The UV index (often written UVI) is a simple number that tells you how strong the sunburn-causing ultraviolet is at a given place and time. Zero is night or a sun that is effectively “resting.” Eleven and above is extreme. The World Health Organization groups the scale into bands: Low (0–2), Moderate (3–5), High (6–7), Very High (8–10), and Extreme (11+).
Basky exists because that number, on its own, is only half the story. The same UV 5 is a different experience for very fair skin than for deeply melanated skin. A safe-sun window has to start with you, then with today’s sky.
UV is not heat, and it is not “how sunny it looks”
Ultraviolet is a band of light you cannot see. Infrared is what you feel as warmth. Visible light is what makes a day look bright. Those three can travel together, but they are not the same thing.
That is why:
- A crisp spring noon can redden skin faster than a sticky August evening.
- Shade, glass, and clothing change UV more than they change the temperature on your phone.
- Snow, pale sand, and water can bounce extra UV toward you even when the air feels mild. See reflection, snow, and water.
If you have ever thought “it doesn’t feel that hot, so I can stay out,” the UV index is the gentle correction.
How the number is defined (without the lab coat)
Scientists weight the ultraviolet spectrum by how likely it is to redden human skin — erythemally weighted UV. The UV index is that irradiance multiplied by 40, so a convenient 0–11+ scale falls out. You do not need to memorize the formula. You only need to know what Basky does with it:
- Look up today’s UV for your location (optional location; there is a generic fallback if you decline).
- Combine it with your Fitzpatrick skin type.
- Return a conservative window of minutes — a fraction of a minimal erythemal dose, clamped to a sensible range, never a marathon, and never a reward for going past the line.
The math lives in Basky’s Exposure Engine. The feeling on the Today screen is simpler: a UV number, a plain-language band, and “about n minutes of safe sun.”
What each band is trying to tell you
| Band | UV index | Plain reading |
|---|---|---|
| Low | 0–2 | Gentle sky. A window can be long; in Basky, very low UV may simply mean the sun is resting. |
| Moderate | 3–5 | Useful daylight for a short bask. Fair skin should still treat it as real UV. |
| High | 6–7 | Strong. Windows shrink. Shade and covering start to matter sooner. |
| Very high | 8–10 | Short windows. This is “find shade after a little,” not “stay until you feel done.” |
| Extreme | 11+ | Rare outside the tropics, high altitude, or midsummer noon. Treat it as a brief, careful visit. |
Bands are labels, not permissions. High UV does not mean “skip the day.” It means the same wellness dose happens in fewer minutes. Low UV does not mean “pointless.” Stepping outside still counts — Basky’s daylight check-in exists for exactly those hours.
Why the index changes through the day
The sun’s path, the thickness of atmosphere the light travels through, ozone, season, and latitude all move the number. Late morning through early afternoon is typically the peak at mid-latitudes. Early and late light can still help a circadian daylight habit even when UV is too low for a baskable window.
Basky surfaces a hint like “best light around 1:00 PM” so you are not guessing from a single snapshot. For the hour-by-hour picture, see UV index by hour.
What Basky will never do with this number
- It will not tell you to skip sunscreen. A window is not a license to burn, and it does not replace clothing, shade, or a doctor.
- It will not stretch the window to keep a streak alive. Over-exposure earns nothing.
- It will not treat vitamin D as a lab result. The friendly estimate is encouragement, capped with the same safe dose.
- It will not sell your location. Coordinates, if you allow them, go only to the weather lookup. They are not stored as a history or tied to an account. There is no account.
A calmer way to use the UV index
Read the number. Read your window. Step outside for a little, not a lot. If the sky is closed, tap that you got some daylight anyway. That is the whole ritual — how Basky works, in one breath.
Now on iPhone. The core is free, forever. General wellness guidance, not medical advice.