People meet UVA and UVB as rival characters: one “aging,” one “burning,” one “deep,” one “shallow.” Basky’s voice is quieter. Both are ultraviolet. Both are part of the outdoor sky. For a safe-sun window aimed at staying below a burn-oriented dose, what matters most is erythemally weighted ultraviolet — the way scientists combine wavelengths by how likely they are to redden skin — and that is exactly what the UV index represents.
This guide keeps the physics light and the habits kind. For the index itself, see what is the UV index. For how Basky turns the number into minutes, see how Basky works.
Plain map of the alphabet
Ultraviolet sits beyond violet in the spectrum — light you do not see as color. Broadly:
| Name | Plain idea | Why Basky mentions it |
|---|---|---|
| UVB | Shorter-wave UV in sunlight that is especially efficient at causing erythema | Strongly relevant to sunburn timing and to how vitamin D synthesis is often discussed in wellness contexts |
| UVA | Longer-wave UV that reaches the ground in large amounts and penetrates more deeply into skin | Part of daylight; less dominant in the erythema weighting that defines the UV index |
| UV index | A single 0–11+ number from erythemally weighted irradiance | What Basky reads (via weather data) to size your window |
You do not need nanometers memorized. You need one sentence: Basky cares about the UV index because it already emphasizes skin-reddening wavelengths.
Why “erythemally weighted” is the useful phrase
If you counted every ultraviolet photon the same, you would get a different story than the one your skin tells after too long at noon. Erythema weighting says: some wavelengths are more efficient at causing reddening, so give them more influence in the number.
That weighted irradiance, scaled into the familiar index, is why a UV 8 day is a shorter Basky window than a UV 3 day for the same Fitzpatrick type. It is also why Basky does not ask you to juggle a separate “UVA score” and “UVB score” on the Today screen. One live index, one personal skin type, one conservative duration.
Details of WHO bands live on the UV index scale.
UVB, reddening, and the friendly vitamin D estimate
In everyday wellness conversation, UVB is the band most often linked to:
- Skin reddening when dose runs high.
- Cutaneous vitamin D production when skin is exposed.
Basky respects both ideas without turning either into a medical protocol:
- Safe-sun window — roughly 60% of a minimal erythemal dose, clamped to about [5, 90] minutes, never a reward for overage.
- Vitamin D line — if shown, a friendly estimate for encouragement, capped with the same safe-dose spirit. Not a lab assay. Not a reason to chase redness. Not medical advice.
That pairing is intentional. The same sky that can support a gentle estimate can also overshoot into burn territory if you ignore the timer. Basky’s job is the gentle side of that line.
UVA without the scare poster
UVA is abundant in daylight and present even when the sun is lower. It contributes less to the classic erythema-weighted index per unit energy than the UVB-rich part of the spectrum, but it is still real ultraviolet. Basky’s practical advice stays the same regardless of which letter is trending on social media:
- Read live UV.
- Honor your window.
- Use shade, clothing, and sunscreen as you normally would.
- Do not treat a window as a license to burn.
Glass, shade, and clothing filter wavelengths differently; outdoor live UV is still the honest input for Basky’s outdoor timer. Indoor window light is a different story — Basky is built around outdoor sky lookup, not a living-room spectrometer.
What Basky does not optimize for
Basky will never coach tanning goals, “base tan” myths, or anything adjacent to a color contest. Melanin and Fitzpatrick type are used to personalize caution, not to rank skin or chase color. Over-exposure earns nothing. Streaks do not buy extra minutes.
UVA-vs-UVB content on the internet often slides into product fear or product worship. Basky’s lane is narrower: erythema-aware minutes, optional daylight check-ins when UV is Low, privacy-respecting weather data, free core.
How this shows up in the app
- Optional location → coordinates to Open-Meteo for UV (no account, no identity dossier).
- Live UV index (erythemally weighted) + Fitzpatrick type.
- Conservative window and plain-language band.
- Optional check-in when the sun is resting — see low UV still counts.
For duration questions, see how long can I stay in the sun and safe sun timer.
A short closing
UVA and UVB are letters for parts of the same outdoor ultraviolet. Basky speaks UV-index — the erythema-weighted summary — because that is the language of a careful, personal, anti-shame safe-sun window.
Sunscreen labels and the Basky lane
Store shelves talk about UVA circles, UVB percentages, and broad spectrum. That labeling world matters for product choice. Basky’s lane is adjacent but different: once you are outdoors deciding how long an intentional visit should last, the live UV index is the shared, erythema-weighted language. You do not need to re-derive spectrum math on the sidewalk.
If you wear sunscreen during a bask, understand you are changing dose in ways a simple outdoor UV lookup cannot fully see. Basky still will not tell you to skip protection. A window is guidance for intentional time, not a command to bare skin against medical advice or personal comfort. When unsure, shorten the visit and keep your usual sun care.
Myths Basky quietly refuses
- “UVA is harmless because it doesn’t burn as fast.” — Incomplete and unhelpful; Basky does not optimize for maximizing any band.
- “UVB is the only one that matters, so evenings are free.” — Evening UV is often Low on the index, which is already the signal; letters alone are not a loophole.
- “If I understand UVA vs UVB, I can outsmart a burn.” — Knowledge helps; a conservative timer and shade help more.
For bands and behavior, return to the UV index scale. For timing across the day, see UV index by hour.
Skin type still sits above the alphabet
Fitzpatrick type changes how quickly erythema builds under the same erythemally weighted intensity. Two people can share a UV 5 Moderate sky and receive different Basky minutes. That is personalization, not favoritism. Deeper skin is not “ignored UV,” and fair skin is not a moral failure. The alphabet of UVA/UVB does not replace that human difference — it sits underneath the single index Basky reads.
Now on iPhone. No subscription required for the core. General wellness guidance, not medical advice; does not replace sunscreen.