Hi All,

I'm a long time lurker who is just interested in the weather, but possibly
more so than your 'average Joe' which is why I enjoy reading all the
interesting stuff from the experts on here. I like to think I've developed a
better understanding of forecasting as a result of lurking here but there's
one thing that puzzles me and would really appreciate some guidance from the
contributors to this group.

One of the sites I regularly look at gives an 3 hour forecast window
including a UV index and this always peaks between 16.00 and 19.00 when, for
example, today and tomorrow (Sunday and Monday) it shows the UV level as 8
during these hours but only 6 between 13.00 and 15.59. I can't fathom out
why this should be - surely the UV index would peak when the sun is
strongest which, in summer time in the UK would be around lunchtime,
wouldn't it? Please bear in mind I'm a complete novice and have no detailed
understanding of weather systems and suchlike but info I've read on this
group has helped me understand a good deal over the last couple of years;
but I just can't understand why the UV level isn't strongest when you'd
expect the sunshine (when we get it) to be strongest.

Thanks

Mike.

On Sun, 8 Jun 2008 19:11:42 +0100, wrote in
m

Hi All,

I'm a long time lurker who is just interested in the weather, but possibly
more so than your 'average Joe' which is why I enjoy reading all the
interesting stuff from the experts on here. I like to think I've developed a
better understanding of forecasting as a result of lurking here but there's
one thing that puzzles me and would really appreciate some guidance from the
contributors to this group.

One of the sites I regularly look at gives an 3 hour forecast window
including a UV index and this always peaks between 16.00 and 19.00 when, for
example, today and tomorrow (Sunday and Monday) it shows the UV level as 8
during these hours but only 6 between 13.00 and 15.59. I can't fathom out
why this should be - surely the UV index would peak when the sun is
strongest which, in summer time in the UK would be around lunchtime,
wouldn't it? Please bear in mind I'm a complete novice and have no detailed
understanding of weather systems and suchlike but info I've read on this
group has helped me understand a good deal over the last couple of years;
but I just can't understand why the UV level isn't strongest when you'd
expect the sunshine (when we get it) to be strongest.

Mike - you are quite right to question the times of 13.00 to 15.59. Solar
radiation, of any kind, will peak an hour or so either side of local noon.
Where I am now local noon is at 13.30 BST, so peak radiation will be
between 1200 and 1500.

--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 08/06/2008 18:26:59 GMT

On Sun, 8 Jun 2008 19:11:42 +0100, wrote in
m

Hi All,

I'm a long time lurker who is just interested in the weather, but possibly
more so than your 'average Joe' which is why I enjoy reading all the
interesting stuff from the experts on here. I like to think I've developed a
better understanding of forecasting as a result of lurking here but there's
one thing that puzzles me and would really appreciate some guidance from the
contributors to this group.

One of the sites I regularly look at gives an 3 hour forecast window
including a UV index and this always peaks between 16.00 and 19.00 when, for
example, today and tomorrow (Sunday and Monday) it shows the UV level as 8
during these hours but only 6 between 13.00 and 15.59. I can't fathom out
why this should be - surely the UV index would peak when the sun is
strongest which, in summer time in the UK would be around lunchtime,
wouldn't it? Please bear in mind I'm a complete novice and have no detailed
understanding of weather systems and suchlike but info I've read on this
group has helped me understand a good deal over the last couple of years;
but I just can't understand why the UV level isn't strongest when you'd
expect the sunshine (when we get it) to be strongest.

Mike - you are quite right to question the times of 16.00 to 19.00. Solar
radiation, of any kind, will peak an hour or so either side of local noon.
Where I am now local noon is at 13.30 BST, so peak radiation will be
between 12.00 and 15.00
--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 08/06/2008 18:30:06 GMT

Hi Mike

As already been said, the UV intensity will generally be proportional to how
high the sun is in the sky, i.e., less atmosphere (and ozone) for the light
to travel through before reaching the earth's surface.

Therefore, the greatest UV levels will generally occur, on a clear day when
the sun is at its highest elevation in the sky. There will be no lag as you
would expect with temperature.

At this time of year (BST daylight saving) and the fact that most of the
British Isles lie west of the Greenwich Meridian, the greatest UV index will
occur from around 1pm in London and the South East through to around 1:30 pm
in the far south west of England, west Wales, western Scotland & Ireland.

Here is a link to a graph showing the UV index for today (8th) and yesterday
for Camborne in Cornwall, note the peak at around 1pm - 2pm.

http://www.btinternet.com/~nickgardn...ex08062008.jpg
________________
Nick.
Otter Valley, Devon
83 m amsl
http://www.ottervalley.co.uk

On Sun, 8 Jun 2008 19:30:06 +0100, Mike Tullett wrote:

Mike - you are quite right to question the times of 16.00 to 19.00.
Solar radiation, of any kind, will peak an hour or so either side of
local noon.

Ish. Cloud cover will make a difference Thick heavy cloud and midday v
clear skies late afternoon... But cloud can probably increase the UV level
as well, high thin sirrusy stuff producing more back scatter?

This would not account for "a UV index and this always peaks between 16.00
and 19.00" though.

--
Cheers
Dave.

"Nick Gardner" wrote...

As already been said, the UV intensity will generally be
proportional to how high the sun is in the sky, i.e., less
atmosphere (and ozone) for the light to travel through before
reaching the earth's surface.

Therefore, the greatest UV levels will generally occur, on a clear
day when the sun is at its highest elevation in the sky. There will
be no lag as you would expect with temperature.

Here is a link to a graph showing the UV index for today (8th) and
yesterday for Camborne in Cornwall, note the peak at around 1pm -
2pm.

http://www.btinternet.com/~nickgardn...ex08062008.jpg
________________

.... this graph shows a representation of the total global radiation
and UV radiation at an un-named spot:-


http://www.cpc.ncep.noaa.gov/product...s/diur_guv.gif

and is confirmed by the HPA site ...

http://www.hpa.org.uk/webw/HPAweb&HP...=1158934607746

These confirm the notes of others: the UV radiation should be
(ignoring effect of varying cloud cover), at a maximum around solar
noon (which does vary w.r.t. 'clock' noon).

If you are using 'Metcheck' then I note that their algorithms don't
seem to predict *any* UV radiation (despite forecast well broken cloud
cover), much before 10 o'clock local time, which at this time of the
year is unrealistic - particularly in Scotland: it may be lower than
midday, but it isn't going to be zero! Also, worryingly, the maxima
for a selection of sites I've just put in seem to coincide with the
day maximum temperature - to which it isn't connected as noted by
Nick.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

On Jun 8, 8:26 pm, "Martin Rowley"
wrote:
"Nick Gardner" wrote...

As already been said, the UV intensity will generally be
proportional to how high the sun is in the sky, i.e., less
atmosphere (and ozone) for the light to travel through before
reaching the earth's surface.

Therefore, the greatest UV levels will generally occur, on a clear
day when the sun is at its highest elevation in the sky. There will
be no lag as you would expect with temperature.

Here is a link to a graph showing the UV index for today (8th) and
yesterday for Camborne in Cornwall, note the peak at around 1pm -
2pm.

http://www.btinternet.com/~nickgardn...lery/weatherim...
________________

... this graph shows a representation of the total global radiation
and UV radiation at an un-named spot:-

http://www.cpc.ncep.noaa.gov/product...ndex/gif_files...

and is confirmed by the HPA site ...

http://www.hpa.org.uk/webw/HPAweb&HP..._C/11976370217...

These confirm the notes of others: the UV radiation should be
(ignoring effect of varying cloud cover), at a maximum around solar
noon (which does vary w.r.t. 'clock' noon).

If you are using 'Metcheck' then I note that their algorithms don't
seem to predict *any* UV radiation (despite forecast well broken cloud
cover), much before 10 o'clock local time, which at this time of the
year is unrealistic - particularly in Scotland: it may be lower than
midday, but it isn't going to be zero! Also, worryingly, the maxima
for a selection of sites I've just put in seem to coincide with the
day maximum temperature - to which it isn't connected as noted by
Nick.

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

Mike wrote :

"One of the sites I regularly look at gives an 3 hour forecast window
including a UV index and this always peaks between 16.00 and 19.00
when, for
example, today and tomorrow (Sunday and Monday) it shows the UV level
as 8
during these hours but only 6 between 13.00 and 15.59."

It would help if you gave us a link to that site. Is it Irish, or
American? It the time in GMT, BST or local time?

Sunshine is as strong in May as it is in July but most peope seem to
get sun burnt later in the year. I have wondered why, but it is
probably because the air is warmer later in the year (and day) so more
people sun bathe then.

Cheers, Alastair.

"Alastair" wrote

It would help if you gave us a link to that site. Is it Irish, or
American? It the time in GMT, BST or local time?


Thanks for all the really helpful replies. Alastair, it's Metcheck which
Martin has indicated seems to be inaccurate. Their forecast for tomorrow has
the same peak as today which is between 16.00 and 18.59 which makes no sense
based on the replies you've kindly given. I do enjoy sitting in the sun but
like to avoid sunburn or over exposure which I've always assumed meant
around the lunchtime (i.e. in London from around midday till 14.00 give or
take a bit depending on the proximity to the longest day, cloud and so on)
but Metcheck had completely thrown me. I use several sources for the
forecasts, several from sites mentioned on this group, though I've no idea
where I got Metcheck from!

http://www.metcheck.com/V40/UK/FREE/...zipcode=london

I now realise it can't possibly be right as it also shows an index of 6 in
the period till 21.59. What sites do you guys use that can be interpreted
by laymen like me?

Thanks,

Mike

"Alastair" wrote in message
...
On Jun 8, 8:26 pm, "Martin Rowley"
wrote:
"Nick Gardner" wrote...

As already been said, the UV intensity will generally be
proportional to how high the sun is in the sky, i.e., less
atmosphere (and ozone) for the light to travel through before
reaching the earth's surface.

Therefore, the greatest UV levels will generally occur, on a clear
day when the sun is at its highest elevation in the sky. There will
be no lag as you would expect with temperature.

Here is a link to a graph showing the UV index for today (8th) and
yesterday for Camborne in Cornwall, note the peak at around 1pm -
2pm.

http://www.btinternet.com/~nickgardn...lery/weatherim...
________________

... this graph shows a representation of the total global radiation
and UV radiation at an un-named spot:-

http://www.cpc.ncep.noaa.gov/product...ndex/gif_files...

and is confirmed by the HPA site ...

http://www.hpa.org.uk/webw/HPAweb&HP..._C/11976370217...

These confirm the notes of others: the UV radiation should be
(ignoring effect of varying cloud cover), at a maximum around solar
noon (which does vary w.r.t. 'clock' noon).

If you are using 'Metcheck' then I note that their algorithms don't
seem to predict *any* UV radiation (despite forecast well broken cloud
cover), much before 10 o'clock local time, which at this time of the
year is unrealistic - particularly in Scotland: it may be lower than
midday, but it isn't going to be zero! Also, worryingly, the maxima
for a selection of sites I've just put in seem to coincide with the
day maximum temperature - to which it isn't connected as noted by
Nick.

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

Mike wrote :

"One of the sites I regularly look at gives an 3 hour forecast window
including a UV index and this always peaks between 16.00 and 19.00
when, for
example, today and tomorrow (Sunday and Monday) it shows the UV level
as 8
during these hours but only 6 between 13.00 and 15.59."

It would help if you gave us a link to that site. Is it Irish, or
American? It the time in GMT, BST or local time?

Sunshine is as strong in May as it is in July but most peope seem to
get sun burnt later in the year. I have wondered why, but it is
probably because the air is warmer later in the year (and day) so more
people sun bathe then.

Cheers, Alastair.

In the northern hemispher, ozone levels are lower in may than in July and
those of northern european descent are relatively intolerant of UV after
winter and cases of severe sunburn are more common in early summer than late
summer. Having said that you are correct that mild/moderate sunburn peaks
later when it's warmer and folks are on their hols.

Dave Liquorice wrote:

On Sun, 8 Jun 2008 19:30:06 +0100, Mike Tullett wrote:

Mike - you are quite right to question the times of 16.00 to 19.00.
Solar radiation, of any kind, will peak an hour or so either side of
local noon.

Ish. Cloud cover will make a difference Thick heavy cloud and midday v
clear skies late afternoon... But cloud can probably increase the UV level
as well, high thin sirrusy stuff producing more back scatter?

Cumulus clouds are more likely to produce an increase due to reflection. I
vaguely recall that they can increase the level by as much as 100%.


This would not account for "a UV index and this always peaks between 16.00
and 19.00" though.



--
Graham P Davis, Bracknell, Berks., UK. E-mail: newsman, not newsboy.
"What use is happiness? It can't buy you money." [Chic Murray, 1919-85]