I have an argue with an ex-Doctor of Medicine about the amount of ultraviolet light reaching us under the clear day sun and under the 100% cloudy sky.
To what extent we can say that the sky clouds stops ultraviolet light from the sun?
Clouds don't absorb light (much), they reflect and refract it, and this applies to uv light in the same way as visible light. So a 100% cloudy sky will block uv light in the same way it blocks visible light. A quick Google suggests that heavy cloud cover will remove 80-90% of uv light. Anyone disputing this should try sunbathing on a cloudy day :-)
I've heard occasional claims that broken cloud cover can actually enhance uv levels in the unshaded areas by reflecting uv light into the breaks. However I've never come across any studies that prove this happens. While it seems vaguely plausible I'd be surprised if the effect was very big.