[ 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
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W: booty.org.uk