Why You Can Get Sunburned on a Cold or Cloudy Day
Sunburn comes from UV, not heat, and the two regularly part ways. Why a cool spring afternoon or a grey overcast sky can still carry high UV, and how to read the UV index instead of the thermometer.
On this page
The thing that burns your skin is invisible, gives off no warmth of its own, and cannot be felt. So the way a day looks and feels is a poor guide to it. A cold, bright afternoon in spring can burn you; so can a grey, overcast one. The mistake in both cases is the same: reading the temperature, or the cloud, instead of the ultraviolet.
Sunburn is caused by UV radiation, not by heat. They usually rise and fall together in high summer, which is why the two get confused, but they are driven by different things and they part ways more often than most people expect. The chart below shows one city's UV and temperature side by side across a year.
Monthly clear-sky (cloud-free) UV index against the average daytime high in Munich, a 2019–2024 average. UV tracks the sun's height, cresting just after the solstice, and is already in the "high" band by May; temperature lags by weeks, staying warmest through August. The same UV level turns up in spring and autumn at very different temperatures. Source: CAMS reanalysis (UV) and ERA5 (temperature), via Open-Meteo.
Sunburn comes from UV, not heat
Reddening, peeling and the long-term damage that follows are the work of ultraviolet radiation. The World Health Organization lists sunburn among the acute effects of UV, and it opens its account of the subject with the detail that matters most here: UV radiation can neither be seen nor felt. The warmth of sunshine on your skin is real, but it is not the UV, and it is not what causes the burn.
That single fact undoes the everyday shortcut of judging the sun by how it feels. Warmth is the cue people trust, and it fails whenever UV and temperature come apart. The US Centers for Disease Control and Prevention puts the practical version bluntly: "UV rays can reach you on cloudy and cool days, and they reflect off of surfaces like water, cement, sand, and snow." A cold day is not a safe day, and neither is a dull one.
The UV index is built from the sky, not the thermometer
Look at what actually sets the UV index, and temperature is simply not on the list. The WHO names the factors that raise or lower solar UV: the sun's elevation, latitude, altitude, cloud cover, the ozone layer, and reflection from the ground. The forecast models work from the same inputs. The US EPA's description of the calculation builds the index from the sun's angle, the amount of ozone, the elevation and the cloud, and nowhere does it read the air temperature.
The reason is physical. UV is the sunburning part of sunlight, and how much of it reaches the ground depends on how directly the sun strikes and what the atmosphere does to the beam on the way down. How warm the air happens to be is a separate result of season, wind and the slow heating of land and sea. The two often move together, but nothing forces them to. When they diverge, the UV index tells you about the burn and the thermometer does not.
Spring feels safe long before it is
The clearest place to watch UV and temperature separate is spring. UV follows the height of the sun, which climbs to its peak at the June solstice and is already high for weeks on either side. Temperature does not keep pace, because land and water take time to warm. This delay has a name, the seasonal lag, and the Royal Meteorological Society explains it simply: the longest day is not the warmest, because the land and the oceans warm up slowly, and water's large heat capacity carries the warmth well past midsummer.
The Munich figures above show the gap plainly. Clear-sky UV averages about 6 by May, already in the "high" band, on days when the daytime high is a mild 17°C. The same UV level does not return until August, by which point the afternoons are running near 24°C. April and September are near mirror images for UV, both close to 5, yet September's afternoons are some 6°C warmer than April's. A May afternoon that feels like a jacket day carries roughly the sun of an August one. The daily shape of that curve, and why the sun's height governs it, is the subject of when is UV strongest.
Cold and bright can be the strongest combination of all
Altitude pushes the split further. UV rises with height because there is less atmosphere overhead to absorb it, by roughly 6% for every kilometre of elevation according to the EPA. Mountain air is thin and cold at the same time, so a freezing ski slope can deliver more UV than a warm beach at sea level. Snow then compounds it, because fresh snow is a strong reflector that adds to the UV arriving straight from the sun; that and the other surface effects are laid out in what affects the UV index.
The extreme case is a place that is high, sunny and near the equator all at once. Quito sits almost on the equator at about 2,850 metres, and in 2024 its clear-sky UV index reached the extreme band, 11 or above, on every single day of the year, with a median daily peak around 15. Its daytime highs over the same year stayed mild, mostly between 13 and 23°C. A city that never feels hot can sit under some of the most intense sunburning radiation on the planet. Cold-climate cities feel the milder version of the same effect: high-altitude, snowy places such as Denver can pair sub-zero mornings with UV worth taking seriously.
Clouds do not switch the UV off
The other half of the illusion is grey skies. Overcast reads as gloom, gloom reads as harmless, and that chain of assumptions is wrong. The WHO notes that UV levels are highest under cloudless skies but can remain high even with cloud cover, and the CDC counts cloudy days among those when UV reaches you. Thin or broken cloud in particular lets most of the UV through, and even a solid overcast passes a meaningful share of it. How much survives each kind of sky, from thin haze to heavy cloud and on through glass and water, is set out in does UV go through clouds, windows and water. The lesson for this page is narrower: a sky that looks soft is not the same as a sky that is safe. Seattle and other cities with cloudy reputations still run their UV into the protection range through the summer half of the year.
What this means in practice
Read the UV index, not the temperature or the sky. The number is built from the sun, ozone, altitude, cloud and reflection, and it already accounts for the things your senses miss. The WHO advises sun protection once the UV index reaches 3, whatever the thermometer says. What each level means is set out in what is a safe UV index.
Treat spring, height and snow with respect. A cool clear day in April or May, a mountain in any season, and bright snowfields all combine low temperature with high UV. These are the situations where trusting the warmth will let you down.
Check the forecast before a grey day fools you. Cloud lowers UV but rarely removes it, so an overcast afternoon can still need protection. A quick look at the hourly forecast settles it; how those forecasts are made, and how far ahead to trust them, is covered in how UV forecasts work.
Methodology and sources
The Munich chart shows the monthly average of each day's peak clear-sky UV index against the monthly average daytime high, over the six years 2019 to 2024. Clear-sky UV comes from the CAMS reanalysis and temperature from ERA5, both retrieved through Open-Meteo and rounded. The Quito figures are drawn the same way from 2024 data. Clear-sky (cloud-free) values are used so the comparison reflects the sun's own seasonal cycle rather than one year's weather; real UV on a given day is lower when clouds are present. Every city page on UVI.today shows the live hour-by-hour UV index from the same CAMS model; how those forecasts are produced is described on the methodology page.
- World Health Organization: Ultraviolet radiation fact sheet — UV cannot be seen or felt; it causes sunburn; the factors that set UV levels; protection advised at UV 3.
- US CDC: Sun Safety — UV rays reach you on cloudy and cool days and reflect off water, cement, sand and snow.
- US EPA: Learn About the UV Index — how the index is calculated from sun angle, ozone, cloud and elevation, and the roughly 6% rise per kilometre of altitude.
- Royal Meteorological Society: Summer solstice and seasonal lag — why the warmest weather arrives weeks after the longest day.
- Copernicus Atmosphere Monitoring Service (CAMS) and Open-Meteo — the clear-sky UV index and temperature data behind the charts.
Frequently asked questions
Can you get sunburned on a cloudy day?
Yes. Sunburn is caused by ultraviolet radiation, and clouds do not block all of it. The US CDC states plainly that UV rays reach you on cloudy days, and the WHO notes that UV levels can be high even under cloud cover. A bright overcast can still leave the UV index in the range where protection is advised. How much different skies let through is covered in the guide to whether UV goes through clouds, windows and water.
Can you get sunburned when it is cold?
Yes. Temperature and UV are set by different things. The UV index depends on the sun's height, the ozone layer, altitude, cloud and reflective ground, and air temperature is not one of its ingredients. Cool clear spring days, high mountains and snowfields can all pair low temperatures with high UV.
Why is UV high in spring when it is still cool outside?
Because UV follows the sun's height, which peaks at the June solstice, while temperature lags by several weeks as land and water slowly warm. This delay is called the seasonal lag. By May the midday sun is nearly as high as in July, so clear-sky UV is already in the high range even though the warmest weather is still weeks off.
Does a high temperature mean high UV?
No. A hot day is not necessarily a high-UV day, and a cold day is not necessarily a low-UV one. The only reliable guide is the UV index itself, which the WHO says calls for sun protection once it reaches 3.
