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