeratitis): damage to the surface of the cornea caused by ultraviolet light and other light bounced off snow. Medically, it is a kind of ultraviolet damage to the cornea (photokeratitis).
- Cornea: the clear membrane covering the front of the eye. It is the window that lets light in, and it absorbs much of the short-wavelength ultraviolet light.
- Reflectance (albedo): the fraction of the light hitting the ground that is bounced back. Fresh snow is high even for ultraviolet light, at up to about 0.8.
Middle schoolHigh schoolCheck with a formula: how many times more UV on a snowy mountain than on flat ground?
We think of the ultraviolet light reaching the eye as the sum of "light from the sky" and "light bounced back from the ground". We take the sky's contribution on flat ground as 1 and calculate relative amounts.
| In symbols | U ＝ k × ( 1 ＋ R ) |
| In words | UV reaching the eye ＝ altitude factor ×(from the sky ＋ reflectance × reaching the ground) |
| Where it comes from | Amounts of light can be added (superposition). Of the light reaching the ground, only the fraction given by the reflectance bounces back up. That is the light budget behind the formula. |
| U | UV reaching the eye (relative amount, with sky UV on flat ground as 1) |
| k | Altitude factor (how UV increases with height; no unit) |
| R | Ground reflectance (0 to 1; no unit) |
| Increase with altitude | About 0.1 (about 10%) per 1,000 metres |
| Reflectance of fresh snow | Up to about 0.8 |
| Reflectance of grass or soil | Under 0.1 (taken here as 0.1, on the high side) |
| Altitude of the snowy mountain | 2,000 metres |
| Flat grassland (k＝1) | 1 ＋ 0.1 ＝ 1.1 |
| Increase at 2,000 metres | 0.1 × 2 ＝ 0.2 |
| Altitude factor k | 1 ＋ 0.2 ＝ 1.2 |
| (1 ＋ R) on snow | 1 ＋ 0.8 ＝ 1.8 |
| UV reaching the eye on the mountain, U | 1.2 × 1.8 ＝ 2.16 |
| Compared with grassland | 2.16 ÷ 1.1 ≒ 2.0 |
| Time to reach the amount received in 1 hour on grassland | 60 ÷ 2.0 ＝ 30 (min) |
On a snowy mountain at 2,000 metres, the ultraviolet light reaching the eye is about twice that on flat grassland. You would get in about 30 minutes what takes an hour on grassland. In reality, the brow and eyelids block much of the light "from the sky", so for the eye the light from the ground weighs even more.
High schoolHigh school+Different wavelengths are absorbed in different parts of the eye
High schoolUltraviolet light is divided, from longer to shorter wavelengths, into UV-A, UV-B and UV-C. UV-C is almost entirely absorbed by ozone in the atmosphere and doesn't reach the ground. UV-B, which does reach the ground, is short-wavelength, high-energy light, and it is almost entirely absorbed at the surface of the cornea. UV-B is the main cause of snow blindness.
High school+UV-A that passes through the cornea is mostly absorbed by the lens behind it. So in adults only a little ultraviolet light is said to reach the retina. The cornea and lens act as stacked filters, each taking different wavelengths. The tissue doing the absorbing is the one that carries the burden.
UniversityPhotokeratitis and the action spectrum
In medicine, snow blindness is called photokeratitis (ultraviolet keratitis). How harmful each wavelength is, is described by a weighting curve called the action spectrum. The ultraviolet intensity reaching the eye, multiplied by this weight and summed, gives the "effective dose". The guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) set an exposure limit for the eyes and skin in terms of this effective dose. The albedo of snow differs between visible light and ultraviolet light; it is highest for fresh snow and falls when the snow gets dirty or melts.
📖 Derivations and further reading: Wikipedia: "Cornea" / Wikipedia: "Albedo"
ResearchWhat is still not clear
- How to measure what actually reaches the eye. The ultraviolet light entering the eye varies greatly with face shape, brow height and gaze direction. Studies that put sensors on mannequin heads continue, but estimating the dose for each person accurately is still said to be difficult.
- Long-term effects of repeated exposure. Ultraviolet light has been linked to a disease in which the white of the eye grows over the cornea, and to cataracts. No clear line has yet been drawn for how much exposure over how many years is dangerous.
- The detailed mechanism of the delayed pain. What happens, and in what order, between the cell damage and the cells peeling away? Researchers are looking at links to the "programmes" by which cells destroy themselves, among other things.
So the contents of this article are also "an explanation of what is known so far". Still, the conclusion "protect your eyes on snow" does not change in any study.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science year 1 "Reflection of light", year 2 "Sense organs" | Snow bouncing light / structure of the eye |
| High school | Physics "Wavelength of light", Biology "Receptors" | UV-A, B, C and where they are absorbed |
| High school+ | Physics / earth science sidebars (ozone layer, ultraviolet light) | Cornea and lens as stacked filters |
| University | Photobiology, ophthalmology, environmental radiation | Photokeratitis, action spectrum, albedo |
| Research | Environmental epidemiology, eye research | Estimating the dose to the eye, long-term effects |
| ― | Links to daily life | Goggles for skiing and climbing, cloudy-day precautions, welding masks |
- World Health Organization (WHO) and others, "Global Solar UV Index: A Practical Guide" (UV reflectance by type of ground, increase with altitude).
- Ministry of the Environment, Japan (環境省), "Manual on UV and Health" (reflection from snow, effects on the eyes, choosing sunglasses).
- Japan Meteorological Agency (気象庁), "Properties of ultraviolet light" (passage through clouds, surface reflection, change with altitude).
- International Commission on Non-Ionizing Radiation Protection (ICNIRP), "Guidelines on limits of exposure to ultraviolet radiation" (action spectrum and exposure limits for the eye).
- Wikipedia: "Cornea"
*This article is a general science explainer. The numbers given are rough guides to help you understand the mechanisms. If you have eye pain or any change in your vision, don't judge for yourself; see an eye doctor. When climbing or skiing, follow the guidance and instructions of the facility or local authority.