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From high to low lookout below

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From high to low lookout below

Old 16th Aug 2016, 17:35
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From high to low lookout below

I kind of asked this question before but I've thought about it more. There is a phrase 'from high to low (temperature or pressure), lookout below'. I thought that if you fly at the same (true) altitude from a high pressure to a low that the temperature would increase, based on the fact the high has cold air and low has warm air. If that is the case, then you should lookout below when flying from right to left in the below diagram, and from left to right. I know that can't logically be true (you'd constantly be looking out below!), so where have I gone wrong?

I have a feeling it might be my statement that the temperature would decrease from low to high (10 to 5 in my example) at a given altitude.

Note, I added the green and pink to the diagram, so those are the bits that are probably wrong somewhere!
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Old 16th Aug 2016, 17:55
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But it isn't flying at the "same true altitude". That's the point. If you're flying from a high pressure area to a low pressure area at the same INDICATED atitude and without adjusting the altimeter setting, then your TRUE altitude is decreasing -so you are lower than you think you are, and terrain that you think you are well above may be a problem. That's what the saying means.
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Old 16th Aug 2016, 19:54
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I think I've found the answer to my own question! Gay-Lussac's law states that if temperature decreases, pressure must decrease. Colder air in lower pressure can be less dense than warm air in higher pressure. Therefore, the air will be less dense to the altimeter at the now incorrect QNH and it will give a higher altitude than is correct.

My pink line is incorrect. It would increase from left to right.
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Old 16th Aug 2016, 21:23
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Originally Posted by gerardflyagain
I think I've found the answer to my own question!
No you haven't, the phrase is purely about atmospheric pressure, not temperature. Have a little think about how altimeters work & read Heston's post again.

Altimeters do not measure your altitude, they sense the static (ie ambient) atmospheric pressure & display it on a scale calibrated in feet or metres. If you are in a floatplane on a calm sea in a high pressure area, the QNH might be, say, 1030 hPa. Now take off & climb to 2,000' asl, the ambient atmospheric pressure at 2,000' will be around 970 hPa. If you maintain an indicated altitude of 2,000' you will be flying at an altitude where the ambient pressure is 970 hPa. If you are flying towards a low pressure area you will be following the pressure gradient downhill. If the sea level atmospheric pressure in the low pressure area is at or below 970 hPa your altimeter will continue to display an indicated altitude of 2,000' even as you fly into the sea.

"From high to low, lookout below" is a cautionary remeinder to either reset your QNH regularly or be aware that your real altitude will be less than your indicated altitude when flying from high to low pressure.
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Old 16th Aug 2016, 22:26
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There's an excellent picture in one of the training books where someone takes off from the south and flies northwards into reducing pressure without resetting their sub scale.

By the time they get to Scotland the pressure has dropped so much the mountain they think they are well above is the one they fly into.

Then it points out the met chart is real, even if the flight wasn't.
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Old 17th Aug 2016, 10:48
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It is not necessarily the case that air temp in a low is higher than the air temp in a high (at any given true altitude). It could be either. Further study of pressure systems and cells would help but just accept it for the purpose of this question.

Give prop swinger's answer some thought. The altimeter can only sense the pressure. It does not sense density.

But density is still relevant. I have seen explanations in terms of 'columns of air' and personally I find it a useful concept. The cool dense column of air is shorter and pressure drops more rapidly with height. The warm less dense column of air is taller and pressure drops less rapidly with height. Therefore, even if the surface pressure is the same in each column, for any given true height above the surface the pressure in the cool column will be lower and so the altimeter will indicate higher. Your altimeter is calibrated to ISA conditions so it will be correct when the OAT is standard. On a cold day it will over-read in proportion to both how cold it is and how high you are and on very cold days it becomes necessary to make adjustments when flying IFR in order to maintain safety margin above obstructions.
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Old 17th Aug 2016, 12:24
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To put this into context, do you fly a Cessna 150 or a Boeing 787 ?
The pressure difference will all depend upon the number of Isobars you cross, just like crossing Time-Zones. If you fly a C150 then you are unlikely to cross many Isobars (and even less Time-Zones !)


So looking at todays Bracknell Synoptic Chart...
Bracknell Synoptic Charts 12 - 120 hours | NOTAM Info


Even if you went from the Wash to Cornwall you would only cross 4 Isobars, a difference of 120ft to your altimeter.
Bracknell also give you the Geostrophic Wind Scale, which shows how close together the Isobars need to be to give a certain wind speed. Unless you fly in wind speeds over 40kts, you are not likely to cross many Isobars in a C150.
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Old 17th Aug 2016, 18:08
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Prop swinger, thanks for your reply. The phrase is not just about atmospheric pressure. It is also about temperature. I have seen it quoted in several places but here is one:

"That is, “high to low look out below” applies to flying from warm air into cold air as it does when flying from high to low pressure."

I understand why flying into lower pressure will mean the pilot needs to lookout below. What I was struggling with was why when flying from high to low temperature they needed to lookout below. Take the example in the attached picture I drew. If the plane flies at a consistent true altitude from 10c to 5c I thought the cold air will be more dense (because cold air is denser than hot air - Charles’s Law). Because density has increased, pressure would increase (Boyle’s Law). On that basis the altimeter would report the plane as being lower than it is so the pilot wouldn't need to look out below. My mistake, I believe was not knowing about Gay-Lussac's law. This states that as the temperature decreases, the pressure also decreases. Cold air at a lower pressure can be less dense than hot air at a higher pressure (see here). Therefore the altimeter can end up showing higher than true altitude and the pilot should lookout below. I believe that is the correct explanation?

oggers, in your columns example, surely all 'columns' of air have to be the same height, as they all go to the same height in the atmosphere?

Phiggsbroadband, I fly a c152. I need to know this for my exams. Truth be told, I know how to answer the questions in my exam already, but I struggle to accept facts at face value and am obsessed with understanding them at a deeper level!
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Old 18th Aug 2016, 14:17
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oggers, in your columns example, surely all 'columns' of air have to be the same height, as they all go to the same height in the atmosphere?
They do not go to the same height because they represent columns of different density. The concept is validated by the real world observation that the tropopause is much lower over the poles than it is over the equator.

I understand your desire to learn each process in detail but sometimes like a jigsaw puzzle the individual pieces in the box make more sense as you assemble the big picture.

Now back to the detail

If the plane flies at a consistent true altitude from 10c to 5c I thought the cold air will be more dense (because cold air is denser than hot air - Charles’s Law
It depends. The airmass it flew into may be cooler but that does not mean the cooler air at the same true altitude is more dense. If both columns are at the same surface pressure the cooler column will be more dense at the surface. But the pressure will fall more rapidly in the cooler column. At some given height the density of the cooler column is less than the warmer column. That is a key concept to understand.
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Old 18th Aug 2016, 17:00
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Thanks for the advice and explanation

Does this diagram I've scribbled sum up what you're saying?
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Last edited by gerardflyagain; 18th Aug 2016 at 18:45.
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Old 19th Aug 2016, 19:25
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Had a quick look and yes I believe you have the concept there.
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Old 19th Aug 2016, 20:11
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Great, thanks!
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Old 24th Aug 2016, 14:35
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Some good info here:
http://www.ifalpa.org/downloads/Leve...orrections.pdf

and here:
Altimeter Temperature Error Correction - SKYbrary Aviation Safety
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Old 24th Aug 2016, 20:37
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Thanks 212man.
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Old 27th Aug 2016, 06:28
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I think it is not correct to state that crossing isobars only matters to jets. I fly in the alps and if there is Foehn (the Chinook of the Alps) then there are pressure differences in excess of 15 hpa between North and South of the mountains. This can become relevant. There is the problem of «from high to low watch out below», but also the wind and resulting downdrafts easily exceed the airplanes capability to climb.
This is indeed relevant to you and your C150.
Also, without mountains in the equiation, it is possible that air pressure drops at extreme rates during a couple of hours.
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Old 27th Aug 2016, 06:46
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Doesn't Gay Lussac's law rely on fixed volume? It's how a pressure cooker works.
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