The UV index scale is the shared language weather apps, public health agencies, and Basky use for sunburn-causing ultraviolet. It is not a fashion metric, not a heat index, and not a score you “win.” It is a 0–11+ number that says how strong the erythemally weighted UV is right now — the part of sunlight most tied to skin reddening.
The World Health Organization groups that ladder into five bands. Once you know the bands, a single digit on your phone becomes a calmer sentence: gentle sky, useful daylight, strong — keep it short, very short window, or extreme — be brief. Basky shows both the number and the band so you do not have to memorize the cutoffs.
If you want the deeper “what is this number?” story, start with what is the UV index. This page is the practical map: what each band is trying to tell you, and what Basky does with it.
The WHO bands at a glance
| Band | UV index | Plain reading | Basky-leaning habit |
|---|---|---|---|
| Low | 0–2 | Gentle sky. Erythema risk is low for most people outdoors briefly. | A window can be long; at the bottom of the band Basky may simply say the sun is resting — still a fine time for a daylight check-in. |
| Moderate | 3–5 | Real UV. Fair skin can redden faster than a “quick errand” suggests. | A short, intentional bask. Treat it as useful light, not background noise. |
| High | 6–7 | Strong. Minutes matter more than “how I feel.” | Windows shrink. Plan shade and covering sooner — see high UV: what to do. |
| Very high | 8–10 | Intense. This is not a linger-until-you’re-done sky. | Short windows. Find shade after a little. Over-exposure earns nothing. |
| Extreme | 11+ | Rare outside tropics, altitude, or peak midsummer noon. | Brief, careful visit. Pair with shade, clothing, and sunscreen as needed. |
Bands are labels, not permissions. Low does not mean “pointless.” Extreme does not mean “stay home forever.” They describe intensity so your minutes can stay kind.
What “erythemally weighted” quietly means
Ultraviolet spans a range of wavelengths. Scientists weight that spectrum by how likely it is to redden human skin — erythema — then scale the result into the familiar UV index. That is why the index tracks sunburn-relevant UV more closely than “how bright it looks” or “how warm the breeze feels.”
Basky’s Exposure Engine uses that same idea: live UV (via Open-Meteo when you allow location), your Fitzpatrick type, and a conservative ~60% of a minimal erythemal dose, clamped to roughly 5–90 minutes. The band helps you read the sky in plain language; the engine turns the number into a personal window.
You do not need the lab formula. You only need: higher band → shorter careful window; lower band → longer window or a gentle “not baskable right now, but daylight still counts.”
Low and Moderate: still real sky
Low (0–2) is the band people dismiss. Winter mornings, late evenings, thick overcast, high latitudes in the dark months — the index can sit near the floor while the day still looks “fine.” Basky’s answer is soft: if there is no meaningful baskable dose, the Today screen can invite a check-in instead of a forced timer. Stepping outside still supports a circadian daylight habit. Low is not a failure; it is honest weather.
Moderate (3–5) is where many mid-latitude days live for hours. It feels ordinary, which is exactly when fair skin can overstay. Basky treats Moderate as useful daylight with a real clock — longer than High, shorter than a Low evening stroll that somehow turns into two hours unprotected.
High, Very High, and Extreme: same wellness, fewer minutes
High (6–7) and Very High (8–10) are not punishments. They are compressed schedules. The wellness idea — a little intentional sun, not a burn — still applies; the minutes simply shrink. Basky will never stretch the window to protect a streak. Past the line earns nothing.
Extreme (11+) deserves respect without panic. It shows up more often near the equator, at altitude, or under a clear midsummer noon. Treat it as a brief visit: shade nearby, skin covered when you are done, sunscreen as your usual practice. Basky’s upper clamp exists so “extreme” never becomes an invitation to marathon exposure.
For beach days and reflective surfaces that can push the felt load higher than a bare number suggests, see reflection, snow, and water and beach UV.
How Basky turns a band into a window
- Read today’s live UV for where you are (location optional; a generic fallback if you decline).
- Combine it with your Fitzpatrick skin type — the same UV 6 is a different minute count for type I than type V.
- Return a conservative window, roughly sixty percent of a minimal erythemal dose, held inside a [5, 90] minute range.
- Never reward going past the line. Never claim the window replaces sunscreen.
That is the opposite of a one-size “everyone gets 20 minutes” rule. It is also the opposite of shame. Missed days, cloudy weeks, and Low winters are part of a real year — Basky’s free core is built for the ordinary ritual, not perfection.
A calmer way to use the scale
Glance at the band. Glance at your minutes. Step out for a little, then step in. If the band is Low and the sun is resting, tap that you got daylight anyway. For the hour-by-hour shape of a day, see UV index by hour. For how long you can stay, see how long can I stay in the sun.
Now on iPhone. Free core, optional one-time Basky Forever cosmetics, no subscription, no account, no ads, no tracking. General wellness guidance, not medical advice — and never a substitute for sunscreen, shade, or a clinician when you need one.