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Graphical Area Forecast Question

  • michaelqian
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michaelqian created the topic: Graphical Area Forecast Question

Hi Bob/Team,

Another question from me regarding the Graphical Area Forecast from one of the practice exams. Kindly refer to attached photo.

The questions asks "The lowest cloud forecast for Roma (YROM) is?"

As YROM is in area C, I selected "1-4 eights Stratus, base 1,200 feet". The only thing that bothered slightly me was SCT is 3-4 OTKAS, rather than 1-4 OKTAS.

However the correct answer is "3-4 eights Stratus, base 1,500 feet".

Two things I don't understand here:

1 - This means that the option I selected only applies to area C1, but I thought the "(BNK C1)" means that in area C it is SCT Status cloud, but in C1 it is Broken (i.e. 5-7 OKTAS, hence the brackets). In other words, the lowest cloud in C is still 1,200 feet.

2 - If the correct answer is indeed "3-4 eights Stratus, base 1,500 feet", then should it be 5-7 eights instead as BKN means 5-7 OKTAS? Refer to 4th line down in the area C forecast "BKN ST 1,500-3,000FT".

Would very much appreciate it if you could clarify this for me.

Many thanks

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Stuart Tait replied the topic: Graphical Area Forecast Question

Gidday Michael
Your answer was correct, and your understanding of (BKN C1) is also correct,
there was an error in the question and the wrong answer selected. I have amended the question in the exam.

Cheers
Stuart
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Carello replied the topic: Graphical Area Forecast Question

Hi Michael
Your interpretation of the GAF is correct. The lowest layer of cloud at Roma would be SCT 1200/3000 on Area QNH (ie above MSL)

That being said, Roma's elevation is 1032' - see ERSA snapshot below. This means that the lowest cloud layer above aerodrome elevation would be 1200' - 1032' = 168' (ie 168' on local QNH if it were available)



The correct answer, 1200' or 168', will depend on the context of the question - are you being asked for the lowest cloud layer above MSL or above the aerodrome. Either way, be aware that for planning purposes (ie alternate requirements) the cloud height above aerodrome elevation will be more important than the cloud height on QNH - assuming that the TAF was not available for what ever reason.

You were also correct about the cloud amounts - see below



Cheers
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Carello replied the topic: Graphical Area Forecast Question

Whoops - spotted a typo

This means that the lowest cloud layer above aerodrome elevation would be 1200' - 1032' = 168' (ie 168' on local QNH if it were available)

Replace (local QNH) with QFE
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michaelqian replied the topic: Graphical Area Forecast Question

Thanks Stuart and Carello.

Throughout the confusion I did learn something new. That is the cloud base is just the lowest base of any cloud you see in the sky, while the "cloud ceiling" is the lowest base of clouds forming a layer of at least 5 OKTAS.
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Carello replied the topic: Graphical Area Forecast Question

Yes, once again you are correct.
That being said, the official definition of "Ceiling" taken from the CARs is as follows:

Ceiling means the height above the ground or water of the base of the lowest layer of cloud below twenty thousand feet covering more than oneā€‘half of the sky.

When interpreting the above definition be careful to note the following detail
1) The ceiling is a height above the ground or water - not an altitude on QNH
2) The cloud amounts are cumulative - ie the ceiling can occur at the base of a SCT layer of cloud

example
If the TAF says BKN020
The ceiling is 2000' because there will be more than 4/8 of cloud below any level above 1500'

If the TAF says: FEW010, SCT015
The ceiling is 1500' because there will be more than 4/8 of cloud below any level above 1500'

If the TAF says: FEW005 and nothing else,
There is no ceiling - this one can be ambiguous as BOM forecasters often omit high level cloud from the TAF
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michaelqian replied the topic: Graphical Area Forecast Question

Thanks again.

So hypothetically if a question asked if an alternate aerodrome was required, and the TAF says FEW008 (vis >8km), then the answer would be no, as there is no "ceiling"?
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Carello replied the topic: Graphical Area Forecast Question

Correct !

For a VFR flight, an alternate would only be required if the ceiling was below 1500' - if there is no ceiling then there is no requirement for an alternate based on cloud. That being said, if vis was < 8km, then an alternate would be required due to low vis.
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