why always high Qnh in mountain airports?
Thread Starter
Join Date: Jun 2000
Location: GRANADA HILLS, CA USA
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
why always high Qnh in mountain airports?
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.
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
ENTREPPRUNEUR
Join Date: Jun 2001
Location: The 60s
Posts: 566
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
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.
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.
ENTREPPRUNEUR
Join Date: Jun 2001
Location: The 60s
Posts: 566
Likes: 0
Received 0 Likes
on
0 Posts
Tin
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?
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?
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.
I wasn't really trying to cover why the 'real' column is different to the assumed lapse rate.
Thread Starter
Join Date: Jun 2000
Location: GRANADA HILLS, CA USA
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
No.
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.
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.
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Thread Starter
Join Date: Jun 2000
Location: GRANADA HILLS, CA USA
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
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!
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
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)
Thread Starter
Join Date: Jun 2000
Location: GRANADA HILLS, CA USA
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
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">
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">
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
Correct.
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.
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.
Join Date: Apr 2000
Location: london
Posts: 30
Likes: 0
Received 0 Likes
on
0 Posts
"This is true even if the actual pressure remains the same"
INCORRECT
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
INCORRECT
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
Join Date: Apr 2000
Location: london
Posts: 30
Likes: 0
Received 0 Likes
on
0 Posts
Huggy
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
Max
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
Max
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Apr 2000
Location: london
Posts: 30
Likes: 0
Received 0 Likes
on
0 Posts
Huggy
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
Max
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
Max
Join Date: Sep 1999
Location: here to eternity
Posts: 577
Likes: 0
Received 0 Likes
on
0 Posts
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">
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">
Join Date: Dec 2001
Location: europe
Posts: 546
Likes: 0
Received 0 Likes
on
0 Posts
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.