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

Shouldn't the equator consider a low pressure area?

FF,

You've got blobs of air moving around the country. On any given day MSP or DEN may have a higher or lower pressure. But to know what is going on, the NWS has a protocol to bring every thing down to sea level and then we mere pilots can use that info to miss the mountains.

R1830 - I have a couple thousand hours behind you. Yeah, it's shocking but we are not the center of the universe.

The atmosphere is much thicker at the equator (extends to a higher alt).

Who still sets a Kholsman window!? Even my standby altimeter has a digital display.

Huh!? I couldn't imagine this, can you explain?

Due to its rotation, the whole earth is fatter at the equator, with a diameter about 27 miles greater than at the poles.

True, but a quick google search about "atmospheric circulation" explains the Low pressure at the equator due to the sun heating

The atmosphere is thicker (extends to a higher altitude) because a warm air mass is less dense than a cold air mass. A warm air mass will expand when heat is added. Since the air at the equator is usually hotter than its surroundings, the air at the equator expends vertically. This extra vertical expansion is why the atmosphere extends to a higher altitude at the equator.

In addition to a higher topping atmosphere, when the air at the equator is heated (from the hot surface), the air rises causing uplift. This rising air causes an area of low pressure to form at the surface as the air rises. This uplift causes air to be sucked in at the surface towards the equator. Air from the north and air from the south "converge" on the equator where this area of low pressure forms. This area of convergence near the equator is called the "InterTropical Convergence Zone" (ITCZ).

The Earth is, in fact, larger at the equator than it is at the poles due to its rotation. Although assumed to be a perfect sphere, the Earth is actual an oblate spheroid. While this is true, this is not the reason why the atmosphere extends to a higher altitude at the equator.

Since pressure changes with elevation, in order to compare relative high and low air pressure areas, they all must account for this elevation-related pressure change. Otherwise, there would be a permanent low pressure area over all the mountains (where the measuring station is at relatively high elevation) and a permanent high pressure in all the valleys (where the measuring station is at relatively low elevation) and no relative comparisons would be able to be made.

The way this is accounted for is by "reducing" all measured atmospheric pressure readings down to sea level. This way, since all pressure readings will be based on the SAME altitude(ie. sea level), all altitude related pressure anomalies will be removed and a pure air mass pressure comparison can be made. This how high and low pressure air masses are identified and quantified.

The way this is accomplished is by using an average assumed atmospheric pressure change for a given altitude. This assumed value (which is the same assumption aviation altimeters work with) is that pressure will decrease 1 inch of Hg for every 1,000ft of altitude above sea level.

An example is on a standard (29.92) day, the actual measured pressure at Denver (assuming 5,000 ft elevation) would be 24.92 because, assuming 1 Hg / 1,000ft pressure decrease, the pressure would be 5 inches of Hg lower than what it would be at sea level due to the 5,000ft elevation above sea level.

So, when you read an altimeter setting in Denver, you are actually reading the measured pressure at Denver REDUCED (using the 1 Hg/1,000ft assumption) to sea level (zero altitude). This reduction done to altimeter settings is what allows us to always know our height above SEA LEVEL (not just the local area elevation).

In the OP' original question, QNH is the altimeter setting reduced to sea level and the QFE is the ACTUAL measured pressure at the field (no sea level reducion = includes altitude-related pressure decreases).

So, on a standard day in Denver (5,000 ft field elevation:

QNH = 29.92 Hg (reduced to sea level)
- Altimeter would read 5,000 ft when set to QNH = Altimeter would read altitude above sea level

QFE = 24.92 Hg (NOT reduced to sea level = ambient pressure)
- Altimeter would read zero ft when set to QFE = Altimeter would read altitude above ground level

Fly_Boy_Knight thanks for the great explanation!

still not sure why at ADDIS ABEBA airport (7700ft elevation) a QNH would always be high as 1028 hPa.. I will convince myself it is a high pressure area as galaxy flyer stated (yet cant figure how a high pressure can occur at equator).

Again really thankful for such great explanation :)

Some airports in some locations are just prone to having a more unique local weather pattern than others in more "normal" locations. A good example is an airport in a valley which gets narrower near the airport would be prone to lower pressure due to the venturi effect of the wind moving down the valley and speeding up (lowering the pressure) at the constriction where the airport is (and the obvious higher wind speeds parallel to the valley direction). Other airports have completely other random local weather patterns too. Sometimes these local-scale weather patterns / events tend to overcome the larger scale weather patterns so sometimes, you really just never know. This is why I think meteorology is really interesting, because not only do you need to understand each scale and their associated parameters, but you also need to identify which pattern will dominate the overall conditions (like whether a high pressure system moving into this valley airport's area will overcome the local scale wind venturi enough to register a higher than normal pressure reading).

As for the high at the equator, while the LARGE (Global) scale weather pattern depicts an average lower pressure at the surface (due to the ITCZ), the equator still gets highs and lows, warm and cold fronts, and other synoptic and local scale weather events so high pressure areas can occur at the equator however the most prominent pressure system at the equator is a low pressure due to heating, convergence, and the resultant lifting action. Earth's rotation and the thickness of the atmosphere at the equator do not really contribute to the equatorial low pressure (THAT MUCH!! They do contribute to global scale weather patterns considerably, but not for this specific topic)

Thanks for throwing a light down here Fly_Boy_Knight

Another question came to my mind. Can you anticipate a "bad weather" which is associated with low pressure with just getting a low QNH from ATC? And stable weather when QNH is high?

Making mini-forecasts can be done using pressure reading alone...but because there are so many factors involved in determining the dominant weather in an area, typically pressure reading isn't enough by itself to anticipate any particular pattern of weather beyond stability.

A good example is Florida in the summer. Pressure is typically higher than standard all summer, day and night, however because of the daily heating and converging sea breezes during the day, thunderstorms develop like clockwork every afternoon in otherwise-stable air / high pressure air. This is another example of how local (mesoscale) weather patterns can overpower larger (synoptic) scale weather patterns.