What the UV index actually means
The UV index measures how strongly sunlight at ground level can damage skin and eyes: specifically the wavelengths responsible for sunburn, premature ageing and, with repeated exposure, skin cancer. It has no fixed ceiling: readings above 11 are routine near the equator and at altitude. It is not a brightness reading and it is not a proxy for how warm the day feels; it measures erythemal irradiance, and it behaves differently from both.
Below UV 3 (low), unprotected skin is not at meaningful risk during an ordinary day outside. From 3 to 5 (moderate), this site's own default warning threshold, sunscreen, sunglasses and some midday shade are worth the ten seconds they cost. From 6 to 7 (high), unprotected skin burns within roughly an hour for most skin types, so plan around midday sun. From 8 to 10 (very high), a full afternoon outside unprotected is a mistake, not bad luck. At 11 and above (extreme), common in the tropics and at altitude, fair skin can burn in under twenty minutes.
Two things fool people into skipping protection. The first is cloud cover: only thick, low, unbroken cloud cuts UV meaningfully. Thin or broken cloud transmits most of it straight through, and light scattered off cloud edges can occasionally push a reading higher than a clear sky would. That is one reason this site blends two independent forecast models and always takes the higher one; cloud is exactly where they disagree most. The second is air temperature, a different quantity entirely: driven by air mass and season, not by the sun's angle. A cold, clear day burns exactly as fast as a warm one at the same reading.
Altitude and snow both raise the real risk for the same underlying reason: thinner air scatters and absorbs less UV on the way down. This site adds roughly 8% to the index for every 1,000 metres of elevation above 300m, which is why Cusco, La Paz, Denver and Addis Ababa read higher than a coastal city at the same latitude and season. Snow compounds it further. Fresh snow reflects up to 80% of incoming UV back upward, so a skier on a sunny day at altitude is being irradiated from above and below at once, a combination no thermometer will ever show.
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Frequently asked questions
By WHO convention, UV 3 and above needs active protection: sunscreen, sunglasses, some shade around midday. That's also why 3 is this site's own default warning threshold. UV 6–7 is high enough that unprotected skin burns within about an hour for most skin types. UV 8–10 is very high, and 11 or above is extreme, where fair skin can burn in under 20 minutes.
It depends on skin type. Unprotected, skin that always burns and never tans (type I) reaches its burn threshold in about 17 minutes at UV 8; skin that burns easily (type II, this app's default) in about 21 minutes; deeply pigmented skin (type VI) in about 1 h 23 min. Those figures come straight from minutes = MED ÷ (UVI × 1.5), the same formula used everywhere on this site. Sunscreen extends the number by its SPF, but real-world application is about a third of the tested layer, so treat any protected time as a ceiling, not a promise.
Only if the cloud is thick, low and unbroken. Thin or broken cloud transmits most of the clear-sky UV straight through, and light scattered off the edges of broken cloud can occasionally push a reading higher than a clear sky would. This is exactly why this site blends two independent forecast models and always takes the higher one. Cloud is the variable they disagree on most.
No. They are different physical quantities. UV is driven by the sun's angle and how much atmosphere the light passes through; temperature is driven by air mass and season. A cold, clear day burns exactly as fast as a warm one at the same UV reading, which is why a cool morning is not a reason to skip sunscreen.
Around solar noon, typically within a 10:00–16:00 local window, though the exact hours shift with season and latitude. This site's danger windows are calculated by interpolating the forecast to the minute rather than rounding to the nearest hour, so the start and end times shown for any location are exact, not approximate.
Usually less, but not never. Beyond about 50° latitude, winter sun sits too low for meaningful UV for months at a time, and the index typically stays under 3. Closer to the equator, at altitude, or on snow, winter UV can still reach the moderate band. Snow is the exception that catches people out: fresh snow reflects up to 80% of incoming UV back upward, so a sunny day skiing at altitude can carry more real risk than a summer day at sea level.
Roughly UV 3 is the threshold: below that, skin makes no meaningful vitamin D no matter how long you stay out. Above it, 10–15 minutes of bare face and forearms is enough once UV reaches 6 or higher, or 15–25 minutes between 3 and 6. Longer exposure doesn't make more vitamin D; it only adds burn risk with nothing to show for it.
Yes. Thinner air scatters and absorbs less UV on the way down, so this site adds roughly 8% to the index for every 1,000 metres of elevation above 300m. At 3,000m that works out to about 24% higher than the same sun angle would give at sea level, part of why Cusco, La Paz, Quito, Denver and Addis Ababa read higher than their latitude alone would suggest.