The UV Index scale explained — and what it leaves out

The UV Index is the one piece of ultraviolet information almost everyone has seen: a number in a weather app, colour-coded, somewhere between 0 and 11. It is a genuinely good piece of science communication. It is also, by design, a measure of one thing — and this page is about both halves of that sentence.

Where the number comes from

The UV Index is not a measurement of ultraviolet light. It is a measurement of ultraviolet light's ability to redden human skin, which is a different quantity.

Sunlight arrives at the ground as a spectrum — a distribution of energy across wavelengths. To produce the index, that spectrum is multiplied, wavelength by wavelength, by the CIE erythemal action spectrum: a standardised curve describing how effective each wavelength is at causing sunburn. The weighted result is integrated across the ultraviolet band and scaled by a constant.

The times-40 factor exists purely so the result lands on a memorable 0-to-about-11 range for conditions on Earth. It carries no physical meaning of its own — exactly like the divisor of six that puts this site's UVA Index on a comparable scale.

The bands, and what the WHO advises

The categories below are the WHO/WMO/ICNIRP standard, used essentially worldwide. The colours are part of the standard, not decoration.

Three is the threshold that matters. It is the value at which the WHO begins recommending protection, and it is the number most national campaigns are built around. Below 3, erythema is unlikely for most skin types under normal exposure durations.

How high does it actually go?

The scale is open-ended. Eleven is where the top band begins, not where the scale stops — a common misreading encouraged by the way apps display "11+".

The extremes come from stacking every amplifier at once: an overhead sun, a thin ozone column, very little atmosphere above you, clean dry air and snow reflecting from below. UVA at altitude covers why the elevation gradient differs by wavelength, and reflected UVA covers the albedo contribution.

What the weighting does — and hides

Here is the structural consequence of erythemal weighting. The action spectrum falls away extraordinarily steeply with wavelength: relative to its peak in the UVB, its effectiveness at 340 nm is on the order of a thousand times smaller. So the index is, in effect, a UVB instrument.

For its intended job — telling you whether you are about to get sunburnt — that is exactly right, and no criticism applies. The problem is that the number then gets used as a general-purpose "how much ultraviolet is there today" signal, which it is not. Because UVB and UVA respond very differently to sun angle, ozone, cloud and glass, the two come apart precisely when people stop paying attention:

• Winter. A long slant path through the atmosphere means far more ozone in the way, which strips UVB while leaving UVA largely intact. The UV Index collapses; UVA falls modestly. See seasonal UVA.
• Behind glass. Ordinary window glass removes nearly all UVB, so the effective UV Index indoors is about zero — while a large fraction of UVA continues through. See does glass block UVA.
• Under cloud. Cloud attenuates the UVB-dominated erythemal band more than it attenuates UVA, so the ratio shifts again. See UVA on cloudy days.
• Early and late in the day. The same air-mass argument as winter, compressed into a few hours. See what time UVA peaks.

In every one of those cases a low UV Index is telling the truth about sunburn risk and giving a misleading impression about ultraviolet exposure in general — which is the entire reason this site computes a separate UVA Index.

Reading the two together

The most useful habit is to treat them as answering two different questions rather than competing for the same one.

That last row is worth dwelling on, because it is the cleanest illustration of the split: vitamin D synthesis is driven by UVB and therefore tracks the UV Index closely, while UVA contributes none of it — a point developed in UVA and vitamin D. The calculator on this site shows the live UV Index alongside the UVA Index for exactly this reason: seeing them together makes the divergence obvious.