UV TrackerThe UV index scale, explained
What the UV index measures
The UV index is a measure of erythemally-weighted solar irradiance — how strongly sunlight at ground level can cause skin reddening (erythema), the earliest visible sign of sunburn. By definition, 1 UV index unit equals 25 mW/m² of that weighted irradiance. Unlike a percentage or a fixed range, the scale is open-ended: readings above 11 do occur at high altitude and near the equator, and simply mean "extreme" gets more extreme.
The WHO bands
| UV index | Band | Guidance |
|---|---|---|
| 0–2 | Low | Minimal protection needed for most skin types. |
| 3–5 | Moderate | Seek shade during midday hours; cover up and wear sunscreen. |
| 6–7 | High | Protection required — shade, clothing, sunscreen. |
| 8–10 | Very high | Extra care; sunburn risk rises fast on unprotected skin. |
| 11+ | Extreme | Take all precautions; sunburn risk is high within minutes. |
What each level means for your skin
The band tells you roughly how urgent protection is; the exact number of minutes depends on your skin type. At UVI 6 (High), a Type II skin has about 28 minutes of safe sun time; a Type V has about 67. See the full tables by skin type or the calculator for your own numbers.
Why the forecast differs from your sky
Satellite-derived UV index retrievals validate well against ground stations across large regions, but they describe an area, not the specific patch of sky above you right now — clouds move on a much shorter timescale than a forecast update. See how a local sky reading adjusts the regional number.
When to check it
The UV index typically peaks within an hour or two of solar noon and falls off sharply toward morning and evening. Checking it once at breakfast can miss a midday spike — the app's sky scan reads the UV where you are, when you ask it to.
UV Tracker measures the UV where you stand — sky scan for local conditions, one photo for your skin type, and your safe sun time on one card. Free forever.
References
- World Health Organization, Global Solar UV Index: A Practical Guide. Accessed 2026-09-01.
- US Environmental Protection Agency, UV Index Scale. Accessed 2026-09-01.
- Kosmopoulos PG, et al. "Real-time UV-Index retrieval in Europe using Earth Observation based techniques and validation against ground-based measurements." Atmospheric Measurement Techniques, doi:10.5194/amt-2020-506.
UV forecast data in the app: Open-Meteo.
This article is general information, not medical advice. Sun-exposure estimates are approximations. If you have concerns about your skin, or a history of skin cancer in your family, see a doctor or dermatologist.