[ I've started a new thread as this relates to coding for the
international exchange of meteorological information for met.
services, rather than aviation. However, it would imply that there was
a change around 1949 regarding the way cloud amounts were observed, at
least within the UK.]

Following the discussion elsewhere re 'oktas' etc., here some
information which I *believe* to be correct, though it would need a
visit to the Archives in Exeter to really dig out the lowdown:
however, I have over the years collected some 'ancient' books that
straddle the period: 1930s to early 1950s, and looking at the
instructions in these regarding cloud amounts, I think that the
following holds good IN THE UK. (Obviously other services may / will
differ, though for international coding of the SYNOP, I believe that
apart from PR China, all countries adopted the 'new' coding, as it
was, at the same time.)

Up until 31st December, 1948, the international code (2459) was based
on the pre-war International Meteorological Organisation version which
ran something like this:-

IIIClCm wwVhNh DDFWN

Nh & N were cloud amount figures (note single figure) for low cloud
and total cloud cover respectively.

The code 60 then gives the following for Nh/N

0 = no cloud
1 = trace of cloud
2 = 1/10 of cloud
3 = 2/10 and 3/10
4 = 4/10 to 6/10
5 = 7/10 to 8/10
6 = 9/10
7 = 9/10 (overcast but with small gaps)
8 = 10/10
9 = obscured by fog, smoke, snow etc.

Instructions associated with observing note that amounts of cloud are
estimated in _tenths_ of cover, and reported to ATC etc., in those
terms, but are coded for transmission as above.

On the 1st January, 1949, the 'Washington' code (as we used to call
it), introduced the SYNOP as it used to be (the one that started iii
Nddff VVwwW .... ). This code then explicitly used oktas (eighths) of
cover, which we all now use.

Now, whether all this has any bearing on the use in _aviation_ coded
reports I don't know, but I do know that by the 1960s, the METAR cloud
groups were of the form NCChshs, where N was the amount of cloud in
oktas, CC the cloud type (i.e. CU = cumulus) and hshs was a straight
copy from the SYNOP (or AERO/SYRED) coding for cloud height, so 1SC20
5AC58 was 1 okta Sc at 2000ft, 5 oktas Ac at 8000ft.

Martin.


--
Martin Rowley
E:
W: booty.org.uk

An appropriate time I think to raise yet again (mainly for the benefit
of the newbies) the perennial irritation of CAVOK reports. Clear and
Visibility OK implies (amongst other things) that there is no
significant cloud below 5,000 feet but it might well be completely
overcast at say 10,000 feet. Pilots cannot know this so are unable to
tell their passengers the basic information as to whether or not their
holiday destination is sunny - a pretty vital piece of information I
would have thought.

Anyway, that no longer really bothers me being a retired professional
pilot. But I am still involved in forecasting for gliding. An
approaching frontal overcast can mean the end of what had been a good
soaring day. But until the first bits of cloud occur below 5,000
feet, the metars keep everyone in ignorance about what is going on and
might continue to report CAVOK.

With so much automation nowadays, it doesn't seem to me to be beyond
the abilities of the aviation authorities to include cloud amounts
above 5,000, eg a metar might read CAVOK, BKN240 OVC300

Captain Jack

On Oct 23, 2:34 pm, "Jack )"
wrote:
An appropriate time I think to raise yet again (mainly for the benefit
of the newbies) the perennial irritation of CAVOK reports. Clear and
Visibility OK implies (amongst other things) that there is no
significant cloud below 5,000 feet but it might well be completely
overcast at say 10,000 feet. Pilots cannot know this so are unable to
tell their passengers the basic information as to whether or not their
holiday destination is sunny - a pretty vital piece of information I
would have thought.

Anyway, that no longer really bothers me being a retired professional
pilot. But I am still involved in forecasting for gliding. An
approaching frontal overcast can mean the end of what had been a good
soaring day. But until the first bits of cloud occur below 5,000
feet, the metars keep everyone in ignorance about what is going on and
might continue to report CAVOK.

With so much automation nowadays, it doesn't seem to me to be beyond
the abilities of the aviation authorities to include cloud amounts
above 5,000, eg a metar might read CAVOK, BKN240 OVC300

Captain Jack

Easier still would be to do away with CAVOK altogether. The
only reason I can think of for its existence is as a kind of
shorthand. But some METARs are quite long and the pilot needs to
absorb all that information so CAVOK cannot be justified on the
grounds of the pilot needing the information in a hurry. I have even
heard (London VOLMET main) a METAR reporting light rain + CAVOK,
showing how misleading they can be to the oridnary enthusiast, though
maybe quite sufficient for aviation purposes.

Tudor Hughes, Warlingham, Surrey.

In article ,
Martin Rowley writes:
Up until 31st December, 1948, the international code (2459) was based
on the pre-war International Meteorological Organisation version which
ran something like this:-

IIIClCm wwVhNh DDFWN

Nh & N were cloud amount figures (note single figure) for low cloud
and total cloud cover respectively.

The code 60 then gives the following for Nh/N

0 = no cloud
1 = trace of cloud
2 = 1/10 of cloud
3 = 2/10 and 3/10
4 = 4/10 to 6/10
5 = 7/10 to 8/10
6 = 9/10
7 = 9/10 (overcast but with small gaps)
8 = 10/10
9 = obscured by fog, smoke, snow etc.
snip

One can see why they changed to oktas. It avoided trying to fit a quart
into a pint pot.
--
John Hall
"Honest criticism is hard to take,
particularly from a relative, a friend,
an acquaintance, or a stranger." Franklin P Jones

On 23 Oct, 17:57, John Hall wrote:
In article ,
Martin Rowley writes:

Up until 31st December, 1948, the international code (2459) was based
on the pre-war International Meteorological Organisation version which
ran something like this:-

IIIClCm wwVhNh DDFWN

Nh & N were cloud amount figures (note single figure) for low cloud
and total cloud cover respectively.

The code 60 then gives the following for Nh/N

0 = no cloud
1 = trace of cloud
2 = 1/10 of cloud
3 = 2/10 and 3/10
4 = 4/10 to 6/10
5 = 7/10 to 8/10
6 = 9/10
7 = 9/10 (overcast but with small gaps)
8 = 10/10
9 = obscured by fog, smoke, snow etc.

snip

One can see why they changed to oktas. It avoided trying to fit a quart
into a pint pot.
--
John Hall
"Honest criticism is hard to take,
particularly from a relative, a friend,
an acquaintance, or a stranger." Franklin P Jones

Apart from the cloud and visibility constraints, CAVOK also requires
there be no precipitation, thunderstorms, shallow fog, or low drifting
snow: so the report on London Volmet must have been a coding error -
probably due to non professional observers now making the reports.

John