On 2010-03-26, Martin Rowley wrote:
snip
The selection of the layers to be reported are as follows:-
1st group: the lowest layer of any amount.
2nd group: the next layer, which covers *more* than 2 oktas (i.e.
SCT
or greater).
3rd group: the next higher layer covering more than 4 oktas (i.e.
BKN
or greater).
Additional group to report CB or TCU if not already reported as
above.
(Reg 15.9.1.4)

Order is from lowest to highest.

BUT .. and this is important for decoding/interpretation purposes
...
from Reg 15.9.1.1, " If there are no clouds below 1500m (5000ft) or
below the highest minimum sector altitude, whichever is greater, no
Cumulonimbus and no restriction on vertical visibility and the
abbreviations CAVOK and SKC are not appropriate, then the
abbreviation
NSC shall be used."

Now, there have been changes to this: for example, 'SKC', sky
clear,
is *not* to be used [though I still keep seeing it] and I believe
(but
can't find the reference) that *any* CB (and possibly TCU)
irrespective of cloud base should be reported ......

.... I've found it! Indeed, the current regulation states that *any* CB
or TCU should be reported, irrespective of cloud base height. I'm
pretty sure also that if a thunderstorm is reported (e.g. TS), then a
'CB' group *has* to be entered whether the cloud can be 'detected' or
not, either visually or otherwise; there can be occasions for example
where thunder is heard from 8 oktas Stratus!

Martin.




--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023