Full cloudcover will block 70% – 90% of UV-B rays (the sunburn causing ones)
Properly applied spf 15 sunscreen will block 93% of UV-B rays for at least two hours.
Partial cloudcover can *increase* UV-B exposure by up to 40%, depending on conditions.
So overcast is substantially less protection than wearing weak sunscreen.
Hopefully useful for everyone, but especially all my fair-skinned friends who suffer from the same rationalizations i have: “oh, it’s cloudy, I’ll be fine.”
(I hate putting on sunscreen, but remembering how miserable I am when sunburned sometimes helps me overcome that, and I’m getting better at it being a thing I just do, like putting on a seatbelt.)