The short answer
The UV index is a live measure of the sunburn-producing ultraviolet radiation reaching the ground where you are, on an open-ended linear scale from 0 to 11+. Linear is the part everyone misses: UV 6 delivers twice the burning energy of UV 3, and UV 9 delivers three times as much. It isn't a rating of how sunny it looks or how hot it feels — it's a speed. It tells you how fast sun damage is arriving right now.
Here's the whole scale in one table, with what each band means for Type III skin — the most common type in the US — and what to do about it.
| Band | UV index | Type III burns in | What to do |
|---|---|---|---|
| Low | Under 3 | ~120 min | "SPF optional." Enjoy the outdoors freely. |
| Moderate | 3–5 | ~60 min | "Lotion up!" SPF 30, sunglasses, reapply every 2 hours. |
| High | 6–7 | ~45 min | SPF 30+, hat, favor shade through the midday hours. |
| Very high | 8–10 | ~30 min | Shade plus coverage — shirt, hat, sunglasses; move plans off-peak. |
| Extreme | 11+ | ~20 min | Keep midday exposure brief and covered. This is holiday-ending sun. |
Approximate onset of mild sunburn for unprotected Type III skin at sea level. Fairer types burn roughly twice as fast; deeper types last longer.
Not a Type III? The shape holds but the minutes shift — the full skin type & burn time table covers all six types at every band.
Where the number comes from
The index isn't a guess at sunshine — it's computed from erythemally weighted irradiance: the UV wavelengths that actually redden skin, each weighted by how effectively it does so, standardized by the WHO and the World Meteorological Organization. That's why UV 7 means the same thing in Denver, Toronto and Sydney. Five things move it:
- Sun angle — season, latitude and time of day. The single biggest factor, which is why the peak lives around solar noon.
- Ozone — the stratosphere absorbs a big share of burning UVB; a thinner ozone column means a higher index.
- Altitude — thinner air filters less. UV climbs about 10% per 1,000 m, so mountain towns out-burn their forecasts' valley cousins.
- Cloud — a heavy storm deck blocks plenty, but up to 80% of UV passes through thin cloud.
- Reflection — water bounces back up to 25% extra, dry sand about 15%, and fresh snow up to 80%.
Why it's not temperature
The index has no idea how warm the air is — and neither does your skin. UV depends on how high the sun stands, not on heat, which is why a crisp 60°F day in late April can out-burn a sweaty 90°F evening in August: the spring sun sits high at 1 PM while the August sun at 6 PM has already collapsed to UV 1–2. Heat is a lagging by-product of hours of sunshine; UV is an instantaneous property of sun angle. The highest-UV window runs roughly 10 AM to 4 PM whether it's beach weather or sweater weather.
The UV index is a speed, not a budget. It never says how much sun you can take today — it says how fast the damage arrives. Your skin type sets the budget; the index sets the clock.
The daily curve: what "UV 7 today" really means
A forecast of "UV 7 today" means seven at the peak — not seven all day. UV traces a bell curve: near zero at breakfast, climbing steeply through mid-morning, cresting in the hour or two around solar noon (about 1 PM in summer with daylight saving), then sliding back down. That same UV 7 day is often UV 3 at 9 AM and UV 2 by 6 PM. Judging your whole day by the daily max is like judging a road trip by its top speed.
How to actually use it
- Check the hour-by-hour, not the daily max. The max punishes your 5 PM walk for a 1 PM peak it never touches.
- Plan around the curve. Run, garden and walk the dog on the shoulders of the curve; save the shade, the SPF, or both for the crest.
- Use 3 as your tripwire. Below 3, sunscreen is genuinely optional for most skin types — UV Buddy literally shows "SPF optional." At 3 it flips to "Lotion up!"
- At 8+, restructure. Shift beach time off-peak and add coverage — a shirt and hat, not just another layer of SPF.
Live UV to one decimal, all day long
UV Buddy takes live UV from Apple Weather and interpolates it minute by minute to one decimal — so you watch the number climb instead of staring at an hour-old stair-step. You get the hourly curve, a 7-day outlook, automatic peak-window detection and a "Best tan time," and you can pin any city, shown in that city's own timezone.
Mini-FAQ
What UV index is dangerous?
It depends on your skin type — "dangerous" is a burn window shorter than your attention span. At UV 3–5, unprotected Type III skin has about an hour; a fair Type I has closer to 25 minutes. By UV 8+ everyone is on a fast clock — around 30 minutes for Type III and single digits for Type I. Below 3, most skin types can be out casually without sunscreen.
Can the UV be high on a cloudy day?
Yes. Up to 80% of UV passes through thin cloud, and a bright overcast sky scatters it at you from every direction. Some of the nastiest burns happen at UV 6 under a white sky that never once felt sunny.
Why does my weather app show a different number than UV Buddy?
Most weather apps show a rounded hourly forecast — often the daily max. UV Buddy shows the current value, interpolated minute by minute between Apple Weather's data points, to one decimal. At 10:40 AM a forecast may still say "4" while the live number already reads 5.2. Forecast versus now — they're answering different questions.
Know the number before you feel it
Live UV to one decimal, a burn window sized to your skin type, and a "Lotion up!" nudge the moment the index crosses 3.
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