alt Hacker NewsIt is a prediction of UV intensity based on specific forecast location, time of day, day of year, prevailing weather, and any anomalies affecting the ozone layer. It is calculated by NOAA.
https://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index...
This depends on the country and the source. Here in Australia BOM UV values are predicted, but cancer council are observed.