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]