On Wednesday, May 27, 2020 at 2:07:49 PM UTC+1, Graham Easterling wrote:

I have not seen any evidence UV levels are linked to high humidity. If UV levels are lower after a cold front it may be there is more cloud.

Nicholas
Meir Heath, Stoke-On-Trent 250 metres above sea level.

If you have not seen the evidence, you haven't looked. There is plenty of evidence that low level humidity & high level UV are linked.

If you don't believe it, I can only suggest you look at Camborne UV over a few summers, & compare it with the synoptic chart. That doesn't prove the exact mechanism (Freddie, any help here) but it proves the strong correlation.

Cloudless skies after the passage of a cold front give lower UV than before. UV is very low under cloud in mP air.

It really irritates may when observational evidence is dismissed as not evidence. The sea fog off the north Cornwall coast in't there because you haven't seen it?
. or you could just spend a moment using Google, I typed UV RH relationship & this came top of the list
" In this article, the ultraviolet radiation and relative humidity (RH) data from ground observations and a radiative transfer model were used to examine the influence of RH on ultraviolet radiation flux and aerosol direct radiative forcing under the clear-sky conditions. The results show that RH has a significant influence on ultraviolet radiation because of aerosol hygroscopicity. The relationship between attenuation rate and RH can be fitted logarithmically and all of the R2 of the 4 sets of samples are high, i.e. 0.87, 0.96, 0.9, and 0.9, respectively. When the RH is 60%, 70%, 80% and 90%, the mean aerosol direct radiative forcing in ultraviolet is −4.22W m−2, −4.5W m−2, −4.82W m−2 and −5.4W m−2, respectively. For the selected polluted air samples the growth factor for computing aerosol direct radiative forcing in the ultraviolet for the RH of 80% varies from 1.19 to 1.53, with an average of 1.31."

I agree that temperature is not linked to high UV levels. There is, of course, an association.

I have to confess I've been a bit of a beach bum. I can lie around all day in the clear sunny polar air, but a couple of hours surfing in shallow sea mist . . . (I'm talking about my younger days!)

There's loads of evidence of the link.

Graham
Penzance

I meant that mention that patches of very low level cloud increase UV over totally blue sky conditions. The EPA say as much in their guide to UV. The sort of cloud that develops quickly as the air rises up cliffs to give cliff top mistiness.

Graham
Penzance