METAR Decode

[DiscoChocolate]

The latest METAR for Liverpool is as follows:
EGGP 101850Z 16013KT 9999 NSC 18/12 Q1007

What does the "NSC" stand for? I've not seen this reported before. A guesstimate tells me its "No Significant Cloud", could some met guru out there shed any light on this?

DiscoC

[India Four Two]

Good guess.

See FAA Aeronautical Contractions and click on the letter "N".

[DiscoChocolate]

Thanks! What a great website!!

[dublinpilot]

I've seen this before, and understand that NSC = No Significant Cloud.

However, I do wonder why, given the 9999 visibility, that this wouldn't be reported as CAVOK? Is it purely down to the met mans personal preference? Is is CAVOK only used when there is some cloud above 5,000ft?

[helicopter-redeye]

CAVOK has a specific definition in terms of a cloud base and presence of CBs, sandstorms, etc so it may have been 9999 but potentially not CAVOK.

CAVOK is visibility greater or equal to 10 km, no cumulonimbus and no cloud below 5000ft or highest MSA (whichever is the greater) and no sig weather.

NSC is No Significant Cloud.

Thus you could be in viz condition of 9999 but have cloud below 5000ft.

[dublinpilot]

Yes, but if the Vis is 9999, and there is No Significant Cloud, then it must be CAVOK.

Is CAVOK only used, when there IS some cloud present, but none belove 5000ft? ie. If there is no significant cloud at all, then don't use CAVOK, use NSC instead?

[helicopter-redeye]

I do wonder why, given the 9999 visibility, that this wouldn't be reported as CAVOK? Is it purely down to the met mans personal preference? Is is CAVOK only used when there is some cloud above 5,000ft?

Returning to the orig. q. CAVOK is more than viz. Consider the CB and other aspects as well.

Popular JAA question in the Met paper ...

h-r

[dublinpilot]

Returning to the orig. q. CAVOK is more than viz. Consider the CB and other aspects as well.

Understood. But if there is no significant cloud, then there can't be any CB action.

[effortless]

If you haven't found it already, can I recommend the Met Office site? You need to register to get aviation stuff but it is free and painless

[BEagle]

A bit strange this. The UK Met Office site implies that 'NSC' is only to be used in TAFs or in the TREND of a METAR, but not in the observation of a METAR. They state:

TAFs:

Cloud:

SKC = sky clear; FEW = 1-2 oktas; SCT = 3-4 oktas; BKN = 5-7 oktas; OVC = 8 oktas; '///' = state of sky obscured (figures after '///' will give forecast vertical visibility in hundred of feet) NSC = no significant cloud (none below 5,000 feet and no CB) CB will be the only cloud type specified

Cloud heights are given in feet above airfield height.

METARS:

Cloud:

SKC=Sky clear (0 oktas), FEW='few' (1-2 oktas), SCT='Scattered' (3-4 oktas), BKN='Broken' (5-7 oktas), OVC='Overcast'. There are only two cloud types reported; TCU=towering cumulus and CB=cumulonimbus. VV///='state of sky obscured' (cloud base not discernable): Figures in lieu of '///' give vertical visibility in hundreds of feet. Up to three, but occasionally more, cloud groups may be reported.

Cloud heights are given in feet above airfield height

Trend:

BECMG=Becoming TEMPO=Temporarily NOSIG=No sig change
NSW=No sig weather AT=At FM=From TL=Until

NSC=No sig cloud

Any of the wind forecast, visibility, weather or cloud groups may be used, and CAVOK. Multiple groups may be present.

So, to my mind, it should indeed have been CAVOK!

[Pierre Argh]

CAVOK... you may be right, the Observer might have been able to use the term (but equally there may have been dustdevils etc etc... anything that constitutes "significant weather iaw the Met Office definition)... end of the day, this has become a circular arguement... CAVOK wasn't used, NSC was... someone got confused... but now has the answer? What's the problem?

[PhilD]

From that METAR it looks like a nice day. I'd stop worrying and go flying...

[dublinpilot]

has become a circular arguement... CAVOK wasn't used, NSC was... someone got confused... but now has the answer? What's the problem?

No argument, and no problem. I was assuming that the report was correct, and done that way for a reason, and I was simply trying to improve my understanding of these things.

The day I stop learning........and all that

[helicopter-redeye]

I've retrieved my met notes what I lent to somebody.

CAVOK
The Viz, RVR, weather and cloud group can be replaced by CAVOK if:

1. Viz 10km or more;

2. No cloud below 5000ft aal or below highest MSA, whichever is the highest;

3. No CBs; and

4. No precip, TS, shallow fog or low drifting snow.

I have a passing recollection from a year ago about an additional caveat "..or forecast".

NSC
No cloud below 5000ft aal or highest MSA, whichever is higest and no CB.

Thus the CAVOK definition is more stringent in terms of viz (horiz) and the items in line 4 (and maybe the 'or forecast' point which I only remember but cannot now find 'in writing')

h-r

[dublinpilot]

Thank you red-eye.

So the conculsion that can be drawn from a metar showing 9999 vis, NSC and no indication of percipitation, rather than CAVOK, is that one of the following must (should) exist:

A) Shallow fog (but must be more than 10km from the airport) or reduced vis in one direction should be reported.
B) A forecast that might change one of the previous factors.

Thank you for helping to increase my understanding!

dp

[SilentHandover]

The rules of reporting weather via metars at aerodromes in the UK changed on Oct 6th this year.
In my opionion the reported metar should have been reported as CAVOK but I'm guessing the new rules have confused the met observers at GP as much as they have us at KK.

The new rules basically mean that cloud above 5000' is no longer reported in a metar. There are other changes too including reporting of RVR's greater than 1500m.

[dublinpilot]

Silent,

Do I understand you correctly, in that now cloud above 5,000 is not currently beingn reported at all?

dp