Clouds do reduce UV Index, but far less effectively than they reduce visible brightness — which is exactly why sunburn on an overcast day catches so many people off guard.
Why clouds are inefficient UV blockers
Thin or scattered clouds may block only a small percentage of UV radiation even while significantly dimming visible light — the eye's sense of 'how sunny it feels' is a poor guide to actual UV exposure.
The cloud-edge amplification effect
Under certain conditions, broken or scattered clouds can actually increase UV at ground level briefly, as sunlight reflects and scatters off cloud edges — a real, documented effect, not just anecdotal.
Only thick, complete overcast meaningfully helps
Full, dense cloud cover does substantially cut UV — the reduction that matters is from thick, complete cloud layers, not the hazy or partly-cloudy conditions many people assume are protective.
Can you get sunburned on a fully overcast day?
Yes, though it's less likely than under clear skies — thick, complete cloud cover does meaningfully reduce UV, but 'overcast' doesn't guarantee UV Index near zero, especially at higher baseline UV locations or seasons.