What does the UV index mean? Every number, explained

The UV index is a single number for how much skin-damaging ultraviolet radiation reaches the ground where you are. The World Health Organization (WHO) developed it with other UN agencies so the scale reads the same in every country: it starts at zero and climbs with no fixed top. The higher it goes, the more harm the sun can do to your skin and eyes, and the less time that harm takes. At 3 or higher, protect your skin.

What do the UV index numbers mean?

Those five names come from the WHO’s practical guide to the index, which also gives each band a color: green, yellow, orange, red and purple. If your weather app colors the number, that’s what the colors mean. The advice comes in three steps (under 3, 3 to 7, 8 and up), and the line that matters most for your mornings is 3.

The scale also climbs in a straight line. By the WHO’s own formula, each point stands for the same amount of sunburn-causing UV, so a UV 10 afternoon carries twice the burning power of a UV 5 one.

What UV index do you need sunscreen at?

From 3. Below that, the WHO says the risk of short-term and long-term UV damage is limited, even for very fair skin that burns easily. That’s the rule Nubia’s Today follows: it shows the UV where you are and only calls sunscreen optional when the day’s peak stays under 3.

A few situations are worth covering up below 3 anyway: if you’re on a retinoid or using exfoliating acids, if you’ll be outside for hours, and on snow or water, where the UV hitting you runs higher than the forecast. On days of 3 or more, sunscreen is one piece of the kit, alongside shade, a hat and sunglasses.

Why doesn’t the UV index tell you how long until you burn?

Plenty of people search for a time-to-burn chart, and some countries used to publish burn times next to the index. The WHO’s guide advises against it, because people read a burn time as a safe amount of unprotected sun. The damage behind skin cancer and skin aging adds up even on days you never turn pink. Cancer Council Australia is blunter still: at UV 3 or above, just 15 minutes of unprotected exposure is enough to start damaging skin.

When is the UV index highest?

Around solar noon, when the sun is at its highest. The WHO puts the daily peak in the four hours around solar noon, which falls somewhere between noon and 2 p.m. depending on where you live and whether the clocks have gone forward. The number in a forecast is usually that peak, so early mornings and late afternoons run lower.

No app handy? Use the shadow rule from the US Environmental Protection Agency. If your shadow is shorter than you are, UV is high, so head for shade and put sunscreen on. A shadow taller than you means lower UV, which is typical early and late in the day.

UV follows the seasons too, peaking in summer outside the tropics, and the closer you are to the equator, the higher it runs. It tracks the sun’s height rather than the temperature, though. Cancer Council Australia notes that temperature doesn’t affect UV at all, which is how a chilly spring afternoon can still burn (spring sun has its own article).

Where does UV run higher than the forecast?

The index describes UV on flat, open ground. What actually reaches your face depends on where you’re standing.

Altitude and snow are the big two. The WHO says UV rises about 10% to 12% for every 1,000 meters you climb, because thinner air absorbs less of it, and fresh snow can reflect up to 80% of UV back up at you.

Water and sand add more. Sea foam reflects about 25% and dry beach sand about 15%, and being in the water gives you only minimal protection. Clouds are a mixed bag: Cancer Council Australia says thick cloud filters a good amount while thin cloud lets plenty through.

Put a ski day together. Say the forecast is for the town in the valley and the lift takes you 1,000 meters higher: add roughly 10% for the height, then fresh snow bounces UV up under your chin and nose. The WHO’s guide says snow reflection can double your overall exposure, especially at high altitude. That’s an SPF 50 day with goggles or wraparound sunglasses, whatever the thermometer says.

Can you tan at UV index 3?

Yes, and that’s exactly the problem: if your skin tans, the UV was strong enough to harm it. The WHO describes a tan as your body defending itself against more damage, and says a dark tan on fair skin gives protection of only about SPF 4. So there’s no safe tanning number on the scale, and a base tan before a vacation can’t stand in for sunscreen.

Safety note: Snow and water can burn your eyes as well as your skin. Wear wraparound sunglasses on bright days out, and see a doctor if your eyes turn red, gritty and painful after time in the sun.