Re: Guessing Cloud Heights.

JPG wrote:

On 27 Jul, 08:45, "Jack (jack.harri...@xxxxxxxxx)"
<jack.harri...@xxxxxxxxx> wrote:
On 27 Jul, 02:12, Weatherlawyer <Weatherlaw...@xxxxxxxxxxx> wrote:

WTF use is it?

Glider pilots use it day in day out. Much more accurate way to get
cloudbases than visual estimates

It isn't quite as simple as you suggest. Dewpoints don't always drop
as the air mixes. Sometimes they might rise as eg fog evaporates.
It's a very complex subject.

I was quite shocked when I first came across the Cloud Base Recorder,
that wonderful device consisting of the spinning rotating shutter and
the magic phase-lock-loop decoder (valve -driven) which magically
managed to see a weak reflection of a searchlight beam on the cloud in
full sunlight. The values it would give for cloud base made my fully-
Stanmore-trained guesstimates look decidedly hopeless.


I was lucky with the first station I joined in 1962 - Wyton - as they had a
CBR for verifying against other measurements. I was a bit lost when I moved
to RAE Bedford the next February, not to mention shell-shocked a year later
when we had a batch of dodgy balloons which burst when they were *almost*
full.

I found the estimation of cloud heights very difficult - and still do - but
suspected within a few months of joining the office that those observers
who thought they were good at it were deluding themselves. The guidance we
got at Stanmore was more of a hindrance than a help as they stuck rigidly
to the guidelines for heights of low, medium and high clouds. If cumulus
looked low you'd be allowed to give it 1500' and, in exceptional
circumstances perhaps, 1200'. I later found it could be a lot lower, but
the CBR shocked me once when I found I had CuSc at 200' and 400'. When it
looked really high you'd be allowed to go all the way to 3000' when it was
probably twice that height.

Also when I was at Stanmore we had high Ac Cas which we put at 15,000'. Once
it had precipitated out, we had a sky full of Ci which had to be below the
Ac but which we had to bump up another 5000'. I suspect the Ac was at least
25000'.

On other occasions I've seen observer's with many years experience report Cs
and Cc at 20,000' and above which was only 4000' - a Sc *** which had
thinned near the edge to mimic much higher clouds. The reverse has happened
where cirrus clouds - or I should say clouds at cirrus heights - have been
mistaken for Sc. These have had the appearance of water-droplet cloud and I
suspect they probably are. Super-cooled water droplets have been recorded
at heights where the ambient air temperature was as low as -60C.

--
Graham P Davis
Bracknell, Berks., UK
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