View Full Version : why always high Qnh in mountain airports?

5th Feb 2002, 21:52
Why is the altimeter setting at high altiude airports, in the mountains, always higher than the surrounding areas? For example Big Bear, CA right now is 30.32 and Ontario airport just at the base of the mountains is 30.16. I think I read an explanation for this years ago but I can't find it now.

5th Feb 2002, 22:26
QNH is not regional pressure for sea-level. Important to remember this. It is the setting that, when the aircraft is on the ground, shows the airfield elevation on the altimeter.

This will differ from the setting if you were to dig a big hole down to sea level and drop the aircraft down it, mainly because of slight "errors" in temperature lapse rates and air density corrections.

Specifically, up a mountain it is cooler than it would be given the same terrain but lower down, hence the difference in QNH.

5th Feb 2002, 22:34
I suspect QNH comes from taking the barometric reading on an altimeter that gives the correct height of the altimeter at the airfield concerned. Since altimeters do not read correct heights there will always be different QNHs between airports at different altitudes.

Now the question is why would it tend to be higher as you go to high altitude airports. Why does the pressure fall off less quickly than the altimeter expects it to? It must be that the airport at the lower level is cooler or the airport at the higher level is warmer than expected?

6th Feb 2002, 07:34
Altimeters convert pressure variation to an altitude/eleveation reading based on an assumed model of the atmosphere.

A QNH set at a high elevation will mean that there must be a larger column height of this 'assumed model' of the atmosphere ie ISA with all its specifications.

The greater this assumed column then the greater the deviation from that day's actual column of air commencing from S.L. The altimeter is calibrated at ~30'/HPa but that isn't necessarily what a column of air extending from S.L. may have on that particular day.

The low elevation QNH has a shorter column of assumed lapse rates than the high elevation with the margin of deviation reducing the closer the elevation is to S.L.

6th Feb 2002, 15:18

It's not just how big the column of air is but also how it is spread. If it is cold I suspect more air is concentrated at lower altitudes so the sea level airfield stays the same (same weight of air) but the higer one shows a relatively lower pressure as some of 'its' air has slipped below it.

Hence my argument that artflyer's fields are warmer than they ought to be. Fohn effect?

6th Feb 2002, 21:21
Quite true - temp. is certainly a factor in the density lapse rate.

I wasn't really trying to cover why the 'real' column is different to the assumed lapse rate.

7th Feb 2002, 22:02
Thanks much for the answers! I don't know what Fohn effect is but I assume it has to do with the slight change in lapse rate. The temperature being cooler in the mountains than in the surrounding valleys makes sense to me. If the temperature (or pressure) is lower, the altimeter will read higher and "cranking up" the altimeter setting will bring the indicated altitude back down to field elevation.

I have also heard that the wind blowing over the mountains may cause a lower pressure at a mountain airport and this would also be a factor.

Thanks again, you guys obviously had more ground school than me!

8th Feb 2002, 05:01

Increasing the altimeter setting increases the altitude shown.

Think about it - you land one evening at relatively low pressure. Overnight the air pressure increases. You come back to it and find that the altimeter, seeing higher pressure, thinks you've descended, so shows lower altitude.

You adjust to the higher pressure, and the altimeter winds back up.

8th Feb 2002, 05:14
To add to that...

Flying towards an area either of low pressure or low temperature, the altimeter will overread. This is not very nice in mountainous areas, 'cos you will be lower than you think.

Reason is, if you were to keep a constant ACTUAL altitude, as the pressure drops the altimeter shows a climb. So you descend to keep the same reading.

The reason for the original phenomenon lies in the universal gas law, which states that P1*V1/T1 = P2*V2/T2 = R (the universal gas constant).

Since we're dealing with (for simplicity's sake) constant volumes, P*T = 1/R.

Hence, as temperature drops, pressure increases to keep the same ratio.

9th Feb 2002, 04:16
Okay, I had half of it right. The altimeter would read higher than field elevation but that would mean a lower altimeter setting. So twistedenginestarter would be correct saying that the mountain airport would have a higher than normal temperature to account for a higher altimeter setting. But unfortunately that blows my theory about the temperatures (or pressures) being lower in the mountains!

9th Feb 2002, 04:54
Temperature IS lower - pressure is higher (allowing for increased elevation - sounds odd, but you need a higher pressure than you would normally expect, to indicate the airfield's elevation, BECAUSE of the lower temperature)

9th Feb 2002, 23:38
Hugmonster, I must be really thick or getting old.

The lower temperature would cause the altimeter to read higher. So my airport is at 7,000' and with the lower temperature my altimeter would read say 7,300'. But as you corrected me, increasing the altimeter setting would raise the indicated altitude instead of bringing it down to indicate field elevation.

Help! <img src="confused.gif" border="0">

10th Feb 2002, 02:49

With decreasing temperature, the altimeter overreads. So as the temp drops, it shows 7,300 instead of 7,000 (although the difference wouldn't be that large).

So to show the correct altitude, you reduce the setting on the subscale.

This is true even if the actual pressure remains the same.

Do a search on Gay-Lussac's Law, Charles Law, Boyle's Law and the Universal Gas Law. Maybe that'll help you.

10th Feb 2002, 13:58
PS - the reason for the above is density.

max motor
10th Feb 2002, 15:28
"This is true even if the actual pressure remains the same"


A Barometric altimeter is calibrated to detect changes in pressure only. In fact great care is taken to ensure that temperature DOES NOT affect the reading.


max motor
10th Feb 2002, 17:19

I have just read the rest of this thread in detail and I am afraid you are also incorrect in your interpretation of the universal gas law:

If the volume is constant and the temperature drops then the pressure will DROP. i.e. with a constant volume pressure is directly proportional to temperature. That is why aerosol cans explode when heated etc...

Anyway this relationship cannot simply be applied to the atmosphere because for most calculations the volume cannot be considered constant and density has to be considered.

USUALLY when the temperature drops density will increase and pressure will increase as a result

i.e. the increase in pressure you refer to is due to the increase in density rather then the reduction in temperature


10th Feb 2002, 17:23
max, your conclusions are based on gas within a closed container.

In that model, the volume remains constant, as does the density. This is not true of the atmosphere, where pressure, density, volume and temperature all vary.

What remains constant is R.

max motor
10th Feb 2002, 18:21

I couldn't agree with you more that was the very point I was trying to make but you were the one who said:

"Since we're dealing with (for simplicity's sake) constant volumes, P*T = 1/R."

What I am saying that your equation should be R=P/T or 1/R = T/P


10th Feb 2002, 20:16
Oops! You're dead right - got my equation the wrong way round. However, you are correct when you say [quote]USUALLY when the temperature drops density will increase and pressure will increase as a result

i.e. the increase in pressure you refer to is due to the increase in density rather then the reduction in temperature<hr></blockquote>in that each causes the next. Hence, generally, a decrease in temperature causes an increase in pressure - although not directly. Or alternatively, an increase in pressure causes an increase in pulse rate and stress! <img src="confused.gif" border="0"> :)

10th Feb 2002, 21:30
How significant is any error so caused?. .If it was indeed significant would the altimeter not have a temperature correction facility as has many ASI's.. .Just as an observation, temperatures are quite often lower in valleys than at altitude.

10th Feb 2002, 21:53
Depends on the age and sophistication of the altimeteter.

As already pointed out, the instruments are designed to reduce errors to a minimum. The actual extent of such an error is something you'd have to ask a specialist engineer.

max motor
11th Feb 2002, 02:51

Altimeters and ASI’s are both designed to react to pressure changes only.

i.e. if the static pressure is A and the Pitot pressure is B then the Altitude will be C and the IAS will be D etc.

Neither instruments ‘cares’ what the temperature is. If as in Huggy’s example a local temperature drop has resulted in an increase in pressure then the altimeter is designed to react to the pressure increase by indicating a lower altitude. That is basically the definition of pressure altitude.

One parameter where we do require temperature to be incorporated in our calculations is True Airspeed (TAS) . To calculate TAS density must be known and this can be calculated by modifying IAS (or CAS to be precise) with temperature. This is the reason that larger aircraft have a Total Air Temp (TAT) probe.

Having said all of that all of these instruments will have some internal temperature compensations that are purely for MECHANICAL REASONS e.g. a lever may be a bit longer when hotter, a capsule may not have the same expansion characteristics at one temperature than another.

It is easy to get the two types of temperature compensation muddled.

In the TAS example all TAS indicators (or more usually an Air Data Computer) will calculate TAS from CAS using the same calculations derived from the Standard Atmosphere model.

In the case of compensation for mechanical reasons individual instruments will differ in the amount of compensation required depending on construction methods material used etc.

Hope This Helps


12th Feb 2002, 09:21
Most ATC these days read the QNH off the dial on the console and have no inkling of what is involved in determining it.

Many moons ago I was instructed in the gentle art of measuring the pressure using a mercury barometer, then applying the corrections. I don't remember what they all were now, but one was historical, a correction for noted differences between ISA and reality for that particular location. All done by mirrors now...

15th Feb 2002, 09:04
One possible correction here: although an altimeter does only respond to pressure (and measures pressure very well) pressure does not correspond to altitude, except at ISA lapse rates.

If you fly from a hotter region to a colder region your altitude will decrease, although air pressure (and altimeter) may very well not change.

For example, take a procedure commencement altitude of 3000 feet indicated, QNH 1013hPa or 29.92inHg (how convenient yes?). .If the temperature is 30°C (North Africa) true altitude will be around 3210 feet. If the temperature is -20°C (Arctic), true altitude will be 2690-ish feet.

My apologies if I have misunderstood you Max Motor. I'm sure you know all the above, but artflyer may not. In any case, anyone can check my working with a CRP1 or Jeppeson slide rule.

To get back to the original question - has anyone answered it conclusively yet? I couldn't actually tell! But if the air is hotter than ISA for a particular pressure altitude, you will have to wind up the subscale over QNH to get True Altitude (or aerodrome elevation if you are on the ground at your mountain strip.) This would make mountain QNH higher than "real" sea level pressure at the bottom of the cliff.

In other words, as per my example, your altimeter says 3000 feet at your local North African 30°+ mountain airfield, but the airfield is actually at 3210 feet AMSL. So wind up the QNH subscale to get the altimeter to read 3210 feet...

cheers,. .O8 <img src="smile.gif" border="0">

[ 15 February 2002: Message edited by: Oktas8 ]</p>

15th Feb 2002, 14:26
One ought not to forget plain old altimeter failures .... the Cause and Circumstance article in BCA, April 2000, pp90 is, I suggest, very good reading for altimeter awareness and crosschecking ....not to mention a few other discipline-related things ....

Long Range Cruise
15th Feb 2002, 16:14
Ok, found this in the dictionary. Should clarify some things...

Altimeter, device for measuring altitude. The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises. It shows height above sea level, but not above such land features as hills, mountains, and valleys. The radio altimeter, or terrain-clearance indicator, is an absolute altimeter; it indicates the actual altitude over water or over terrain, however uneven. It operates by first sending either continuous or pulse radio signals from a transmitter in an aircraft to the earth's surface. A receiver in the aircraft then picks up the reflection of the signals from the surface. The time it takes for the signals to travel to the earth and back is converted automatically into absolute altitude that can then be read from a calibrated indicator. The radio altimeter is used in the automatic landing systems of aerospace vehicles; systems developed from radio altimeters can automatically control military aircraft flying at high speeds and low altitudes.

Sounds right to me <img src="smile.gif" border="0">

max motor
15th Feb 2002, 22:32

I completely agree with you conclusions the altimeter will rarely display true altitude. Indeed this is the point I was trying to make. There seemed to be some suggestion in this thread and previous threads that the altimeter would take temperature into consideration and correct this error. As you point out that is not the case.

Just to reinforce this. To use your example, in each case the pressure acting on the altimeter capsule will be 26.82in Hg and the altimeter will indicate 3000 ft. As you point out this will only be the true altitude on the standard day. But 26.82in Hg = 3000 ft period. The altimeter will not attempt to correct for true altitude by taking temperature into account, that is up to you the pilot (or anyone else who maybe interested.)

What the altimeter will try and do is ensure that 26.82 ALWAYS equals 3000ft regardless of the temperature. For example in the Artic the lower temperature makes the capsule become more elastic and it will expand slightly more than it would do normally, even though it has the same pressure acting on it. If uncorrected this would cause the altimeter to overead. But at the same time a bi-metallic spring acting on the capsule slightly increases its force to compensate, so the net movement is the same as it was in standard conditions.

As Long Range Cruise has pointed out an altimeter is nothing more than an aneroid barometer (except that is calibrated to display altitude instead of pressure)


[ 15 February 2002: Message edited by: max motor ]</p>

17th Feb 2002, 00:02
The rule of thumb I have for correcting the indicated altitude for non-standard temperatures is: 4' per thousand for each degree C the temperature varies from ISA. Example: At 5,000' indicated OAT is 0. Thats 5 degrees less than ISA. So 5(thousand)X 4 = 20'. 20(-5) = -100'. Actual altitude 5,000 - 100 = 4,900'.

Since the difference is more significant at higher altitudes I read that some approach charts at higher altitude airports make adjustments in the MDA for very cold temperatures.

No, I never did find out why the QNH settings in the mountains around here always seem to be higher. Maybe it is just a local trend.