Thread

1. avidiax · 1790114317 · focus · HN ↗

   Does anyone know how UV indexes reported by weather apps (including forecasts) work?

   I've found that some sources report lower UV index on cloudy days, whereas some seem to indicate the same index as a sunny day.

   It's not clear if these sources all have a different model for UV, some being more sophisticated or conservative, and whether any of them are backed up by a UV meter.

   1. dlcarrier · 1790144142 · focus · HN ↗

      Some phones and watches have UV sensors in them, and you'll usually see a 20%+ drop in live readings when under an overcast layer, compared to direct sunlight, but prediction models can't easily model when clouds will be where, so they either ignore them or use an average affect.

Unofficial Hacker News client; not affiliated with Y Combinator.