Yes, genuinely — cloudy-day sunburn is one of the more common ways people get caught off guard, precisely because visible cloud cover doesn't correlate as closely with UV Index as most people assume.
Clouds reduce visible light more than UV
The specific wavelengths of UV radiation aren't blocked as efficiently by thin or scattered cloud as visible light is — a hazy, overcast-feeling day can still carry a meaningfully high UV Index.
Real-world pattern
Overcast-day sunburn is a well-documented, common experience specifically because people spend longer outside without protection, assuming (incorrectly) that the lack of direct sun means lack of UV risk.
The reliable fix
Checking actual UV Index — not visible cloud cover — before deciding whether SPF is warranted removes this specific blind spot entirely.
Does light rain reduce UV Index significantly?
Rain itself doesn't directly block UV the way thick cloud does — if rain is accompanied by dense, complete cloud cover, UV does drop meaningfully, but light rain from thin or broken clouds may not reduce UV as much as expected.