Clouds do tend to diminish UV, but not significantly so as to make you UV-invulnerable. In the UV tracking app I use, the largest cloud-effects I’ve seen are i

Clouds do tend to diminish UV, but not significantly so as to make you UV-invulnerable. In the UV tracking app I use, the largest cloud-effects I’ve seen are in the range of 1-3 points decrease. So, UV index of 5-7 instead of 8. Depending on the baseline UV level during exposure, that difference could be significant (5 index down to 2) or not that significant (14 down to 11), in terms of expected minutes until sunburn.