# Converting QNH to height knowing QFE

1. Mar 22, 2006

### itsjustme

Say i have an acurate barometer that in this case reads QFE, is there any way that if i called up an airport's ATIS service and got the QNH (Altimiter setting) be able to somehow work out the altitude of where i a m standing ASL. Thx.

2. Mar 25, 2006

### HallsofIvy

I take it that QFE is the barometric pressure at the airplane's altitude and QNH is the barometric pressure at the airport. What you could do is subtract the QNH-QFE (the airports pressure should be greater than the airplanes) and then calculate the height of a cylinder of air that will give that pressure: if $\rho$ is the weight of air per cubic meter, h is the height of a one meter by one meter column of air, then its total weight would be $\rho$h and so the pressure on the bottom (divide by the area on the bottom which is 1) would be $\rho[/tex]h also. h= (QNH- QFE)/[itex]\rho$ in meters.

3. Mar 26, 2006

### itsjustme

yeah, you kind of got the picture, is p suposted to be a constant?

4. Mar 27, 2006

### quark

Density variations should be considered in these calculations. There are some empirical equations to calculate the atmospheric pressure from altitude and vice versa.

The simplest equation is P = 29.92 - 0.001H, where P is atmospheric pressure at the given altitude in inches of Hg and H is altitude in ft.

An equation(perhaps curve fitting the observed data) which can be used upto 10 kilometers upto a 3% accuracy is

P = [(44331.5 - H)/4946.624]^(1/0.190263), P is ambient pressure at the altitude in Pascals and H is altitude in kilometers.

5. Mar 29, 2006

### itsjustme

Hmm... because QNH is the pressure at sea level can i change that 29.92 into the QNH reported and work from there? if i changed the 29.92 from "Hg to Hpa or Kpa i take it that the -0.001H would also change, if it does can someone tell me what it will change to. Thx.

6. Mar 30, 2006

### quark

Better thing is to do the calculation with the given units and then finally converting "Hg to the required pressure units.

I strongly recommend you to use the second equation.

7. Apr 1, 2006

### itsjustme

In the second equation, where does the QNH (other pressure) come in?

8. Apr 2, 2006

### quark

You need not have to know two pressures to calculate barometric pressure when you know the altitude and vice versa. Just plug in the barometric pressure(P) of the area where you stand and you will know H. The reference point is mean sea level with a barometric pressure of 101.325 kPa and 0 km altitude.

9. Apr 3, 2006

### itsjustme

What if the pressure at sea level is not 101.3Kpa? The way the airports do it is they take a reading of the pressure at sea level make it the QNH, then if you set that pressure on a subscale in the altimeter you get an acurate reading of the altitude.

10. Jan 25, 2009

### xajgyk

quark gave very good formula in post #230. However H is not (km) but (m)

from where (P in hp) we get height in feet H' =((44331.5-4946.624 x (P x 100)^0.190263)/0.3048

Itsjustme should have no problem with QNH.
First calcul H' for certain P in hp, then subtract from it next H' that you get from same formula where P=QNH.
This H' will be + or - according to QNH high or low.