People wait for the day to feel hot. Or they wait for the clock to pass noon. Or they wait until the shadow is a stump at their feet.

Look down sooner. UV follows the sun’s height, not the thermometer and not the wall clock. When the shadow at your feet is already shorter than you are, the sun is more than 45° up. That is a high-UV hour. Cover that starts then is late.

Equal height, 45 degrees

Stand in the open on flat ground. Heels on level pavement or grass. Look at the shadow of your body—not a wall, a lamp-post, or a hillside. A slope lies about the angle. Wright and colleagues are blunt: the rule is for a horizontal surface.

When that shadow is as long as you are tall, the sun is at 45°. Higher than that, it drops shorter than you. Lower, it stretches out.

Public guidance maps that angle onto the index, with a spread:

• ICNIRP (outdoor-worker statement, 2010): the shadow rule is a rough flag for when the UV index exceeds about 4, if a shadow exists. At that 45° / UV-4 point, sensitive unadapted skin can show a noticeable burn after about two hours.
• Te Ara / NIWA scientists (New Zealand): shadow more than twice your height (sun under 30°) — UV index generally under 3. Shadow shorter than you (sun over 45°) — the index can exceed 7.
• A 2025 modeled set for six South African cities (clear sky, typical ozone of 300 DU): shadow shorter than half you sat in about UV 10–14. Between half and equal height: about UV 5–10. Shadow still 1–2× your height sat in about UV 2–5.

Those are planning bands, not a promise for every lawn. Ozone, haze, altitude, and cloud still move the number. The direction does not: shorter shadow, stronger UV. Do not wait for a stump. Half-height is already Very High to Extreme in that South African run.

The trap is waiting for the stump

WHO’s sun-protection line is UV index 3 and above—Moderate, not High. That often arrives while you still look taller than your shadow.

Three everyday misses:

1. Clock noon—and daylight saving. Cancer Research UK puts the UK’s strongest hours around 11 a.m.–3 p.m. WHO puts solar noon between local noon and 2 p.m. once longitude and daylight saving slide the wall clock. The shadow does not jump when the clocks do. Ten to four is not a global clock is the printed-hours cousin of this test—memorized blocks, not the angle at your feet. Do not merge them.
2. “I still have a long shadow.” Long is not Low. The 1–2× band is where Moderate UV lives. WHO already wants cover there if you will be out more than a few minutes.
3. Heat as the cue. Cool air with a short shadow is still a high sun. That split is the same one in UV peaks before the heat—this check is the angle, not the lag.

EPA’s public wording is blunt: shadow taller than you (early morning, late afternoon) — UV likely lower. Shadow shorter than you (around midday) — higher UV; seek shade and cover skin and eyes. WHO’s Global Solar UV Index guide is the same idea: if your shadow is short, protect yourself.

A sharp shadow in puffy cloud still counts as sun. That sky can even sit above a clear hour—see broken clouds. No shadow you can pick out usually means the disc is hidden; UV is down, not zero.

Look down, then check the number

Use the shadow when you are already outside. Use the live UV index when you can—it is the better meter.

1. Do the test on flat ground. Heels together on level pavement or grass. Look at your shadow, not a tree or a wall. A hillside lies. If you cannot find flat, check the index.
2. Shorter than you — treat the hour as High or worse. Hat, shirt, shade, sunscreen. Do not wait for it to feel hot, and do not wait for a stump.
3. Shorter than twice you, still longer than you — already in the WHO 3+ zone on many days. Cover if you are staying out.
4. More than twice you — usually Low. Still glance at today’s peak if the outing runs toward midday.
5. Recheck after a stretch of work. Morning shadows shrink fast. Afternoon ones grow. If the clocks jumped last weekend, ignore yesterday’s “after 4” habit. The shadow did not move.

Altitude still raises the dose at the same sun angle. Snow and pale ground still bounce extra UV into shade. Those are separate paths: altitude and shade.

Sources

A shadow shorter than you means the sun is above 45°—plain trigonometry. WHO’s protect-from-3 line, solar noon falling between noon and 2 p.m. with daylight saving, and “if your shadow is short… protect yourself”: UV index Q&A and the Global Solar UV Index: A Practical Guide. ICNIRP’s UV-index ~4 at 45° and the two-hour burn note: Protection of workers against ultraviolet radiation, 2010. NIWA’s twice-height / under-3 and shorter-than-you / can-exceed-7 bands: Te Ara, the shadow length rule. EPA’s taller-than-you / shorter-than-you wording: UV Index Scale. The South African clear-sky bands (half-height UV 10–14, 1–2× UV 2–5) and the flat-ground caveat: Wright et al., 2025. A wall or doorway distorts the test; this is not a lab reading.