Most people check temperature and rain. UV is a different meter. A cool, bright morning can still sit at Moderate or High. This is the routine we built the homepage for.

Three numbers, not one

Each number answers a different question. Mixing them up is how 8 a.m. becomes the whole day.

1. Current UV index — approximate open-sky intensity near the location we detected. Use it for this hour: a ten-minute errand, whether you need the hat now. It is not a forecast.
2. Today’s peak — the high for the day, with an approximate time. That is the packing number. If the peak is 6 or above, treat late morning through mid-afternoon as a protection window even if current still looks Low. WHO puts the daily maximum in the four-hour period around solar noon—when the sun is highest, not when the clock says 12:00.
3. Next few hours — whether a Low now becomes a High at 11, or a High drops behind thick cloud and then comes back. A single snapshot is how people get caught.

WHO’s protection line is 3, not High. Moderate already wants cover. The jump at 8 is the second line: Very High and Extreme means minutes of bare skin, not hours.

Match the kit to the band

WHO’s public table is two steps. 0–2: you can usually enjoy being outside. 3–7: seek shade in the midday hours; shirt, sunscreen, and hat. 8 and above: avoid midday if you can; shade, shirt, sunscreen, and hat are a must. EPA’s U.S. wording is close: extra care from 8+.

We keep the five colours because they map to minutes:

• 0–2 (Low): Short errands are usually fine. Sunglasses still help in open glare. WHO: even for very fair skin, short-term risk below 3 is limited under normal conditions.
• 3–5 (Moderate): Shirt, hat, SPF 30+ if you will be out more than a walk. See what UV index requires sunscreen.
• 6–7 (High): Shorter unprotected stretches. Reapply if you sweat.
• 8+ (Very High / Extreme): Shade plus clothing plus sunscreen. Do not pick one.

Skin type, medicines, and altitude change how aggressive you should be inside a band. They do not move the line at 3.

Feed the calculator the right hour

The SPF calculator divides a skin-type factor by the UV you give it. Type I uses 67 minutes at UV 1; type II uses 100. At a live 8 that is about 8 minutes unprotected versus about 13. Those are planning estimates, not a lab burn on your arms. The split is skin type changes burn minutes, not the index.

Feed it current for a short outing now. Feed it today’s peak if you will still be out through solar noon. Feeding 8 a.m.’s 2 into a noon picnic hides the rate the film has to fight.

Then name the ground

The index is an open-sky rate. Dose goes up on sand, water, snow, and at elevation. A patio over pale concrete is not a park lawn. Bounce: shade does not cancel UV. Height: altitude raises UV.

When to recheck

Glance at the next few hours before you decide “no sunscreen today” at a kids’ game, a long run, or a job with outdoor breaks.

Cloud. Thick cover that hides the solar disc cuts UV. Thin cloud that still throws a shadow does not—and can even raise it. WHO: up to about 80% of solar UV can pass light cloud; haze can even increase exposure. If you still have a sharp shadow, keep the kit on. The enhancement case is broken clouds can raise UV.

Travel. A reading in the hotel car park is not the beach an hour south, and it is not the ridge above the valley. CDC’s Yellow Book still tells travelers to treat strong midday sun as a cover window. Check again when you arrive.

Altitude. WHO: UV rises about 10–12% per 1,000 m. Cooler air on the climb is not a lower index. Recheck when you gain a thousand metres, not only when the weather icon changes.

Two traps that skip the meter

1. Checking only temperature. Heat is infrared. UV is not. A mild morning can sit under the day’s peak while the air is still climbing. That lag is UV peaks before the heat.
2. Treating 8 a.m. as the day. Low at breakfast does not cancel a High at noon. Hours at Moderate still add dose—hours at UV 4 can outdose minutes at UV 8. Read today’s hours, not a remembered morning.

Use current for now, peak to pack, next few hours to stay honest. Then go.

Sources

WHO protect-from-3, the 0–2 / 3–7 / 8+ action table, solar-noon peak in the four hours around noon, and limited risk below 3: UV index Q&A. Light cloud passing up to ~80% of UV: Global Solar UV Index: a practical guide (WHO, WMO, UNEP, ICNIRP, 2002). Altitude 10–12% per 1,000 m: same Q&A. EPA bands and 8+ extra care: UV Index Scale. Traveler midday cover: CDC Yellow Book, sun exposure. Calculator minutes are this site’s planning factors. Readings are estimates for sun-safety planning. FAQ-style answers: Guide.