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. When the shadow is as long as you are tall, the sun is at 45°. Higher than that, the shadow 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): shadow shorter than you sat in about UV 5–14, depending how short. 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.

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. Cancer Research UK puts the UK’s strongest hours around 11 a.m.–3 p.m. Travel, daylight saving, and latitude slide that window. The shadow does not care what the clock says.
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 INTERSUN mnemonic is the same idea: short shadow, seek shade.

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. Stand on flat ground and look at the shadow of your body, not a wall or a slope. Tilted pavement lies.
2. Shorter than you — treat the hour as High or worse. Hat, shirt, shade, sunscreen. Do not wait for it to feel hot.
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. The clock will not tell you when you crossed 45°.

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 is in the UV index Q&A. Shadow-rule bands also appear in Te Ara / NIWA public notes. A wall or doorway distorts the test; this is not a lab reading.