YWG
Topic Author
Posts: 1055
Joined: Wed Feb 07, 2001 11:29 am

QFF Barometric Pressure

Thu Oct 15, 2009 5:08 pm

Hey guys
Doing some studying here and came across a term I've never seen before in any of my studies.

"QFF" - barometric pressure at a place reduced to msl.

This has confused me. Could someone please explain it?

Would I be correct if I said:

"Say we are at an airport of 2000ASL. The temperature at mean sea level is +15c, while at our airport it is 20c. Using the lapse rate of -1.98c/1000ft, could I then say that msl at 2000 would be ~11c and the pressure should be______"?
Contact Winnipeg center now on 134.4, good day.
 
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TripleDelta
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RE: QFF Barometric Pressure

Thu Oct 15, 2009 8:12 pm

Quoting YWG (Thread starter):
"QFF" - barometric pressure at a place reduced to msl.

The definition is very ambiguous and woefully incomplete... it is true that QFF is pressure at some place reduced to MSL, but that's not the whole story.

The QFE - that barometric pressure mentioned - read at some place can be reduced to MSL in two ways:

1. using standard temperature lapse rate, in which case you'd get QHN. This value is what you've described here (emphasis mine):

Quoting YWG (Thread starter):
"Say we are at an airport of 2000ASL. The temperature at mean sea level is +15c, while at our airport it is 20c. Using the lapse rate of -1.98c/1000ft, could I then say that msl at 2000 would be ~11c and the pressure should be______"?

2. the second method is by using the actual temperature lapse rate at that place and that point in time, in which case you get QFF. This is not used operationally in aviation, though the pressures on the weather chart are expressed in it.

[Edited 2009-10-15 13:15:39]
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SB
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RE: QFF Barometric Pressure

Fri Oct 16, 2009 8:37 am

If memory serves QFF is the actual sea-level pressure based on the current atmospheric conditions whereas QNH is a value which will make your altimeter read field elevation when on the ground.

Interestingly Canadian METARs report both: QNH in inches of mercury (A3020 = 30.20" hg) and QFF in hectopascals / millibars (SLP 239 = 1023.9 mb). The difference can be quite noticeable at times, in the example below 30.20" hg is 1022.6 mb.

METAR CYWG 160800Z 32002KT 15SM OVC058 00/00 A3020 RMK SC8 SLP239=

S.
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YWG
Topic Author
Posts: 1055
Joined: Wed Feb 07, 2001 11:29 am

RE: QFF Barometric Pressure

Sun Oct 18, 2009 6:56 pm

Thanks guys, I really appreciate the help. I knew the answer was easy, but I've just never used the hectopascal system in altimeter setting procedures. This has cleared up the issue completely.
Contact Winnipeg center now on 134.4, good day.

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