The definition of "No Significant Cloud"
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The definition of "No Significant Cloud"
Perhaps a silly question (for which I do apologise), but I would appreciate some clarity here.
Quite often in METARs, mostly on hazy days, I see "No Significant Cloud" displayed on the METAR; occasionally even when vis. is >10k I see NSC displayed on the METAR.
What is the definition of "No Significant Cloud"? Is it a case of there is cloud in the vicinity but not enough to say e.g. "Few at 2500 feet"?
There again, whilst not actually cloud, I would've thought that Haze would qualify as "significant" especially to us VFR folks. So why on hazy days is "HZ" rarely specified on the METAR, instead say vis. at 7K and "NSC"?
What gives?
Smithy
Quite often in METARs, mostly on hazy days, I see "No Significant Cloud" displayed on the METAR; occasionally even when vis. is >10k I see NSC displayed on the METAR.
What is the definition of "No Significant Cloud"? Is it a case of there is cloud in the vicinity but not enough to say e.g. "Few at 2500 feet"?
There again, whilst not actually cloud, I would've thought that Haze would qualify as "significant" especially to us VFR folks. So why on hazy days is "HZ" rarely specified on the METAR, instead say vis. at 7K and "NSC"?
What gives?
Smithy
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Take a look at AIP Gen 3.5.10 pars 7.4
7.4 When there is no cloud below 5000 ft or below the highest minimum sector altitude (whichever is the greater) and there is no
towering cumulus or cumulonimbus, 'NSC' (no significant cloud) is reported. However, the amount, height of cloud base and cloud type
of towering cumulus or cumulonimbus shall be reported, irrespective of the cloud base height.
AIP Gen 3.5 is the best reference for Aviation Met meanings and definations.
Cheers***
7.4 When there is no cloud below 5000 ft or below the highest minimum sector altitude (whichever is the greater) and there is no
towering cumulus or cumulonimbus, 'NSC' (no significant cloud) is reported. However, the amount, height of cloud base and cloud type
of towering cumulus or cumulonimbus shall be reported, irrespective of the cloud base height.
AIP Gen 3.5 is the best reference for Aviation Met meanings and definations.
Cheers***
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As you know, CAVOK is reported when visibility is greater than 10km (and cloud above 5,000 feet, not CB TCU etc). When vis falls below 10k, it is reported as such. When it falls below 5km the type of obscuration is additionally reported, eg BR, HZ, FU, IC, DU or SA (mist, haze, smoke, ice crystals, dust or sand).
The type of poor vis is important in a pilots assessment of how those conditions actually relate to his plan to continue the flight and how it relates to other conditions (eg early winter evening temp/dewpoint similarity and 3000m BR is a warning sign, Lagos in December during the Harmattan and a SA warning, Delhi in January at dinner time with FU in the METAR all help the experienced pilot build up a better image of what's happening, and likely to happen as they get near to the ground).
3km vis for people who are used to 100 mile vis can be quite alarming but still very workable as long as situational awareness and lookout is not compromised.
The type of poor vis is important in a pilots assessment of how those conditions actually relate to his plan to continue the flight and how it relates to other conditions (eg early winter evening temp/dewpoint similarity and 3000m BR is a warning sign, Lagos in December during the Harmattan and a SA warning, Delhi in January at dinner time with FU in the METAR all help the experienced pilot build up a better image of what's happening, and likely to happen as they get near to the ground).
3km vis for people who are used to 100 mile vis can be quite alarming but still very workable as long as situational awareness and lookout is not compromised.
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Thanks for your help, everyone.
I once encountered poor-ish visibility during a lesson, about a couple of months back, around the Edinburgh area. Can't remember the exact weather but the EDI METAR was CAVOK (think there was a high overcast at what looked like 10000 feet. Vis. at ground level looked fine but climbing up through 1000' the vis. deteriorated rapidly to something more like 7-8k. Was quite alarming at first. Luckily that day's lesson was low-level Nav techniques! Vis. was much better out towards East Lothian but around Edinburgh the vis. was mince. Interesting experience though. As you say FL it was alarming at first but as long as you keep situational awareness it is okay. Not sure how I'd cope with 3k however! I can see how people can get lost very easily because all it takes is a moments distraction to lose your situational awareness and you are goosed!
Smithy
I once encountered poor-ish visibility during a lesson, about a couple of months back, around the Edinburgh area. Can't remember the exact weather but the EDI METAR was CAVOK (think there was a high overcast at what looked like 10000 feet. Vis. at ground level looked fine but climbing up through 1000' the vis. deteriorated rapidly to something more like 7-8k. Was quite alarming at first. Luckily that day's lesson was low-level Nav techniques! Vis. was much better out towards East Lothian but around Edinburgh the vis. was mince. Interesting experience though. As you say FL it was alarming at first but as long as you keep situational awareness it is okay. Not sure how I'd cope with 3k however! I can see how people can get lost very easily because all it takes is a moments distraction to lose your situational awareness and you are goosed!
Smithy
