1. Why Eye Pain from Snow Arrives at Night, Not Right Away
You might play in the snow all day and feel fine until evening, when your eyes begin to feel gritty, watery, and sensitive to light. This condition is known as snow blindness, or photokeratitis. It is caused by UV rays reflected by the snow damaging the cornea, the clear surface at the front of the eye[3].
The tricky part is that the pain is delayed. One medical source notes that symptoms appear a few hours after UV exposure[3]. The MSD Manual states that symptoms usually do not appear for 8 to 12 hours and then last for 24 to 48 hours[4]. Because sources vary, it is safe to assume the delay is anywhere from a few hours to half a day. Since there is no pain while you are being exposed, it is easy to think, "Today is fine."
However, the surface of the cornea naturally regenerates within 24 to 48 hours, and permanent vision damage is very rare[4]. If you experience severe symptoms, you should see an eye doctor. But first, how much UV light is the snow actually reflecting?
2. How Much UV Fresh Snow Reflects Compared with Sand, Grass, and Asphalt
The Japan Meteorological Agency lists UV reflection rates by ground type. A sandy beach reflects about 25%, while fresh snow reflects up to 80%. Grass and asphalt reflect 10% or less[1]. If you imagine UV strength as 100, fresh snow can reflect about 80 units back upward.
Dividing the 80% reflection of fresh snow by the 25% of sand gives a ratio of 3.2 times. Compared to grass at 10%, it is more than 8 times higher. Note that the 80% figure refers specifically to fresh snow, likely on a clear day, as suggested by the context of the Agency’s page[1].
Snow reflects so much light because ice absorbs very little UV radiation. In snow, where ice particles are packed closely together, light bounces between the particles and returns upward without being absorbed. Research indicates that the finer the particles, the higher the reflection[5]. However, other studies report snow reflection rates of 50 to 60%. Why do the numbers differ for the same substance?
3. Why Studies Report Different UV Reflection Rates for Snow
A study from Lithuania reports the UV albedo (reflectivity) of natural surfaces, stating that snow is at most 50 to 60%, sand is 10%, and grass is 2 to 3%[6]. This differs from the Japan Meteorological Agency’s 80% for fresh snow.
Research explains that snow’s reflection rate is high when the particles are fine and fresh. It decreases as time passes, particles become coarser, dirt gets in, or the snow melts and mixes with water[5]. The Lithuanian value is described as the maximum for snow under specific conditions. The sources do not specify exactly which conditions cause the gap between 50–60% and 80%.
Another number often confused is the total albedo of fresh powder snow, which can be close to 95%[7]. However, this refers to the reflection of all sunlight, not just UV rays, so it is different from the 80% UV figure.
Therefore, it is difficult to pin down "how much UV snow reflects" with a single number. Yet, all sources agree that snow reflects far more UV than sand or grass. So, where does this reflected light hit your body?
4. How UV Rays Reach You from Both the Sky and the Snow
On flat ground, UV rays mainly come from the sky. But on fresh snow on a clear day, your body is hit by light from the sky and light reflected by the snow surface. Snow scientists explain that this means you get sunburned from two directions: above and below[7].
Consider this: the brim of a hat blocks light coming from the sky. However, light coming from below can easily wrap around under the brim. This might be one reason why faces and chins are prone to burning on ski slopes. Note that this is an inference based on the two-direction explanation, not a conclusion stated directly in the sources.
There are numbers for this effect too. The Japan Meteorological Agency writes that in a snow-covered area like Antarctica, the UV Index increases by 40 to 50% compared to a scenario with no snow, due to reflection and re-scattering in the atmosphere[1]. Light reflected by snow hits air particles and heads back toward the ground.
However, this value applies to areas covered entirely in snow. It cannot be directly applied to Japanese ski resorts, which have mixed forests and buildings. There is another factor to consider: being high up in the mountains.
5. How Higher Altitude Increases UV by About 10% per 1000m
At higher altitudes, there are fewer atmospheric particles to weaken UV rays. As a result, the UV rays reaching from above are stronger[7].
According to the Japan Meteorological Agency, the UV Index increases by about 10% for every 1000m (about 3280 feet) increase in altitude[2]. The same page provides an observation example: on a clear day, a certain location was about 40% stronger than Tsukuba, which is at an altitude of 31m (about 102 feet). Calculating this per 1000m yields about 15%. This shows that 10% is a rough guide, and the actual increase varies with weather and day.
Let’s put these factors together. Fresh snow on a clear day reflects more than three times the UV of a sandy beach. Light comes from two directions. At higher altitudes, the amount from the sky increases. Furthermore, there is no eye pain to warn you while you are being exposed. The reason to be careful on snowy days is not one single factor, but the combination of all these elements.
The sources do not state exactly how strong the UV becomes when all these factors overlap. Since they cannot simply be added together, we will not assign a final number here.
6. How to Check Reflected Light at Home with White and Black Paper
You can test this concept at home. Place a white sheet of paper and a black sheet of paper side by side on the ground in the sunlight. Hold your palm face-down, about 10cm (about 4 inches) above the paper. Since your palm is facing away from the sun, only the light reflected from the paper should hit it. The area above the white paper should appear brighter.
This test checks visible light, not UV rays. However, it demonstrates the idea that light reflected from below hits your body. Do not look directly at the sun.
To investigate further, the Japan Meteorological Agency’s "Knowledge of UV Rays" includes pages on reflection rates and altitude. You can check how the figures of 80% for snow and 25% for sand are written, along with the observation comparing Tsukuba at 31m. Use this data to think for yourself about whether snow’s reflection can be defined by a single number.