The number on the homepage is a burn rate. It is not a count of all the ultraviolet that hits you.

WHO built the UV index on the CIE erythema curve: how fast unprotected skin reddens. That curve is steep. Short UVB is weighted near full strength. Long UVA is weighted down to a sliver. Most of the UV at the ground is UVA—about 95% in WHO’s split. The index can fall while that larger band is still working.

A meter for reddening

WHO’s Global Solar UV Index is 40 times the erythemally weighted irradiance on a horizontal surface. One index point is 0.025 W/m² of that weighted UV. Hours at UV 4 is how long that meter runs. This is which wavelengths it even hears.

WHO’s bands:

• UVB — 280–315 nm. Delayed tanning and burning. Most of it never reaches the ground.
• UVA — 315–400 nm. Immediate tanning, deeper reach, aging. Not filtered as significantly.

The CIE curve WHO uses is almost 1.0 through short UVB, then collapses. NOAA’s tabulated McKinlay–Diffey weights—the same family of curve:

• 298 nm — 1.0
• 315 nm — about 0.025 (~2.5% of peak, at the UVB/UVA line)
• 330 nm — about 0.0014 (~0.14% of peak)
• 360 nm — about 0.00048 (~0.05% of peak, roughly 1/2,000)

A 360 nm photon still arrives. The index barely hears it.

ICNIRP: UVA alone can redden skin, but only at very high radiant exposures—about 10 J/cm² (100 kJ/m²). That is a different clock from the UVB burn that writes the live number.

Most of the sky is UVA

WHO’s 95% figure is the absorption split. All UVC and about 90% of UVB is absorbed by ozone and other gases. UVA is less affected. EPA’s RadTown line matches: the atmosphere does little to shield UVA; how much UVB you get still depends on latitude, altitude, and time of year.

The photons are mostly UVA. The index is mostly the UVB sliver, because that sliver reddens you fastest.

EPA also puts UVA as more constant than UVB. UVB is highest when the sun is highest—the same solar-noon peak as UV peaks before the heat. When the disc drops, the burn-weighted number falls fast. UVA falls less. Late afternoon can look “done” on the index and still be a UVA sky. That is not a second peak. It is a change in mix.

The trap is reading the index as all UV

Three everyday misses:

1. "It's only a 2, so no UV." WHO’s protect-from-3 line is erythemal. Low means a slow burn tap. It does not mean the 95% UVA band switched off. Eyes and aging do not wait for a Moderate label.
2. "The number dropped after lunch." Moving a run to 4 p.m. can dodge the solar-noon UVB spike. It does not empty the UVA. The UVA-to-UVB ratio is higher when the sun is lower—not because UVA got stronger, because UVB fell more.
3. "SPF 50, so I'm covered." SPF is a sunburn test—primarily UVB. FDA’s Broad Spectrum test is the UVA half: a critical wavelength of 370 nm or greater, where 90% of the product’s UV absorbance from 290–400 nm has been counted. A high SPF without that stamp is a burn product, not a full-UV product. What SPF stands for is the ratio; the bottle SPF is the milligrams.

Two meters, one outing

Use the live UV index for the burn clock. Then read the UVA stamp on whatever is covering skin:

1. Take today’s peak. If it is 3 or above, WHO wants cover. Clothing and shade first; sunscreen for what they miss.
2. Do not treat a drop from High to Moderate—or Moderate to Low—as a UVA all-clear. Same shirt. Same broad-spectrum film. The mix changed. The photons did not vanish.
3. Find the UVA line on the bottle. AAD: broad spectrum and SPF 30 or higher. US Broad Spectrum is the 370 nm test. Cancer Council: TGA sunscreens must be broad-spectrum, with a UVA protection factor at least one-third of the labelled SPF under AS/NZS 2604:2021. The digit 30 is not that fraction.
4. Prefer cloth for both bands. UPF is a fabric test of UVA and UVB—see a cotton T-shirt barely blocks UV. A dense sleeve does the job the index cannot describe.
5. Recheck the next few hours, then keep the UVA cover on until you are inside. Glass is a UVA leak more than a UVB leak—see does glass block UV.

The homepage tells you how fast you burn. It does not tell you the rest of the spectrum went home.

Sources

WHO defines the index with the CIE erythema spectrum and k_er = 40 m²/W, protect-from-3, and the formula, in the UV index Q&A and Global Solar UV Index: a practical guide (also ~90% of UVB absorbed). UVA ~95% of surface UV, bands, immediate vs delayed tanning: Radiation: ultraviolet (UV). CIE weights at 298 / 315 / 330 / 360 nm: NOAA GML tabulation of McKinlay and Diffey 1987, doserates.CIE.txt. UVA-alone erythema near 10 J/cm²: ICNIRP UV guidelines (2004). Atmosphere does little to shield UVA: EPA RadTown. UVA more constant than UVB: EPA, UV Radiation. Broad Spectrum = critical wavelength ≥ 370 nm: 21 CFR 201.327. AAD broad spectrum + SPF 30: How to select a sunscreen. UVA-PF at least one-third of labelled SPF: Cancer Council Australia — sun protection. Those are label and spectrum facts, not a personal UVA forecast.