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Considering that

Minimum sector altitude of the airport = 10500 feet

Airport elevation from MSL = 2500 feet

What is the level of clouds of operational significance in METAR for this airport? Should we subtract the airport’s elevation from MSA?

For more explanation

If we have a broken altostratus clouds at 9000 feet height relative to the ground, which coding is correct?

1- …..7000 -RA BKN090…

2- …..7000 -RA NSC…

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1 Answer 1

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Codes used:

Abbreviations used:


In your case:

The cloud group is usually composed of two parts, NNNhhh:

  • NNN: Cloud cover = BRK
  • hhh: Height of the base / 100, AAL = 090

So the default cloud group would be BRK090 however, from WMO in Aerodrome Reports and Forecasts - A Users’ Handbook to the Codes:

When there are no clouds below 1 500 m (5 000 ft) or below the highest minimum sector altitude, whichever is greater, no restriction to vertical visibility and the abbreviation CAVOK is not appropriate, the abbreviation NSC (Nil Significant Cloud) is used.

In your case:

  • Airport: 2,500 ft AMSL.
  • Cloud base at 9,000 AAL + 2,500 = 11,500 AMSL.
  • Highest MSA: 10,500 ft AMSL.

Cloud base is 1,000 ft above the MSA, use NSC in place of BRK090.

In addition if:

  • the horizontal visibility is 10 km or greater and
  • there are no CB/TCU at any altitude,

CAVOK is usable in place of visibility, present weather and cloud groups.

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