QNH
 

Hello everyone

I understand that the QFE is the pressure at the airfield and the QNH is the actual pressure at airfield reduced to sea level using ISA. But I can't understand why at high elevation airports the QNH value is high (i.e QNH 1030). Any ideas?

You said it yourself. It is the airport pressure reduced to sea level.

Sorry it might look dumb but I think I need an example of "how the actual pressure is reduced" so it become a big value at high elevation airport.

Corrected to sea level would be better way of saying it. The normal pressure lapse rate down low (say sea level to 10,000 feet) is about 100 for every .1 of an inch of Hg. So on a standard day the actual pressure in New Orleans is 29.92 and in Denver it'll be about 24.32. The Met people in Denver have a correction factor they add to the actual pressure to bring it up to 29.92. We could use the local pressure and everything would be QFE.

Believe it or not, the world does not revolve around pilots. The weather wizards use pressure to figure out where fronts are and what's going to happen. If everyone just used their local pressure uncorrected for altitude, drawing isobars on a weather map would be useless.

:eek::eek:Say it's not true!

If you set the altimeter to read "0" (zero) while parked at the gate, the pressure setting in the kholsman window = QFE

That's the procedure AA used for years until thy drug a few planes through the trees and approach lights. Most line pilots I've spoken to about it say they liked it.

The RAF (UK) still like to use QFE, and QFE is also used in some parts of Eastern Europe and Asia. A friend of mind requested QNH somewhere in the 'stans but was given QFE, and luckily caught on before he bent some metal.

I got it. But I was thinking in reality and in non standard conditions the QNH at Denver would always be higher that the QNH at New Orleans (because Denver is higher elevation). In your example both QNH would be the same at Denver and New Orleans

Thats my question.. Because at Addis Abeba airport for example the elev is 7700ft the QNH is always high as 1030 hPa or something

Flyfly,

There are many locations where a fairly permanent high pressure atmosphere exists, Addis near the Equator is likely one. Quito and La Paz, Bolivia is also under that high pressure influence. What you are seeing is an atmospheric effect, not a QNH effect.

GF