Why can you get sunburned
on a cloudy day?
"It's cloudy and cool today, I'll be fine" β you spend the whole day outside thinking that, and by evening your skin is stinging. It's a mistake almost everyone makes at least once, and there's a clear reason for it. The UV rays that cause sunburn are not the same as visible light, and they're not the same as heat, either.
An autumn sports day. The sky is thinly overcast all day, the light is soft, the breeze is cool. Even standing in the open, it's neither glaring nor hot. "I won't need sunscreen for this," you think, and spend the day on the sidelines with nothing on.
That evening, your arms sting in the bath. You look in the mirror and find your face and neck are faintly red β even though you barely noticed any sun all day.
The truth is, the "heat" your skin feels and the "UV" that causes sunburn are measured by completely different things. On cool days, on cloudy days, the UV keeps coming down all the same.
There are just two reasons
Clouds block visible light well, which is why it gets dark and cool. But the UV that causes sunburn is different β through thin cloud, around 80% of it is thought to get through. "How dark it looks" is no gauge of UV at all.
UV easily scatters in every direction off air molecules, so it rains down from the entire sky, not just from the sun's direction. That's why it never drops to zero β not in shade, not even when the sun is hidden behind cloud.
Put these two together, and you get the opening scenario: a slow, unnoticed burn on a cool, cloudy day. Let's look at each one in turn.
"Heat" and "UV" are measured differently
Sunlight is a mix of several kinds of light: visible light (what makes things bright), light you feel as heat on your skin (what makes things warm), and UV, which neither your eyes nor your skin's nerves can detect.
The tricky part is that these three behave differently around cloud. Both the heat-carrying light and visible light are weakened a lot by cloud, so cloudy days feel dark and cool. But UV has a shorter wavelength, and most of it slips through thin cloud. In other words, a cloudy day is exactly the kind of condition where your "heat sensor" alone gets caught off guard.
Air temperature isn't a reliable guide either. UV strength is mostly determined by how high the sun sits in the sky (season and time of day), and temperature barely factors in. In Japan, UV peaks from May through August, and on a sunny day in May β before it's even properly hot β UV levels can already be close to midsummer strength.
Even on a sunny day, roughly half of the UV reaching the ground is thought to be this "scattered from the whole sky" portion. A parasol blocks direct sun, but it barely stops the scattered UV coming from the rest of the sky. The accurate rule of thumb: stepping into shade roughly halves your UV exposure β it doesn't zero it out. On sand or snow, reflection off the ground adds even more (fresh snow is said to reflect around 80% of UV, which is why faces burn at winter ski resorts).
So what should you actually do?
The logic behind protection is simple: judge by "is the sun high, in a high-UV season and time of day" rather than "does it feel hot." In Japan, that means May through August, and within the day, roughly 10 a.m. to 2 p.m., when UV peaks. On days that fit this window, spending a long time outside β cloudy or cool or not β is said to call for a hat, long sleeves, and sunscreen. The most reliable approach is to check the "UV Index" (a measure of UV strength) that the Japan Meteorological Agency publishes daily alongside the weather forecast.
Summary
There are two reasons you can get sunburned on a cloudy day: β Unlike visible light or heat, UV passes through roughly 80% of thin cloud. β‘ UV also falls as scattered light from the whole sky, so it never reaches zero in shade or under cloud. Your skin's "hot" or "glaring" sensors simply can't detect UV.
Sunburn is decided by "how high the sun is," not "how hot it feels."
A cool, cloudy day is exactly when to stay alert.
"Shorter wavelengths of light scatter more off air molecules" β the very property that lets UV fall from the whole sky is actually the same physics behind why the sky is blue. We explain this in more detail in this article.
Also, even though the outdoors feels dark on a cloudy day, it's actually about 20 times brighter than indoors. We explain why your eyes don't notice that gap in Why does a bright room and sunny outdoors feel about equally bright?
- Put a sheet of deeply coloured origami paper or newspaper by a sunny window
- Place a coin or paperclip β anything with a clear shape β on top, and leave it for several days to a week
- Remove the object and compare the colour of the covered part against the exposed part
Only the exposed part fades, leaving a crisp outline of the coin's shape. The main culprit behind the fading is UV, which breaks down the pigments in the paper. If you see a difference even after a run of cloudy days, that's visual proof that "UV gets through even when it's cloudy."
Want to go deeper? β Terms, formulas, and where this fits in the curriculumWe've marked which level each part belongs to, from middle-school science to university-level subjects
- MSCovered in middle-school science
- HSCovered in "Basic Physics" / "Basic Biology" in high school
- HS+Advanced high-school "Physics," or textbook sidebar material
- Univ.Not taught in high school β university-level content (photobiology, atmospheric science)
- ResearchNot yet settled even at university level β an active research question
MSTerms: this light has names and types
- Ultraviolet (UV): light with a shorter wavelength than visible violet light. Divided by wavelength into UV-A, UV-B, and UV-C.
- UV-B: the main culprit behind skin reddening (sunburn). Much of it is absorbed by the ozone layer, so only some reaches the ground.
- UV-A: longer wavelength than UV-B, and there's more of it. It reaches deep into the skin and is thought to be involved in slower, cumulative damage (wrinkles, sagging). It also passes through window glass.
- UV-C: the shortest wavelength and highest energy, but it's almost entirely absorbed by the atmosphere and never reaches the ground.
- UV Index: an international scale from 0 to 11+ for ground-level UV strength. The Japan Meteorological Agency issues a daily forecast for it.
- SPF / PA: sunscreen effectiveness ratings. SPF is a guide to UV-B protection, PA to UV-A protection.
MSHSCheck the numbers: comparing cloud, shade, and sunscreen
Let's set the UV strength of direct sun on a clear day at 100, and compare various conditions against it.
| In symbols | I = Iβ Γ TγγοΌγγt = tβ Γ SPF |
| In words | UV reaching the ground = UV on a clear day Γ the fraction cloud lets through (transmittance). Time until skin reddens with sunscreen on = time until bare skin reddens Γ SPF |
| Where these come from | The first comes from "transmittance" β the idea that light passing through cloud gets through at some fixed fraction. The second comes from how SPF is defined: the ratio of UV needed to redden skin with sunscreen versus bare skin. |
| Symbol | Meaning and unit |
|---|---|
| Iβ | UV strength on a clear day (set here to 100) |
| T | fraction of UV that cloud or shade lets through (0β1) |
| tβ | time until bare skin starts to redden (minutes) |
| Thin cloud (said to let through 80%) | 100 Γ 0.8 = 80 |
| Shade (blocks direct sun, only scattered light from sky remains) | 100 Γ 0.5 = 50 |
| Fresh-snow ski slope (direct 100 + 80% snow reflection) | 100 + 80 = 180 |
"Cloud 80, shade 50" β these numbers are in the same ballpark as direct sun on a clear day. On snow, it can actually exceed clear-sky levels at ground level. That's why, on a winter mountain or spring ski trip, it's specifically your face that ends up burned β this addition is the reason.
| Time until bare skin starts to redden (rough guide) | said to be around 20 minutes (varies greatly by skin type and season) |
| Theoretical value with SPF 30 applied correctly | 20 Γ 30 = 600 (minutes) |
| Converted to hours | 600 Γ· 60 = 10 (hours) |
SPF is a multiplier on how much longer it takes skin to redden. But this is a theoretical value assuming the recommended amount (more than you'd think) applied evenly. In practice people apply too little and it washes off with sweat, so reapplying every 2β3 hours is recommended. Reapplying and applying enough is said to matter more than chasing a bigger SPF number.
HSHS+Why does UV come from "the whole sky"?
HSHow much light scatters off air molecules depends heavily on wavelength. Shorter wavelengths scatter far more, and since UV has a shorter wavelength than any colour of visible light, it scatters enormously throughout the atmosphere.
HS+This "shorter wavelength scatters more" rule is called Rayleigh scattering (scattering strength is inversely proportional to the fourth power of wavelength) β and it's exactly the same law that explains why the sky looks blue (blue light scattering across the whole sky). UV scatters even more than blue light, so the entire sky is, in effect, glowing faintly with invisible UV. That's the true source of the scattered UV that reaches even the shade.
Univ.The biology of sunburn: how light damages DNA
UV-B energy is absorbed directly by DNA in skin cells, causing neighbouring bases to bond incorrectly (a classic example is a lesion called a thymine dimer). Cells have repair enzymes that fix most of this damage, but with high enough exposure, repair can't keep up, leading to inflammation (the redness of sunburn) and, over the long term, an accumulation of mutations. Skin darkening is a defence response β the body increases pigment (melanin) production to shield DNA from UV. Sunscreen ingredients are either molecules that absorb UV or particles that reflect and scatter it, intercepting this attack before it reaches the skin.
ResearchWhat's still not fully understood
The relationship between UV and health resists a simple "less exposure is always better" answer.
- The "optimal" amount of exposure is still unsettled. UV damages skin, but it's also what the body needs to produce vitamin D. The required amount varies greatly with latitude, season, and skin tone, and some research points to vitamin D deficiency from "over-protecting" β quantifying the right balance is an ongoing question.
- Monitoring of ozone-layer recovery and long-term UV trends continues. International regulation has put the ozone layer on a recovery trend, but how regional UV levels will change going forward, given interactions with climate change, remains an active area of observation and forecasting.
- Assessment of sunscreen ingredients' environmental impact is ongoing. Some UV-absorbing compounds have been reported to affect marine life such as coral, and some regions now restrict certain ingredients. Quantifying the scale of the impact and developing alternatives are current research topics.
In other words, this article too reflects "what's understood so far." How to live alongside light we can't see is an ongoing question spanning physics, biology, and environmental science.
Where this fits in the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science γ» properties of light | Visible vs. invisible light, differing behaviour with cloud |
| HS | Basic Physics γ» wave properties; Basic Biology γ» body mechanisms | Wavelength and scattering, melanin defence |
| HS+ | Physics γ» scattering of light (advanced) | Shorter wavelength scatters more β same law as a blue sky |
| Univ. | Photobiology γ» atmospheric science | DNA damage and repair, ozone layer and UV |
| Research | Public health γ» environmental science (unresolved) | Optimal balance with vitamin D, environmental impact of sunscreen |
| β | Everyday connections | Using the UV Index, reading SPF, caution on snow and in shade |
- Japan Meteorological Agency (ζ°θ±‘εΊ), "Properties of UV Radiation" and "What Is the UV Index" (cloud transmittance, proportion of scattered light, snow reflection, seasonal and time-of-day variation).
- World Health Organization (WHO), Global Solar UV Index: A Practical Guide (definition of the UV Index and protection guidelines).
- Ministry of the Environment, Japan (η°ε’η), "Manual on UV Radiation and Health" (η°ε’ηη΄«ε€η·η°ε’δΏε₯γγγ₯γ’γ«) (properties of UV-A/B/C, meaning of SPF/PA, how to use sunscreen).
- Standard photobiology textbooks and review articles (formation of thymine dimers and DNA repair, melanin's photoprotective role).
- Downs, C. A. et al., Toxicopathological effects of the sunscreen UV filter oxybenzone, Archives of Environmental Contamination and Toxicology 70, 2016 (report on UV filters and effects on coral).
β»This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the underlying mechanism. If skin symptoms are severe or persistent, we recommend seeing a dermatologist. For choosing and using sunscreen, follow product instructions and expert advice.