How do we know when there’s “really high UV” on any given day?

Question

I was just looking at my weather app and it said, “very high UV – limit sun exposure”. It’s mostly cloudy right now, so how do we know there’s high UV and also, does the sun’s UV rays really change in intensity?

In: 2

Answer 1

It can change with Ozone layer thickness, cloud cover and Sunspots.

Probably more, but those are the ones I remember for sure.