AGL? or MSL?

N

n51222

Filing Flight Plan
Joined
May 7, 2006
Messages
2
Display Name
Display name:
Kevin
Is ceiling height in Area Forecasts (such as the broken layer at 3,500 in the following example) given in AGL or MSL? (Ceiling height only. Not tops, not other layers that are not ceilings, such as scattered layers. Only ceilings.)

Please DO NOT look up the answer first; this is just a short quiz to see if *most* pilots routinely read ceilings in Area Forecasts as AGL or MSL. If you look up the answer, it will invalidate the "first response" hypothesis of this quiz. (And although I know the correct answer now, I'll be the first to confess to being confused before I looked it up).

NY LO
NERN NY...BKN035 TOP 060. 15Z SCT060. OTLK...VFR.

 Kevin
 
Many of our field are only 100 - 800 MSL and often clear, so it almost doesn't matter.
I vote for MSL due to the wideness of the area.
 
Gotta be MSL, because it's an AREA forecast, and the MSL height of the terrain may vary significantly.

IIRC, if it's AGL it will be preceded by "CIG" which your example is not.
 
I'll give y'all a hint...

MT
W OF CONTDVD...SCT-BKN060 BKN080 TOPS 150. SCT -SHSN. BECMG 0204 BKN090-100. OTLK...VFR.
SWRN MTNS...SCT-BKN070-080 BKN110-120 TOPS 150. SCT -SHSN. BECMG 0104 SCT120 SCT160. OTLK...VFR.

Anybody know how high the mountains are in Montana ?
 
Ron Levy said:
I'll give y'all a hint...

MT
W OF CONTDVD...SCT-BKN060 BKN080 TOPS 150. SCT -SHSN. BECMG 0204 BKN090-100. OTLK...VFR.
SWRN MTNS...SCT-BKN070-080 BKN110-120 TOPS 150. SCT -SHSN. BECMG 0104 SCT120 SCT160. OTLK...VFR.

Anybody know how high the mountains are in Montana ?
Click to expand...

Not all that high. I just drove through MT and I don't think we got above 6,000. Granted, the peaks are higher but...

If the answer were always AGL, then the FA wouldn't have the notation NON MSL HGTS DENOTED BY AGL OR CIG...
 
I was gonna say MSL, too. So ya...what everyone else said. I'm curious, what's this poll for?
 
flyingcheesehead said:
Not all that high. I just drove through MT and I don't think we got above 6,000. Granted, the peaks are higher but...
Click to expand...
I'm sure if you keep looking, you'll find FA's for MT/CO/etc with cloud bases of 010 or less. The weather was just too good there today.
 
Let me be more direct...

The cloud bases in the CLDS/WX Area Forecast are always reported AGL, whether or not they constitute a ceiling. This enables proper use of the FA for confirming compliance with flight rules for minimum ceilings for VFR flight as well as IFR alternate requirements (1-2-3 and 600/800-2) when no TAF is published, and prevent confusion when there the bases are/are not reported as a ceiling ("CIG"). Cloud tops are always reported MSL. Compare the forecast below for Buckley ANGB (elevation about 5000 MSL) with the FA for Eastern Colorado:

CO
ERN PLNS/FOOTHILLS...
NRN PTNS...SKC. GRDLY BECMG SCT CI. OTLK...VFR.
CNTRL AND SRN PTNS...BKN070-090. TOPS 100-120. AFT 17Z...SCT
CI. OTLK...VFR.

KBKF 220024 VRB10G20KT 9999 VCSH BKN070 BKN110 OVC220 QNH2978INS
BECMG 0001 24012KT 9999 NSW FEW070 SCT110 SCT220 QNH2980INS LAST NO AMDS AFT 2201 NEXT 2212

If the FA were MSL, it would have the first layer at 120, not 070.
 
Last edited:
Captain Ron's response is perfect! I only wish the NWS agreed with him.

That's exactly why I posted this question after I had a discussion with the folks at the National Weather Service (headquarters in DC). They contend that cloud bases in an Area Forecast are ALWAYS reported MSL UNLESS preceded by AGL or CIG (Ceiling). There's a statement to that effect shown near the beginning of every FA issued.

And I argued that many pilots would interpret a ceiling in an Area Forecast as AGL, not necessarily because most other ceilings are reported as AGL, but also because of that standard statement, that it would only be AGL if preceding by AGL or CIG.

AGL or CIG almost never appears just before a forecast ceiling in an FA (wouldn't CIGBKN35 look funny?) but any layer of broken or overcast that is not thin or partial is, by definition, a ceiling. Right?

Captain Ron's argument ("The cloud bases in the CLDS/WX Area Forecast are always reported AGL") makes perfect sense, but the NWS disagrees with him, citing AC 00-45F, which gives such definitions.

Captain Ron, could you give me a source for your argument that I could use to persuade the NWS that there is, at least, some confusion?

Kevin
 
Ron Levy said:
Let me be more direct...

The cloud bases in the CLDS/WX Area Forecast are always reported AGL, whether or not they constitute a ceiling. This enables proper use of the FA for confirming compliance with flight rules for minimum ceilings for VFR flight as well as IFR alternate requirements (1-2-3 and 600/800-2) when no TAF is published, and prevent confusion when there the bases are/are not reported as a ceiling ("CIG"). Cloud tops are always reported MSL. Compare the forecast below for Buckley ANGB (elevation about 5000 MSL) with the FA for Eastern Colorado:

CO
ERN PLNS/FOOTHILLS...
NRN PTNS...SKC. GRDLY BECMG SCT CI. OTLK...VFR.
CNTRL AND SRN PTNS...BKN070-090. TOPS 100-120. AFT 17Z...SCT
CI. OTLK...VFR.

KBKF 220024 VRB10G20KT 9999 VCSH BKN070 BKN110 OVC220 QNH2978INS
BECMG 0001 24012KT 9999 NSW FEW070 SCT110 SCT220 QNH2980INS LAST NO AMDS AFT 2201 NEXT 2212

If the FA were MSL, it would have the first layer at 120, not 070.
Click to expand...

If the FA were AGL for the bases only, that first layer (bases 7-9K AGL, tops 10-12K AGL) wouldn't exist as 0 AGL is roughly 5000 MSL so the first layer would be at 12-14K with tops at 10-12K.

As for why the FA and TAF above show otherwise - I dunno, maybe BKF was at the border of the clouds. The latest TAF supports that theory:

KBKF 221212 19012KT 9999 FEW140 SCT220 QNH3000INS
BECMG 1718 14014G24KT 9999 FEW070 SCT140 BKN220 520004 QNH2992INS
BECMG 0102 16012KT 9999 FEW140 FEW220 QNH2984INS

The FA is still the same as previously posted.
 
n51222 said:
Captain Ron, could you give me a source for your argument that I could use to persuade the NWS that there is, at least, some confusion?
Click to expand...

Yes...

Ron, I know I've heard something about some product somewhere which used your exact reasoning WRT VFR/IFR and alternates required - Is there such a product which we are confusing with the FA's, maybe?

Or do I need to lay off the crack? :D
 
I would like to solve the puzzle, please.

Quoting directly from AC 00-45F, Aviation Weather Services, Page 7-3, a direct cut & paste from the section about Area Forecasts:

7.1.1.1 Standardization
The CONUS FA follows these standards:

• All referenced heights or altitudes are referenced above mean sea level (AMSL),
unless otherwise noted, and annotated using the height in hundreds of feet, consisting
of three (3) digits (e.g., 040). For heights at or above 18,000 feet, the level will be
preceded by FL (e.g., FL180).

• Messages are prepared in abbreviated plain language using contractions from the
Federal Aviation Administration (FAA) Order 7340.1Y for domestic products and
International Civil Aviation Organization (ICAO) document 8400 for international
products issued for Oceanic FIRs. A limited number of non-abbreviated words,
geographic names and numerical values of a self-explanatory nature may also be
used.

• Weather and obstructions to visibility are described using the weather and
abbreviations for surface airways observations (METAR or SPECI).

7.1.1.1.1 Height Reference
All heights are referenced to Mean Sea Level (MSL) except when prefaced by AGL, CIG or
CEILING. Tops are always referenced to MSL.

Examples:

SCT030 BKN100
Scattered at 3,000 feet MSL, broken at 10,000 feet MSL

AGL SCT030 CIG BKN050
Scattered at 3,000 feet AGL, broken at 5,000 feet AGL

AGL SCT-BKN015-025. TOPS 070-090
Scattered to broken at 1,500 to 2,500 feet AGL. Tops 7,000 to 9,000 feet MSL.
Click to expand...
 
skyflyer8 said:
I would like to solve the puzzle, please.

Quoting directly from AC 00-45F, Aviation Weather Services, Page 7-3, a direct cut & paste from the section about Area Forecasts:
Click to expand...

Great reference, Kate! Thank you so much for shedding some much needed (verifiable) light on this subject.

As PJ Gustafson noted, METARs are reported in AGL while FAs are reported in MSL unless otherwise noted ... I was looking for the METARs myself ...

In the very same document (PDF titled Part 1, Page 45), Section 3.1.3.9, AC Page 3-12: "[METAR] Sky Condition Group: The sky condition group is based on the amount of sky cover (the first three letters) followed by the height of the base of the sky cover (final three digits). No space is between the amount of sky cover and the height of the layer. The height of the layer is recorded in feet Above Ground Level (AGL)"
 
You must log in or register to reply here.
Back
Top