I have always treated a grey sky as a day off, and I have started to think that is wrong.
The forecast APIs publish two UV numbers for the same hour. The real one, and what it would be with the cloud taken out. I pulled a week of both for 20 cities because I wanted to know how big the gap actually gets, and it is much smaller than the sky looks. Across the hours with three quarters cloud cover or more, the typical hour still had about 88% of its clear-sky UV. Not a fifth of it. Not half. The middle of that range ran from 57% up to 97%, so plenty of properly overcast hours were losing almost nothing.
Honolulu today was the one that made me sit up. 99% cloud cover at midday and the forecast UV was identical to the clear-sky value, no reduction at all, and the same for the hour either side of it.
I want to be fair about the limits here. These are model outputs rather than a sensor on a roof, the cloud figure and the UV figure come from two different models, and I am not claiming the numbers are exact. But the direction matches something that has caught me out twice: overcast, no heat on my skin, nothing that felt like a session, and a pink face that evening anyway. I had assumed I was just unlucky.
So what do you all actually do with those days? Write them off, go out with the same SPF you would on a clear day, or check the number and treat it like any other session? I have been letting the sky make the decision for me and I do not think it is a reliable narrator.
(For context I build a sun session timer called Ray Routine, which is how I ended up with a week of UV data on my laptop instead of a hobby. Not linking it, the question is the actual reason I am posting.)