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 :-)

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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