The number you will see cited is about 43.3, recorded on 29 December 2003 at the summit of Licancabur on the Bolivia–Chile border, around 5,920 m (19,400 ft). The instrument paper is Cabrol et al., 2014 in Frontiers in Environmental Science. That is not a weather-app max. Public forecasts stop at 11+ because advice does not change above “extreme.”

Why that day stacked so high

Thin air, Southern Hemisphere summer, a high sun, clear skies, and regional ozone thinning. WHO-style rules of thumb put UV up about 10–12% per 1,000 m of elevation. All of those ran together on a remote cone, not in a downtown park.

Some later discussion has asked whether ground-level values above the mid-20s are easy to reproduce. Treat 43.3 as a published extreme from one campaign, not a daily planning number. The lesson that survives the debate: altitude, ozone, and sun angle can push UV far past what a sea-level summer feels like.

What you will actually see on this site

The WHO UV index used in forecasts runs 0 to 11+. Populated cities rarely sit at 11 for long; high Andes towns can see the teens and 20s. Where those places are: what place has the highest UV index?

For a hike or ski day, read the live index and then add elevation—see higher elevation, stronger UV. You will not meet 43. You can still burn on a cool ridge at 11.