QFF and QNH met Question

[Fastair345]

Hi All,

I understand the differences in QFF and QNH in tems of defining the differences with varitions in Height and Temp. Also understand these can be quite significantly different Hot high etc. What I am unclear on is why do I need to worry about this as I'm not going to set my Alt QFF for an IFR departure using a met chart and looking at the isobars?..I set QNH given by ATC.

Also why do the Met boys use QFF (ie Isothermal Temp) to give a better weather forcast than using a ISA corrected measure such as QNH. Surely this is more accurate !

Sorry If I am being a bit thick but I don't see the answer in the manuals or searching google forums etc.

Appreciate some help so I can fully understand it.

Cheers Fastair !

[Mariner9]

QFF is the "true" pressure at sea level corrected from QFE at actual temperature. QNH is a "theoretical" pressure corrected from QFE at ISA. Obviously, QNH can only be "correct" when the conditions are exactly ISA.

Therefore, QFF is used for weather forecasts and charts.

For your ATPL's you're typically asked questions such as which will be higher given certain temps.

A simple aide memoire is

(For airfields above MSL)

Hotter than ISA QNH is higher
Colder (Froid in french) than ISA QFF is higher

Reversed if Airfield below MSL

[Fastair345]

ok so when the met boys measure the QFF (from the station QFE) do they use the Actual Temperature on the Station or actual at the point they are predicting the Pressure ? ..just don,t get this bit.

Thanks for the Aid memoire in terms of changes in temp / ht ..that is clear.

Fastair

[Mariner9]

AFAIK, QFF is derived from the calculated average temperature of the column of air between between the observation point and MSL. But you don't actually need to know that. Just compare the temp given in the question with ISA.

You'll prolly get a question along the lines of:

Pressure observations are measured at an airfield 1000' above MSL, where the temperature is recorded at +10C. Which of the following statements is correct?

a) QNH is greater than QFF
b) QFF is greater than QNH
c) QFF is the same as QNH
d) The question cannot be answered without knowing QFE

To answer this, calc ISA temp at airfield elevation (1000' therefore +13C), compare with obs (+10C). Temp therefore colder than ISA. Therefore, from aide memoire, Froid = QFF greater, therefore answer = (b)

[Fastair345]

Marriner9 cheers mate alles Clar.

Now you mentioned from the column of air I understand. eg the actual lapse rate etc...compared to the ISA.

Thanks for the help !

Fastair

[380Mrat]

If Temp is higher than ISA how does QNH compare with QFF? Q

Originally Posted by Mariner9

AFAIK, QFF is derived from the calculated average temperature of the column of air between between the observation point and MSL. But you don't actually need to know that. Just compare the temp given in the question with ISA.

You'll prolly get a question along the lines of:

Pressure observations are measured at an airfield 1000' above MSL, where the temperature is recorded at +10C. Which of the following statements is correct?

a) QNH is greater than QFF
b) QFF is greater than QNH
c) QFF is the same as QNH
d) The question cannot be answered without knowing QFE

To answer this, calc ISA temp at airfield elevation (1000' therefore +13C), compare with obs (+10C). Temp therefore colder than ISA. Therefore, from aide memoire, Froid = QFF greater, therefore answer = (b)

QFF located 400m ASL is1016 hPa. Air Temperature is 10C higher than standard. Is QNH higher pr lower than QFF?

I would have thought QNH would be lower (QFF higher) but answer shows QNH is higher!

Please advise