To find the effects of atmospheric water vapor on UV here:
http://rain.atmos.colostate.edu/AT622_section10.pdf and look at the upper right hand figure on the first page. It is a plot of the absorption coefficients for several atmospheric components (e.g. CO, CO2, ozone, NO2, H2O, etc.) The horizontal axis is wavelength in nm (nanometers). UA, UB are roughly in the range 2 to 4 nm. Look at the curve labeled H20. You will see that water vapor (H2O) has an absorption of nearly 100% in at some parts of the UV spectrum. Therefore water vapor can have a significant effect on UV.
Now as to the response from the Cancer Institute, realize that they want people to know that cloud cover is not perfect protection from UV. Of course it's not... BUT IT HELPS! Also, my comparison was meant to be between the SW (e.g. Arizona) and the SE (e.g. Alabama, NOT Michigan), two states at the same latitude, but with vastly different levels of average UV.
Here's a common sense answer to the issue: If you have to be outside naked for 3 hours in the mid day, would you rather it be a cloudy day?
Here's a common sense answer to the issue: If you have to be outside naked for 3 hours in the mid day, would you rather it be a cloudy day? Thanks for the link, but it doesn't change anything. You don't seem to understand that Arizona is not dry 100% of the time. The deadliest UV ray index readings come through here when our dewpoints reach in the 60 degree range.
As for your question, all it would do would be more comfortable while being out there under cloud cover, rather than in the sun. The damage would still be the same.