QFF

[BECMG]

The explanation given regarding QFF and QNH in my book is as follows:
The QFE reduced to mean sea level using the temperature at the station which is an isothermal lapse rate.

The QNH is the QFE reduced to mean sea level using the standard lapse rate of 1.98/1000 ft.QNH is only used by Pilots and ATC

It further states:
QFF is used by meteorologists and is the pressure recorded on synoptic charts.

My question is:
a)What do they mean by an isothermal lapse rate?
b)In altimetry we are taught to find the QNH from a given QFE by using 1 hpa=27ft then isnt the QNH reduced to sea level using the standard pressure lapse rate instead of 1.98/1000 ft ?
c)Why dont the meteorologists use the QNH?

Thanks.

[kme]

I will answer in reverse order,

c) Because if you use QNH for drawing meteorological maps the pressure lines will be influenced by local temperature and elevation - This is highly undesirable

QNH is not a "correct" pressure at MSL. The question should actually be asked in reverse - Why does the pilot then use QNH?

The answer is that the onboard instruments is "assuming" that ISA is present, therefore keeping to ISA in the calculations will avoid adding any temperature error between ground and MSL.

Simply (on the ground): The pressure at the airport is measured and MSL pressure is calculated according to ISA (QNH) and exact elevation. The altimeter also measures the pressure at the airport and you input QNH - The altimeter then uses ISA and calculates to find your altitude (in this case elevation)

It is the same calculation with the same values which means that the result should be correct and without temperature influence (except instrument errors)



a) + b) The isothermal lapserate is referring to the lapserate of the temperature:
ISA = 1,98 degrees/1000´ , ISOTHERMAL = 0 degrees /1000´

Explanation:
The ISA pressure lapse rate (27´/HPa) based on ISA temperature which also has a lapse rate. If you change the temperature the ISA pressure lapserate will no longer be correct. This is why you correct min useable FL for temperature.

Example coming up, hoping I didn´t confuse you already..

[Jumbo Driver]

Originally Posted by BECMG

My question is:
a)What do they mean by an isothermal lapse rate?
b)In altimetry we are taught to find the QNH from a given QFE by using 1 hpa=27ft then isnt the QNH reduced to sea level using the standard pressure lapse rate instead of 1.98/1000 ft ?
c)Why dont the meteorologists use the QNH?

a) "Iso-thermal" means "same-temperature" - so isothermal lapse rate means constant temperature.

b) Pressure reduces with height and, at the surface (ISA), it approximates to 27'/mb. However, reduction with height is not linear and. as height increases, so the difference in height corresponding to each millibar increases. 1mb approximates to 27' at the 1000mb level, to 50' at the 500mb level and to 100' at the 200mb level. A more accurate formula to show the relationship between height h (feet) and pressure P (mb) at mean temperature T (degrees Kelvin) is:

h2 - h1 = 221.1T (logP1 -logP2)

c) QNH is calculated using ISA because the altimeter is designed to assume ISA conditions and QNH is therefore likely to produce the most accurate indication of the altitude on the altimeter. QFF is obtained by taking the airfield pressure (QFE) and reducing it to sea level, assuming the actual temperature at the airfield is constant (isothermal) down to that level. This value is used by meteorologists on surface weather charts because it is more likely to accurately reflect actual conditions.

Hope this helps ...


JD

[kme]

Example

I would suggest drawing while reading this...
(The calculations are not exact but normal "round" values are used to simplify)

Lets first assume some conditions: Airfield elevation 1000´
QFE: 993 HPa
Temperature: 34 degrees


Question one, calculate the QNH
27´/HPa (This is ISA and assumes 15degrees at MSL and 13,02 degrees at 1000´)
1000´
gives 1000/27=37 HPa

QNH: 993+37 = 1030HPa


QFF
The problem for calculation of QFF is that you have to use the real pressure lapserate which takes temperature into account. Normally equations are used for this but to show how it works i will use a simplified model which goes well with the rest of the pilot theory.

The pressure lapse rate changes about 4% for each 10 degrees off ISA temperature.

In this case the temperature is 20 degrees higher than ISA. (The mean ISA temp for 0-1000` is 14,01 degrees, in the isothermal calculation 34 degrees is used)

This means we correct the ISA lapse rate with 8%: 27*1,08=29,16´/HPa

1000/29,16=34 HPa

The QFF will be = 993+34= 1027 HPa


The difference in th calculations is simply what temperature are used for calculating the pressure lapse rate. In QNH ISA temp is used (in this case 14 degrees) for QFF the real temperature is used. (34 degrees)


Hope i made it clearer, there is always the risk of overcomplicating matters when discussing basics.

[BECMG]

Many thanks guys.You have made it very clear.Cheers!

[kme]

Just to join the responses of me and JD:

The 4% rule is based on JD´s formula h2 - h1 = 221.1T (logP1 -logP2)

as you can see the pressure difference between true height h2 and h1
is proportional to T which is the temperature in Kelvin (normally meanvalues for the layer are used).

For "normal" aviation atmosphere (~200-300 degrees K) a change of ten degrees (Celsius or Kelvin) will change the absolute temperature (T) with about:
(mean value 250 used) 10/250 = 4%

Happy landings

[GATZY]

Qff<>qnh

for the met exams you dont needs to go that deep as the exams only ask you which one is greater.
i.e

Q QFF at an airfield located 400 meters AMSL is 1016Hpa the air temp is 10 degree c higher than standard atmosphere what is QNH?

A 1016 hpa
B it is not possible to give a definitive answer
C more than 1016 Hpa
D less than 1016 Hpa

for this problem think of a XNOR logic gate (sorry for my ASCII pictures it meant to be a XNOR Gate!)i.e

Above MSL =1 ___________
Temp above ISA=1________))----1= QNH>QFF (QNH more than QFF)

or if it was the other way i.e

Below MSL =0___________
Temp below ISA =0______))------1= QNH>QFF (QNH morw than QFF)

so in the above question it can be seen that QNH is more than QFF so C is correct.


or i.e.

the QNH california is 69m below MSL and is 1018Hpa the air temp is ISA+10 what is QFF?

A 1018
B not possible to answer
C more than 1018
D less than 1018


draw XNOR logic gate

Below msl=0___________
above ISA=1__________))----------QNH<QFF=QNH more than QFF

so C is correct!

try it!
and if you need a refresh on logic gates look here

Logic gate - Wikipedia, the free encyclopedia

[manucordier]

To summarize

To summarize all what has been said (just a few years later):

True Pressure Laps Rate (feet/hPa) = 27 * (1 + 4*ISADEV/1000)

ISADEV = OAT(at airfield) - ISA T°(at airfield elevation/2)
It is the difference between the actual OAT at airfield and ISA temperature at mid height between SL and airfield.

QNH (hPa) = QFE + AirfieldElevation/27

QFF (hPa) = QFE + (AirfieldElevation / True Pressure Laps Rate)

Units :
Temperatures in C°
Elevations in Feet
Pressures in mb or hPa (it's the same)

Emmanuel Cordier.

[galaxy flyer]

Guys,

If these questions are on a PILOT'S met exam, what is the point?

GF