What Is the UV Index? Scale, Calculation, and Forecast Methodology
The Ultraviolet Index (UVI) is an international standard measurement of the intensity of ultraviolet radiation from the sun at the Earth's surface. It was developed jointly by the World Health Organization (WHO), the United Nations Environment Programme (UNEP), the World Meteorological Organization (WMO), and the International Commission on Non-Ionizing Radiation Protection (ICNIRP), with the first international version published in 1994.
The WHO UV Index Scale
The UV Index is a dimensionless linear scale that starts at 0 (no UV radiation) and has no upper limit — values above 11 are designated as Extreme and are typical in tropical regions and at high altitudes. The WHO organises the scale into five exposure categories:
| UVI Range | Category | Color Code |
|---|---|---|
| 0–2 | Low | Green |
| 3–5 | Moderate | Yellow |
| 6–7 | High | Orange |
| 8–10 | Very High | Red |
| 11+ | Extreme | Violet |
These categories are documented in the WHO Global Solar UV Index: A Practical Guide, co-published by WHO, UNEP, ICNIRP, and WMO, and are also summarised in the WHO ultraviolet radiation fact sheet.
What Factors Affect the UV Index?
The WHO UV fact sheet identifies several environmental factors that determine the UV index value at any given location and time:
Solar elevation angle — the primary driver. UV radiation is strongest when the sun is highest in the sky (solar noon) and weakest near sunrise/sunset, when sunlight must travel through more atmosphere before reaching the surface.
Stratospheric ozone concentration — the ozone layer absorbs most incoming UVB radiation (280–315 nm). According to WHO documentation, a 1% decrease in stratospheric ozone concentration leads to approximately a 2% increase in ground-level UV.
Cloud cover — dense overcast conditions can reduce UV by 50% or more. Thin or broken cloud cover may have minimal effect, and reflected UV from clouds can occasionally elevate ground-level UVI above clear-sky values.
Altitude — UV intensity increases with elevation. The WHO cites an approximate 10% increase per 1,000 metres above sea level, due to reduced atmospheric path length and scattering.
Surface albedo — highly reflective surfaces increase effective UV exposure. Snow can reflect up to 80% of incoming UV; dry sand reflects approximately 15%; calm water approximately 10%.
Aerosols and air pollution — tropospheric particles scatter and absorb UV radiation, generally reducing UVI in heavily polluted urban environments.
The WHO Standard Reference Spectrum
The UV Index is calculated from a spectrally weighted integral of incoming solar irradiance, using the CIE standard erythema action spectrum — a biologically derived weighting function representing the relative effectiveness of different UV wavelengths in causing skin reddening (erythema). A UVI value of 1 corresponds to a UV irradiance of 25 mW/m². This standardisation, defined jointly by the WHO and ICNIRP, ensures that UV Index values measured anywhere in the world are directly comparable.
How UV Index Forecasts Are Generated
Modern UV index forecasts are produced by combining numerical weather prediction (NWP) models with real-time satellite observations. The Copernicus Atmosphere Monitoring Service (CAMS), operated by the European Centre for Medium-Range Weather Forecasts (ECMWF), produces global UV radiation forecasts up to five days ahead. These incorporate:
- Total column ozone data from Copernicus Sentinel satellites
- Aerosol optical depth forecasts from global atmospheric composition models
- Cloud cover and transmission from NWP model runs updated multiple times daily
The CAMS UV forecast outputs are disseminated via the CAMS API and are used by open-data services including Open-Meteo, which exposes hourly UV index forecasts for any location worldwide under an open-source, commercial-use licence.
Where to Find UV Index Data
UV index data is freely available from several authoritative sources:
- Copernicus Atmosphere Monitoring Service — global daily and hourly UV forecasts from ECMWF
- Open-Meteo — open-source weather API with UV index as a variable
- UVI Today (uvi.today) — hourly UV index display for any location, powered by Open-Meteo/CAMS data
For WHO's complete guidance on ultraviolet radiation and health effects, see the WHO Ultraviolet radiation fact sheet.
