Graham Easterling wrote:

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

Those are conditions that can result in my fair skin burning very badly
if I don't take precautions. I get the same effect here in Tideswell
when there's a layer of low stratus with clear sky above it.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
twitter: @TideswellWeathr