Just watched that giggly woman on BBC news channel having showed the
radar picture for the UK which shows a large area of cloud covering much
of Eastern England followed by the usual warning to slap on high factor
sunscreen because of the strong sunshine
Having seen the heavy Cu build up to virtually 8/8 cover here in the
south-east since late morning ,what could possibly cause this overcast
to dissipate and therefore come into line with what is obviously an
incorrect forecast ?

RonB

Hello Ron,

I'm not commenting on forecast accuracy, merely observations and current satellite imagery.

Fair point on the basis of your local conditions now (though UV levels will be quite high anyway), but there's hardly a cloud in the sky south of a line joining Winchester to the Thames Estuary. If this clear marine air pushes a little further north with the developing sea breeze, then it will become very sunny around London. Or a slight change in wind direction. A delicate balance. There will also be sunny spells further inland.

Cheers Julian

On 09/07/17 13:18, Ron Button wrote:
Just watched that giggly woman on BBC news channel having showed the
radar picture for the UK which shows a large area of cloud covering much
of Eastern England followed by the usual warning to slap on high factor
sunscreen because of the strong sunshine
Having seen the heavy Cu build up to virtually 8/8 cover here in the
south-east since late morning ,what could possibly cause this overcast
to dissipate and therefore come into line with what is obviously an
incorrect forecast ?



I was told years ago that UV levels when Cu is around can be higher than
when there is no cloud. I think what happens there is that you get a lot
of reflected radiation.

Another point is that I assume the cloud was CuSc (I haven't really been
noticed as I've been watching the cricket) as you can't really have more
than 4/8 cover of Cu. It may look like more but, when assessing the
amount of Cu, you have to ignore the sides of the cloud and merely add
up the area of the bases.

--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
Web-site: http://www.scarlet-jade.com/
“Like sewage, smartphones, and Donald Trump, some things are just
inevitable.� [The Doctor]

Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
Just watched that giggly woman on BBC news channel having showed the radar
picture for the UK which shows a large area of cloud covering much of
Eastern England followed by the usual warning to slap on high factor
sunscreen because of the strong sunshine Having seen the heavy Cu build up
to virtually 8/8 cover here in the south-east since late morning ,what
could possibly cause this overcast to dissipate and therefore come into
line with what is obviously an incorrect forecast ?



I was told years ago that UV levels when Cu is around can be higher than when
there is no cloud. I think what happens there is that you get a lot of
reflected radiation.

Another point is that I assume the cloud was CuSc (I haven't really been
noticed as I've been watching the cricket) as you can't really have more than
4/8 cover of Cu. It may look like more but, when assessing the amount of Cu,
you have to ignore the sides of the cloud and merely add up the area of the
bases.

If you've got a ruddy great Cu sitting right on top of you with a base at 800ft
and it's peeing down with rain then I think it's reasonable to call it 8/8 Cu
:-)

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

On Sunday, 9 July 2017 13:32:02 UTC+1, wrote:
Hello Ron,

I'm not commenting on forecast accuracy, merely observations and current satellite imagery.

Fair point on the basis of your local conditions now (though UV levels will be quite high anyway), but there's hardly a cloud in the sky south of a line joining Winchester to the Thames Estuary. If this clear marine air pushes a little further north with the developing sea breeze, then it will become very sunny around London. Or a slight change in wind direction. A delicate balance. There will also be sunny spells further inland.

Cheers Julian

It was mostly sunny here (12 mi S of London) No large Cu. Max 27.3°C

Tudor Hughes, Warlingham, NE Surrey.

On 09/07/17 19:44, Norman Lynagh wrote:
Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
Just watched that giggly woman on BBC news channel having showed the radar
picture for the UK which shows a large area of cloud covering much of
Eastern England followed by the usual warning to slap on high factor
sunscreen because of the strong sunshine Having seen the heavy Cu build up
to virtually 8/8 cover here in the south-east since late morning ,what
could possibly cause this overcast to dissipate and therefore come into
line with what is obviously an incorrect forecast ?



I was told years ago that UV levels when Cu is around can be higher than when
there is no cloud. I think what happens there is that you get a lot of
reflected radiation.

Another point is that I assume the cloud was CuSc (I haven't really been
noticed as I've been watching the cricket) as you can't really have more than
4/8 cover of Cu. It may look like more but, when assessing the amount of Cu,
you have to ignore the sides of the cloud and merely add up the area of the
bases.

If you've got a ruddy great Cu sitting right on top of you with a base at 800ft
and it's peeing down with rain then I think it's reasonable to call it 8/8 Cu
:-)


That may be true but I can't recall ever observing a complete cover of a
single Cu cell, even in a shower. Mind you, it's been a long time since
I've felt like standing outside, looking up at the clouds when it's
peeing down, when I could be taking cover. ;-)

Most if not all the reports I've seen of 7-8/8 Cu, where I've been near
enough to judge for myself, have been when no rain is present and what
they have reported as Cu has been either CuSc or Sc type 4.

When I started as an observer, I was corrected on an occasion when I'd
wrongly reported something like 7/8 Cu. The old scientific law of "what
goes up must come down" was used to explain how at least half the sky
would be be cloud-free due to the descending air and that when
estimating the cloud amount the sides of the cloud must be ignored. When
you do that, the estimate of the amount of cumulus drops to at least
half of the initial estimate.

--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
Web-site: http://www.scarlet-jade.com/
“Like sewage, smartphones, and Donald Trump, some things are just
inevitable.� [The Doctor]

On Monday, 10 July 2017 15:34:24 UTC+1, Graham P Davis wrote:
On 09/07/17 19:44, Norman Lynagh wrote:
Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
The old scientific law of "what
goes up must come down" was used to explain how at least half the sky
would be be cloud-free due to the descending air
The descending air doesn't necessarily occur in the vicinity of the cloud formed by the ascent of the air, though. I used this argument when I was being trained, but was told to carry on using the rule of thumb (I didn't!). I can see the logic behind what you are saying, but I would argue that it only applies to cumulus of limited vertical and horizontal extent - which is probably more often than not. I can't see any problem with reporting 8/8 Cu, but I would expect to only do it when the air was unstable to some depth, and I would expect to be reporting precipitation at the same time.

--
Freddie
Fishpool Farm
Hyssington
Powys
296m AMSL
http://www.fishpoolfarmweather.co.uk/
https://twitter.com/FishpoolFarmWx for hourly reports

On Mon, 10 Jul 2017 08:35:41 -0700 (PDT)
Freddie wrote:

On Monday, 10 July 2017 15:34:24 UTC+1, Graham P Davis wrote:
On 09/07/17 19:44, Norman Lynagh wrote:
Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
The old scientific law of "what
goes up must come down" was used to explain how at least half the sky
would be be cloud-free due to the descending air
The descending air doesn't necessarily occur in the vicinity of the cloud
formed by the ascent of the air, though. I used this argument when I was
being trained, but was told to carry on using the rule of thumb (I didn't!).
I can see the logic behind what you are saying, but I would argue that it
only applies to cumulus of limited vertical and horizontal extent - which is
probably more often than not. I can't see any problem with reporting 8/8 Cu,
but I would expect to only do it when the air was unstable to some depth, and
I would expect to be reporting precipitation at the same time.


As a fully trained observer I can say that if the sky is 8/8 full of cloud and
it is raining you cannot report Cu as you cannot see what cloud it is. In
observing you observe what you see not what you expect or what you know from
the situation. 8/8 Cu and raining would normally be reported as 8/8 St or 8/8
Ns. Actually 8/8 Cu would be a socking big Cu :-)

---
This email has been checked for viruses by AVG.
http://www.avg.com

On 10/07/2017 17:14, Will Hand wrote:
On Mon, 10 Jul 2017 08:35:41 -0700 (PDT)
Freddie wrote:

On Monday, 10 July 2017 15:34:24 UTC+1, Graham P Davis wrote:
On 09/07/17 19:44, Norman Lynagh wrote:
Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
The old scientific law of "what
goes up must come down" was used to explain how at least half the sky
would be be cloud-free due to the descending air
The descending air doesn't necessarily occur in the vicinity of the cloud
formed by the ascent of the air, though. I used this argument when I was
being trained, but was told to carry on using the rule of thumb (I didn't!).
I can see the logic behind what you are saying, but I would argue that it
only applies to cumulus of limited vertical and horizontal extent - which is
probably more often than not. I can't see any problem with reporting 8/8 Cu,
but I would expect to only do it when the air was unstable to some depth, and
I would expect to be reporting precipitation at the same time.


As a fully trained observer I can say that if the sky is 8/8 full of cloud and
it is raining you cannot report Cu as you cannot see what cloud it is. In
observing you observe what you see not what you expect or what you know from
the situation. 8/8 Cu and raining would normally be reported as 8/8 St or 8/8
Ns. Actually 8/8 Cu would be a socking big Cu :-)

---
This email has been checked for viruses by AVG.
http://www.avg.com

I agree with you Will, at best it would be 8/8 Cb rather than Cu. This
assumes that previous obs were reporting convective cloud.

Will Hand wrote:

On Mon, 10 Jul 2017 08:35:41 -0700 (PDT)
Freddie wrote:

On Monday, 10 July 2017 15:34:24 UTC+1, Graham P Davis wrote:
On 09/07/17 19:44, Norman Lynagh wrote:
Graham P Davis wrote:

On 09/07/17 13:18, Ron Button wrote:
The old scientific law of "what
goes up must come down" was used to explain how at least half the sky
would be be cloud-free due to the descending air
The descending air doesn't necessarily occur in the vicinity of the cloud
formed by the ascent of the air, though. I used this argument when I was
being trained, but was told to carry on using the rule of thumb (I didn't!).
I can see the logic behind what you are saying, but I would argue that it
only applies to cumulus of limited vertical and horizontal extent - which is
probably more often than not. I can't see any problem with reporting 8/8
Cu, but I would expect to only do it when the air was unstable to some
depth, and I would expect to be reporting precipitation at the same time.


As a fully trained observer I can say that if the sky is 8/8 full of cloud and
it is raining you cannot report Cu as you cannot see what cloud it is. In
observing you observe what you see not what you expect or what you know from
the situation. 8/8 Cu and raining would normally be reported as 8/8 St or 8/8
Ns. Actually 8/8 Cu would be a socking big Cu :-)

---
This email has been checked for viruses by AVG.
http://www.avg.com

I'll have to disagree with you on that, Will. What about night-time obs when it
is often impossible to see the cloud at all. Manual SYNOPs will almost always
include cloud type, which is nothing more than the observer's best estimate. If
it is showering then Cu (or Cb) is likely to be reported, even though the cloud
may well be invisible.

I often record 6/8-8/8 Cu here in showery situations. We get a lot of
broadscale orographic uplift, especially in W-NW airstreams, resulting in near
overcast cumuliform cloud. It isn't St or Ns. Fair weather Cu is a very
different scenario and probably one in which 6/8-8/8 cover wouldn't happen.

The real world often doesn't fit very well with the text books :-)

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