great question! My understanding is the UV index shown online and in apps is “modeled” data, not realtime.

Basically it takes into account things like ozone measurements from satellites, latitude, forecasted cloud cover, and distance from sun (time of year).

See section “Calculating the UV index” here:

https://www.epa.gov/sunsafety/learn-about-uv-index

It’s not intended to be realtime. It’s more about understanding, generally, what the risk is.