SunticHighest UV Index in the World: Where the Sun Is Strongest
The UV index is usually described on a scale that runs from 0 into the 11+ "extreme" range, which can make it sound like 11 is the ceiling. It isn't. In the right conditions, at high altitude, near the equator, under a thin ozone layer and clear skies, the measured UV index climbs well beyond that. This guide looks at what pushes UV to its highest, where on Earth it gets most extreme, and the sunny cities where everyday UV runs high.
What makes UV extreme?
A handful of factors stack up to drive the UV index higher. The more of them line up, the more extreme the reading:
- Latitude: near the equator the sun passes high overhead, so its rays travel through less atmosphere and arrive stronger.
- Altitude: UV rises by roughly 10 to 12 percent for every 1,000 metres of elevation, because there is less air above you to absorb it.
- Time and season: UV peaks around solar noon and over summer, when the sun is highest. See when the sun is strongest.
- Ozone and clear skies: a thinner ozone layer and cloudless skies let more UV through to the ground.
- Reflection: snow, water and sand bounce UV back up, adding to your dose. See how surfaces reflect UV.
How high can the UV index actually go?
Although forecasts often top out at "11+," the real scale has no fixed ceiling. Researchers studying high-altitude sites in the Bolivian Andes reported a UV index above 40, measured near the Licancabur volcano at extreme elevation close to the tropics. That is far beyond anything you would experience at sea level, but it shows what the combination of altitude, latitude and thin ozone can produce. For most people, anything in the very high (8 to 10) or extreme (11+) range already calls for full protection.
Where on Earth is UV highest?
The most extreme readings on the planet come from high-altitude tropical locations, where thin air and a near-overhead sun combine. The Andean altiplano of Bolivia, Peru and Chile, and the high deserts like the Atacama, top the list. Parts of the Southern Hemisphere also see unusually high UV because the Earth is slightly closer to the sun during the southern summer and the ozone layer has historically been thinner, which is one reason Australia records some of the world's highest populated-area UV.
Sunny cities where everyday UV runs high
You don't need a mountaintop for high UV. Plenty of popular cities see very high or extreme midday UV for much of the year. A few from our local guides:
- Deserts: Dubai, Phoenix and Las Vegas get relentless clear-sky sun and extreme summer UV.
- Tropics and equator: Singapore, Honolulu, Phuket and Cancún run high all year round.
- Southern Hemisphere: Sydney, the Gold Coast and Rio de Janeiro reach extreme UV over their summer.
Browse all of our city UV guides for the local picture, then check the live reading wherever you actually are.
High UV isn't only in hot places
It is easy to assume extreme UV means extreme heat, but the two don't always travel together. Snowy mountains at altitude can deliver very high UV while feeling cold, because snow reflects UV and thin air absorbs less of it. Skiers and mountaineers regularly burn for exactly this reason. The lesson is the same everywhere: judge the risk by the measured UV index, not the temperature.
Know your real number, wherever you are
Because UV swings so much with place, altitude and time, no global figure tells you your risk. Suntic shows the live, location-accurate UV index for your exact spot on your iPhone, with a 10-day forecast, so you can see when it climbs into the high or extreme range and protect your skin accordingly. This is general information, not medical advice.
Frequently asked questions
What is the highest UV index ever recorded?
Researchers measured a UV index above 40 in the high-altitude Bolivian Andes, far beyond the usual "11+" extreme range. Such readings need a rare mix of high altitude, a near-overhead tropical sun, thin ozone and clear skies, so they are not seen at sea level.
Which city has the highest UV index?
Among popular cities, hot desert and tropical spots like Dubai, Phoenix, Las Vegas, Singapore and Honolulu see very high or extreme midday UV for much of the year, and Australian cities such as Sydney record some of the highest populated-area UV in summer.
What is the maximum on the UV index scale?
There is no true maximum. Forecasts often label everything from 11 upward as "11+" or "extreme," but the underlying measurement can go much higher in extreme conditions. For practical purposes, anything 11 or above calls for full protection.
Where is the sun strongest in the world?
The most extreme UV is measured at high-altitude tropical sites such as the Andes and the Atacama Desert, where thin air and a high sun combine. Australia and other clear-skied Southern-Hemisphere regions also see exceptionally high UV in summer.
