The homepage number is the burn rate on flat ground. A cheek stands closer to a wall. It does not collect that noon digit.

WHO and the WMO define the UV index as erythemally weighted ultraviolet on a horizontal surface, using the CIE erythema spectrum. The meter lies flat. Which wavelengths that curve counts is the UV index is a burn meter.

A quarter of the ground reading

When the sun is high, the direct beam prefers surfaces that face up. A vertical cheek takes a smaller share of the same sky.

Downs and Parisi measured erythemal UV on facial sites in Toowoomba from 2005 through 2008, scored against horizontal ambient UV in the same period. The median for the face:

• Sun within 30° of overhead — 26% of the ground reading
• 30–50° off overhead — 39%
• 50–80° off overhead — 48%

The share rises as the sun drops, while the ground reading falls, so a larger slice can still be a smaller dose. Hold the high-sun fraction still for a worked example, not a Toowoomba forecast: 26% of a ground index of 10 is about 2.6 on that face median. Forehead and nose tilt up more than a cheek, so a cheek sits toward the low end of the average.

Facing the sun, the cheek’s hour shifts

Wang and colleagues asked when the cheek’s own reading peaks. They put a manikin on a roof near Shaoxing, about 30°N, on two clear days in late May 2010. At solar noon the sun stood about 82° up. They logged ultraviolet from 300 to 400 nm — a broadband meter, not the index — and weighted those spectra with the ICNIRP occupational skin-damage curve, the occupational cousin of the CIE erythema spectrum. The joules below are not homepage points.

With the face held toward the sun, the cheek’s broadband reading at a solar elevation of about 54° matched the cheek’s own reading at that 82° noon: about 1,584 against about 1,525 microwatts per square centimetre. The ground meter was still climbing. The cheek pointed at the disc had already leveled off.

On that skin-damage weighting, still facing the sun, the cheek’s one-hour total peaked at 10:00–11:00 (about 50 J/m²) and again at 13:00–14:00 (about 43 J/m²). The overhead hour belonged to the ground.

A turning head collects a different day. Over a full rotation, that cheek near noon took about a quarter of the horizontal hour — 0.25 from 11:00 to 12:00 and 0.26 from 12:00 to 13:00. At 7:00–8:00 the fraction was about 0.36 of an ambient hour near 52,000 J/m²; the noon hour’s 0.25 came from about 175,000 J/m², so the turning head’s broadband dose was still larger near noon. The off-noon peak belongs to a cheek aimed at the sun. Two days, one roof, a manikin: those clock times are that experiment.

Noon is the lawn’s peak

1. The day’s peak is the face’s worst hour. It is the worst hour for the flat meter. Skin aimed at the sky sits nearer it. That horizontal dose is already in hours at UV 4.
2. A falling index means the face is finished. Walk toward the sun — a commute, a sideline, a path that keeps the disc on one cheek — and the Shaoxing manikin still had a high skin-damage hour after the sun was highest.

A visor covers some forehead and leaves the cheek, ear, and neck open. That gap is a baseball cap leaves the ears.

Cover the cheek on its own hour

Use the live UV index as the ground rate, then ask which way the face points.

1. Read the peak as flat ground. A high noon integer means cover.
2. If the outing faces the sun, cover the cheek on the way up and the way down, not only under the overhead hour.
3. If you turn, stop, and bend, expect a share closer to the quarter. Standing up does not turn a high index into a low one.
4. Sky-facing skin keeps the noon rule. Lying flat, or bare shoulders at midday, sits nearer the meter.
5. Recheck the next few hours. Cloud moves the ground number on its own.

What UV index requires sunscreen is the floor, from 3 up on that ground meter. Skin type, a brim, and shade change how fast any of this reddens. These fractions are a planning rule, not a personal burn timer.

Sources

UV index defined for a horizontal surface with the CIE erythema spectrum: WMO Codes Registry, UV index and WHO, Global Solar UV Index: a practical guide. Face median 26% / 39% / 48% of horizontal ambient erythemal UV at solar zenith 0–30° / 30–50° / 50–80°, Toowoomba, 2005–2008: Downs and Parisi, 2009. Shaoxing manikin, two late-May days, cheek broadband UV at solar elevation ~54° matching the cheek at ~82° (~1,584 vs ~1,525 µW/cm²), skin-damage hour peaking 10:00–11:00 (~50 J/m²) and 13:00–14:00 (~43 J/m²) when facing the sun, and a full-turn cheek ratio of about 0.25 on an ambient hour near 175,000 J/m² at noon, against about 0.36 of an hour near 52,000 J/m² at 7:00–8:00: Wang et al., 2017. Wang’s weighting is the ICNIRP occupational skin spectrum, not the CIE curve inside the UV index. A brim, a turn of the head, and cloud move a real face off both experiments.