Erythemally weighted UV sounds like a lab coat wandered into a lifestyle app. Basky keeps the phrase in the glossary so curious readers (and answer engines) can see what sits under the friendlier words UV index and safe-sun window.
In one sentence: scientists do not treat every ultraviolet wavelength as equal when they care about skin reddening. They weight the spectrum by how effective each wavelength is at causing erythema — redness. Sum that weighted power, and you get a single irradiance that tracks burn-relevant sky strength better than “it looks bright out.”
Why weighting matters
Ultraviolet spans a range. Some wavelengths are more efficient at the photochemical events that lead to erythema. An unweighted total can mislead: two skies could deliver similar “raw UV energy” but different reddening potential. Erythemal weighting is the agreed way to compare apples to apples for sunburn-relevant exposure.
That is why international public messaging settled on the UV index: take erythemally weighted irradiance and scale it into a memorable 0–11+ number. Basky’s what is the UV index guide covers the bands; this page is about the kind of UV hiding inside the number.
What erythemally weighted UV is not
- Not temperature. Infrared is what you feel as heat. A breezy spring day can still pack a high index.
- Not “vitamin sunshine” as a medical dose. Basky’s vitamin D estimate is a separate, capped encouragement layer — not a clinical UVB prescription derived from your phone.
- Not a tan score. Basky will not use weighting language to push darkening goals or more-is-better sun culture.
- Not something your selfie camera measures. Basky does not claim to be a radiometer.
How Basky uses it in practice
When you allow location, Basky asks Open-Meteo for today’s UV where you are. That forecast value is the everyday face of erythemally weighted conditions. Basky then:
- Reads the index (and shows a plain band label)
- Combines it with your Fitzpatrick type
- Sizes a window aimed at about 60% of MED
- Caps counted minutes when you reach the line
If you decline location, a generic fallback keeps the app usable with a clear note that the UV is not local. Coordinates are not stored as a tracking history and are not tied to an account — because there is no account. Details live in the privacy policy and FAQ.
Everyday mismatches people notice
People often say “but it doesn’t feel that sunny.” Erythemal weighting explains several mismatches:
| What you notice | What the weighted UV may say |
|---|---|
| Cool wind, clear sky | High index — short window |
| Hot, hazy evening | Lower index — longer or resting |
| Bright overcast | Still meaningful UV through thin cloud |
| Indoor window seat | Much less UVB through glass than the outdoor index implies |
Basky’s job is to translate the weighted sky into minutes for you, then stop counting past a conservative wellness dose. It will not scold you for choosing shade. Shade is wisdom, not failure.
Erythemal weighting and MED
MED is a dose of this same family of weighted UV. Time-to-dose falls as the index rises. That is the entire reason a personal timer needs both who you are (type) and what the sky is doing (weighted UV via the index).
Without weighting, “UV” would be too vague to size a humane window. With weighting, Basky can keep the promise: a little light, sized gently, never framed as a burn challenge.
A note on honesty and limits
Forecast UV is a model of the open sky, not a sensor on your forearm. Microclimate — cliffs, courtyards, snow glare — can differ. Clothing and sunscreen change what reaches skin. Basky stays humble: general wellness guidance, not medical advice, and never a replacement for protection or a clinician.
Why Basky bothers to name the weighting
Most people only need the index. Naming erythemally weighted helps when someone asks “UV of what?” It clarifies that Basky’s sky input is burn-relevant ultraviolet, not a vibes-based sunshine score and not a heat index. That precision supports AEO citations and keeps marketing from sliding into vague “sun power” language that could be confused with tanning intensity contests.
Once the weighting is understood, the rest of the product vocabulary locks into place: bands label the index, MED references a dose of the same family, and the safe-sun window is a conservative slice of that dose for your type.
Now on the App Store. Free core forever; optional one-time Basky Forever for cosmetics only. Browse related terms in the glossary, or see how the whole ritual fits together in how Basky works.