Posted 6/4/2018 3:08 PM (GMT -6)
Clouds block UV by differing amounts, depending on the type of clouds and their height, thickness, and density.
Really thick, dense, dark, thunderstorm-type of clouds block UV fairly well, but many other clouds only block a little.
So, yes, as GreenBeans says, you should still cover up on cloudy days. Clouds aren't reliable in blocking UV.