Source
UV forecasts are produced by NOAA’s National Weather Service Climate Prediction Center (CPC) and published by the EPA as hourly and daily forecasts for every ZIP code. CPC computes the index with a radiative transfer model using forecast ozone, cloud attenuation and aerosols, adjusted for elevation. NOAA notes that surface UV depends mainly on the sun’s elevation in the sky, stratospheric ozone and clouds.
How we use it
- Each city uses the forecast for its central ZIP code (the ZIP code area nearest the Census point for the city).
- Every six hours, we check for missing forecasts for each ZIP’s local day. We reuse an already valid daily forecast and preserve its original retrieval time. The recent history shows up to seven available past forecast dates; these are archived forecasts, not measurements, and missing dates are not estimated.
- Exposure categories follow the WHO scale; sun protection advice is quoted from the EPA.
- Sunrise, sunset and solar noon are calculated from the city’s coordinates with standard NOAA solar equations.
Limitations
- It’s a forecast: actual UV depends on the clouds overhead at the time.
- Sand, water and snow reflect UV and can increase exposure; elevation increases it too.
- City pages cover places with 25,000+ residents for now.
Sources: CPC: UV Index, how it is computed; EPA Envirofacts UV web services; EPA UV Index scale.