YWG From Canada, joined Feb 2001, 1147 posts, RR: 2 Posted (5 years 4 months 2 weeks 4 days ago) and read 18158 times:
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______"?
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.
SB From United Kingdom, joined Aug 2006, 216 posts, RR: 0
Reply 2, posted (5 years 4 months 2 weeks 3 days 8 hours ago) and read 18072 times:
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.