About this site

uvaindex.org exists to give UVA radiation a number of its own. The familiar UV Index is genuinely useful, but it answers only one question — how fast will the sun burn my skin? — and in doing so it all but ignores UVA, the part of sunlight most responsible for skin aging and deep, cumulative damage. This site fills that gap with a dedicated, easy-to-read UVA Index.

Why a UVA Index?

The standard UV Index is erythemally weighted — tuned to sunburn — which makes it dominated by UVB. UVA barely moves it, even though UVA is the overwhelming majority of the UV that actually reaches you, stays high through more of the day, penetrates cloud and glass, and drives photoaging and indirect DNA damage. So you can have a comfortable, low UV Index while still taking a substantial UVA dose.

A UVA-specific measure makes that invisible exposure visible. It looks at the unweighted UVA band (≈315–400 nm) — the real long-wave irradiance hitting the surface — and puts it on the same friendly 0–11+ scale you already know from the UV Index. For the full argument, see why the UV Index isn't enough.

This is not an official index

We want to be completely upfront: there is no official UVA Index. The UV Index is an international standard defined by the WHO, WMO and others. UVA has no equivalent — no agency publishes one, and no free weather API serves pure UVA data.

So everything here is an estimate:

• The UVA irradiance is derived from a physical model, not measured by an instrument. We compute the sun's position and apply live corrections for altitude, aerosols, cloud and ground reflectance. The full method is in how the UVA Index is calculated.
• The 0–11+ scale and the colour bands are our own pragmatic choice. We reuse the WHO UV Index bands so the number feels familiar, but they are not an official UVA standard.
• Values are best treated as a relative guide — great for comparing times, places and conditions — rather than an instrument-grade absolute reading.

Why we do it anyway

Because the gap is real and the awareness matters. People reasonably assume a low UV Index means "the sun can't hurt me right now," and for aging and long-term UVA damage that simply isn't true. A transparent, free, no-login estimate — with every input and every step of the calculation shown — is more useful than no UVA information at all. We'd rather give an honest, clearly-labelled approximation than leave UVA invisible.

Transparency is the whole point: the calculator shows the inputs and the model breakdown for every reading, the methodology is written out in plain language, and the source code is open for anyone to inspect or improve.

How to use it responsibly

• Use it alongside the official UV Index, not instead of it — they answer different questions.
• Treat the bands as guidance, not precise thresholds.
• For medical or skin-health decisions, talk to a qualified professional. This site is educational information only, not medical advice.

How it's built

It's a small, fast, dependency-free website that runs entirely in your browser. Atmospheric data comes from the free Open-Meteo APIs (no key, no tracking), the solar maths runs locally, and the whole thing works offline once loaded. It's free, ad-free and open source.

Open source

This whole project is open source and released under the permissive MIT License. The complete source code — the calculator, the physical model and every word of the Knowledge Base — lives on GitHub, where you can read exactly how each reading is produced, file an issue, or open a pull request to improve it. We chose to make it open because the credibility of an unofficial index rests entirely on transparency: if you can't trust an agency to stand behind the number, you should at least be able to inspect and challenge the maths yourself.

Want to help? Model refinements, content corrections, accessibility fixes and bug reports are all welcome — see the contributing guide to get started.