On 27/05/2020 16:03, Nicholas Randall wrote:
I am not dismissing it as no evidence. I have not read the article but I have searched for it today. I have looked at the World Health Organization website and humidity is not mentioned as a factor with high UV levels so this made me think it was not. There are a lot of factors and all of them are not listed. There are also things that reflect UV levels that make them higher. One of these is water so the sea would affect them.

As Graham has said at length, the UV is highest in humid air, especially
near a slack low pressure when in a warm sector. Sea fog adds to the UV
as well as mist. Under these conditions (fairly common in July in the
South West), the UV rockets to 9 or 10, or even higher.

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.

Also, people have mentioned ozone depletion above warm sectors.

Here a typical UV in maritime polar air would be 6.5 at this time of
year. Introduce Atlantic air from the south and that UV rises to between
7.5 & 8.5. Warm sector air and you're between 8.5 and 9.5.

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