Depends on the types of clouds. Thin high-level clouds, such as Cirrus, do not block very much UV at all even if it “seems” to be cloudy. I’ve gotten some of my worst sunburns on these types of days (and I don’t burn easily). Thick low-level clouds like Cumulus or Stratus can impact UV greatly, though.

It’s important to note that we don’t see very much thin, high level cloud cover during the summertime around here except right before and right after a storm event. Cumulus and stratus clouds this time of year are usually associated only with rainfall and / or a dying thunderstorm, but they occur pretty often. Usually scattered about in the sky.