On Tuesday, 26 May 2020 20:09:50 UTC+1, Graham Easterling wrote:
I agree with most of what you state about factors that affect UV levels except humidity. I do not think it affects them.

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

Equally, I am sure it does. If Norman reads this I know he'll agree. In fact, anyone that spends a lot of time outdoors, certainly on the coast of SW England where high humidity is common, fully appreciates the fact.

How else do you explain the big drop in UV when a cold front goes through? Certainly lower high level ozone sometimes being associated with a warm sector ridge is partly the cause, but the whole thing? If you don't except it's the humidity, despite the evidence, you need to come up with an alternative explanation why it's so high in air which has none of the clarity of that behind the cold front.

Somewhere I'v got a list of times when the Camborne UV has hit 9, never in clear, clean sunny mP conditions.

Graham
Penzance

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.

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.

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

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

On Wednesday, 27 May 2020 14:07:49 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 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.

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

On Wednesday, 27 May 2020 16:03:08 UTC+1, Nicholas Randall wrote:
On Wednesday, 27 May 2020 14:07:49 UTC+1, Graham Easterling wrote:

One of these is water so the sea would affect them.

So cloud and mist droplets would come into this category too.

--
Freddie
Dorrington
Shropshire
115m AMSL
http://www.hosiene.co.uk/weather/
Stats for the month so far: https://www.hosiene.co.uk/weather/st...cs/latest.xlsx

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

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