Our Data Sources & Methodology

Primary Data Source: Open-Meteo

All UV index data on uvindex.now is sourced from the Open-Meteo API, an open-source weather forecast API. Open-Meteo aggregates data from leading national meteorological services and numerical weather prediction models, including:

• ECMWF (European Centre for Medium-Range Weather Forecasts) — the primary global model used for UV forecasting, with 0.25° resolution
• DWD (Deutscher Wetterdienst / German Weather Service) — provides ICON model data for enhanced European coverage
• NOAA GFS (Global Forecast System) — US-based global model providing additional forecast verification

How the UV Index Is Calculated

The UV index is a standardized measure developed jointly by the World Health Organization (WHO), the United Nations Environment Programme (UNEP), and the World Meteorological Organization (WMO). The calculation integrates several atmospheric and geographic variables:

• Solar zenith angle — the sun's elevation relative to directly overhead, determined by latitude, time of day, and date. This is the primary driver of UV intensity — UV is strongest when the sun is highest.
• Stratospheric ozone — measured via satellite (Sentinel-5P, OMPS). Ozone absorbs the majority of UV-B radiation; thinner ozone layers (common over Australia, New Zealand, and polar regions) result in higher surface UV.
• Cloud optical depth — clouds attenuate UV transmission. Thick clouds can block 70–80% of UV, but thin or broken cloud cover may only reduce UV by 10–25%.
• Surface altitude — UV intensity increases approximately 8–10% per 1,000 meters of elevation gain due to reduced atmospheric absorption.
• Aerosols and particulates — air pollution, dust, and smoke can scatter or absorb UV radiation, modifying surface levels.

The resulting UV index is a linear scale where 1 unit equals 25 milliwatts per square meter (mW/m²) of erythemal UV radiation — the wavelengths most responsible for sunburn.

Update Frequency

UV index data on uvindex.now updates every 15 minutes via client-side API calls to Open-Meteo. Each page request fetches fresh data directly from the API — we do not cache or store UV readings. This means the data you see is as current as Open-Meteo's latest model run.

Forecast models typically update every 6 hours (00:00, 06:00, 12:00, 18:00 UTC) with intermediate updates for rapid-refresh models. Hourly UV forecasts are interpolated between model run times.

Sunburn Calculator Methodology

Our sunburn time estimates are based on the Fitzpatrick skin phototype classification system combined with the Minimal Erythemal Dose (MED) for each skin type:

• Type I (very fair): MED ~200 J/m² — burns in 5–10 min at UV 10
• Type II (fair): MED ~250 J/m² — burns in 10–15 min at UV 10
• Type III (medium): MED ~350 J/m² — burns in 15–25 min at UV 10
• Type IV (olive): MED ~450 J/m² — burns in 25–35 min at UV 10
• Type V (brown): MED ~600 J/m² — burns in 35–50 min at UV 10
• Type VI (dark): MED ~800 J/m² — burns in 50–70 min at UV 10

Burn time is calculated as: burn_minutes = MED / (UV_index × 0.4167). SPF multiplies the unprotected burn time (e.g., SPF 30 at 10 minutes unprotected = ~300 minutes theoretical, though reapplication every 2 hours is recommended regardless).

These are estimates for direct midday sun exposure. Actual burn times vary with altitude, surface reflection, cloud cover, medication photosensitivity, and individual skin characteristics.

Coverage

uvindex.now currently provides UV data for 150+ cities across 6 continents. Our coverage includes the United States (72 cities), Europe (29 cities), Asia and the Pacific (21 cities), the Americas and Caribbean (18 cities), and the Middle East and Africa (10 cities). The search tool supports free-form location lookups via geocoding, meaning any location on Earth with Open-Meteo coverage can be checked.

Weather Data

In addition to UV index, each city page displays current weather conditions sourced from the same Open-Meteo API request: temperature (actual and apparent), humidity, wind speed, cloud cover percentage, and sunrise/sunset times. These are provided for contextual awareness — cloud cover and temperature influence sun exposure behavior even when they don't directly determine UV intensity.

Limitations & Disclaimers

• UV index values are model-based forecasts, not ground-truth measurements from pyranometers or radiometers. Actual UV at your specific location may differ.
• The UV index does not account for microclimate effects: urban canyon shading, tree canopy, building shadows, or indoor/vehicle window UV filtering.
• Surface reflection (water, sand, snow, concrete) is not factored into the reported UV index but can increase effective UV exposure by 10–80%.
• Sunburn time estimates assume direct, perpendicular sun exposure on unshaded skin. Real-world conditions vary significantly.
• This tool provides general information, not medical advice. Consult a dermatologist for personalized sun protection guidance.

Open-Meteo License

Open-Meteo weather data is provided under the Creative Commons Attribution 4.0 International License (CC BY 4.0). Attribution: weather data by Open-Meteo.com.

Contact

Questions about our data or methodology? Contact us.