Altimeter

[fireflybob]

Firefly I appreciate your answers, but can you give me an example of how actual pressure is reduced to sea level to get the QNH? Or even lead me where I can find some calculation about it? Thanks in advance

Ok you asked!

The formula for the altimeter is

h = 221.1 Tm (log p1 - log p2)

Where h is height in feet, Tm is mean temperature in degrees absolute and p1 is pressure at bottom of the layer and p2 pressure at the top of the layer (in Hectopascals).

So let's say you want the QNH at an airfield which is 5,000 feet amsl where the temperature is 20 degrees Celsius and the pressure at airfield level is 843 Hpa.

We have to compute the mean temperature of the layer using a lapse rate of 1.98 deg C per 1,000 feet. (Lets call that 2 deg C to keep the math simple!).

The mid point of the layer would be at 2,500 feet - so the temperature at that point would be 2.5 X 2 degrees less than 20 deg C which = 15 deg C = 288 deg Absolute.

So substituting in the formula:-

5,000 = 221.1 X 288 (log QNH - log 843)

0.07852153 = log QNH - log 843

log QNH = 0.07852153 + log 843

log QNH = 0.07852153 + 2.92582757462

log QNH = 3.00434910462

Antilog 3.00434910462 = 1010.0644932

Therefore QNH = 1010 hpa to nearest hpa

Next question?

PS Hope the math is right!