It's even more overcast here (AL).
If you can, open this .pdf file
http://nadp.nrel.colostate.edu/UVB/publications/uvb_primer.pdf Page eight (8) shows UV measurements for a clear day and a cloudy day, same place. Pretty much lays the issue to rest... Except: thin clouds and thick clouds can have different effects.
can you post it? I'm unable to open this link. thanks