UV index today across the US

Every metro we track

What does the UV index scale mean?

The UV index is a measure of sunburn-causing ultraviolet radiation on an open-ended scale starting at 0. 0–2 is Low, 3–5 Moderate, 6–7 High, 8–10 Very High and 11 or above Extreme. The bands are the US EPA’s; the number rises with altitude, thins with cloud, and peaks about an hour after solar noon rather than at 12 on the clock.

The scale is open-ended: 11 is not the maximum, it is where the named bands stop. Values above 12 occur at altitude and in the tropics.

Why is UV strongest at 1pm, not at noon?

Latitude does most of the work. The further a place sits from the equator, the lower the sun stands at midday and the more atmosphere its light crosses before reaching the ground. Across the metros we model that runs from 21°N to 48°N, and today's peaks line up with it: 10 in Honolulu at the top, 2.7 in St. Paul at the bottom. A city page states its own latitude, its rank, and where today's peak fell in that spread.

The clock lies about noon. Solar noon drifts across a timezone — a city at the eastern edge reaches it earlier by the clock than one at the western edge — and daylight saving pushes the sun's high point roughly an hour later than most people expect. That is why peak UV usually lands between 12pm and 2pm local rather than at noon, and why each city page names its own hour instead of repeating a rule of thumb.

What the bands mean for a plan. EPA treats 3 as the point where protection starts being worth it and 6 as the point where shade and cover are worth planning around. The useful figure is not the peak but the length of the window above those thresholds, which is why every city page reports its own hours at or above 3 and 6 rather than a single number.

Modelled, not measured — and why we say so

Our air-quality numbers come off EPA regulatory monitors: instruments, in places, with identifiers. The UV figures on this site do not. They are computed by the Copernicus atmosphere model and delivered through Open-Meteo's free tier, which is licensed for non-commercial use — which is what this project is.

The honest limits: cloud cover is the model's weakest point, and broken cloud is exactly the condition where a modelled figure and the sky above you diverge. Altitude and reflective ground — snow, water, sand — push real exposure the other way, above what the index implies. Neither correction is applied here, because we do not have the ground truth to apply it with.

We publish it anyway because it is the only free source covering every US location, and a labelled model beats no answer. What we will not do is let it sit next to monitor data and quietly borrow its authority. Our method →

Questions