On Tuesday, 26 May 2020 15:22:04 UTC+1, Freddie wrote:
On Monday, 25 May 2020 14:25:39 UTC+1, Nicholas Randall wrote:
I look at the Met Office website and the temperature forecasts are too low and this makes me think the UV forecasts are also too low.

You should be aware that there isn't a link between temperature and UV level, so the above logic is incorrect. UV level is a function of the amount of absorption that occurs in the atomosphere, and this is largely determined by the angle of the sun's rays.

The simplest demonstration of this is the fact that UV level peaks around local noon, whereas temperature peak is 2-3 hours later.

Also, the summit of, say, Snowdon will experience a higher UV level than, say, Llandudno, as there is less thickness of atmosphere above to filter out incoming UV.

Factors such as low-level moisture, the presence of some cloud, and ozone also play a part.

--
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

I am aware there is no link between temperature and UV levels. I was stating the Met Office maximum temperature forecasts are low and I think the UV forecasts are also too low. Today the maximum temperature was forecast to be 18 C and the UV forecast was changed from 6 to 5.
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.

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

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 can't comment on actual UV measurements but, from personal
experience, for any given temperature I find that I burn more readily
in high humidity conditions than in dry conditions. In coastal, fairly
humid conditions, in this country with the temp in the low 20s I burn
much more than I do in 40° in very low humidity in inland Australia.
Indeed, I have to be very careful in this country on summer days when
there is a complete cover of low cloud but nothing above it. On such
days, if unprotected, I would burn very badly.

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

On Wednesday, 27 May 2020 10:38:20 UTC+1, Norman Lynagh wrote:
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 can't comment on actual UV measurements but, from personal
experience, for any given temperature I find that I burn more readily
in high humidity conditions than in dry conditions. In coastal, fairly
humid conditions, in this country with the temp in the low 20s I burn
much more than I do in 40° in very low humidity in inland Australia.
Indeed, I have to be very careful in this country on summer days when
there is a complete cover of low cloud but nothing above it. On such
days, if unprotected, I would burn very badly.

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

Where in inland Australia are you referring to? The temperature is not linked to UV levels. It is about how high the sun is in the sky and the latitude. Lower latitudes will have higher UV levels than higher latitudes.

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

Nicholas Randall wrote:

On Wednesday, 27 May 2020 10:38:20 UTC+1, Norman Lynagh wrote:
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 can't comment on actual UV measurements but, from personal
experience, for any given temperature I find that I burn more
readily in high humidity conditions than in dry conditions. In
coastal, fairly humid conditions, in this country with the temp in
the low 20s I burn much more than I do in 40° in very low humidity
in inland Australia. Indeed, I have to be very careful in this
country on summer days when there is a complete cover of low cloud
but nothing above it. On such days, if unprotected, I would burn
very badly.

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

Where in inland Australia are you referring to? The temperature is
not linked to UV levels. It is about how high the sun is in the sky
and the latitude. Lower latitudes will have higher UV levels than
higher latitudes.

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

Many places, including wide areas in the Northern Territory and North
Queensland. Also some places in nothern WA.

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

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.