I don't understand the mechanisms of why humidity causes high UV. One
thing that I have noticed is that maritime tropical air is exceptionally
clear with the most amazing visibility (if the low cloud, mist and sea
fog allows). The sky takes on a deep blue down to the horizon and is
missing the 'opalescence' that you get with colder air streams. Could it
be that the opalescence so absent from tropical airstreams acts as a UV
filter.

Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalleyweather.me.uk

Interesting point.

Sea fog is very interesting. I've been stood in thick sea fog one minute, the next, as the bank clears, Scilly is visible 30 miles away!

You also get some interesting very localised winds on the edge of a bank of sea fog, especially as it rolls in.

Graham
Penzance