The altimeter error is what we are going to pay special attention to and why it is important to be aware of the changes as we fly across country.
Change in Pressure
If we take a look at the picture below we can see when going from an area of high pressure to an area of low pressure our aircraft will be lower than what it indicates if we do not change the sub-scale setting on our altimeter. This happens because as the area pressure decreases you try and keep the plane reading the required altitude (in this case 5000 ft) and therefore willingly descend. So to keep you plane at the correct altitude above the ground you would have to change the sub-scale to the lower setting, in this case from 1018 to 1016.
HIGH TO LOW LOOK OUT BELOW
High to Low Look Out Below
Change in Temperature
Similar to the above scenario, when we fly from a hot area to a cold area our aircraft will be lower than what it indicates if we do not change the sub-scale setting on our altimeter. A good way to think about this is if you were in a elevator “floating” above a column of air which can only expand/contract vertically, when that air cools down the column shrinks and therefore the elevator sinks. If you had an altimeter in the elevator it will still be reading the same, giving you the impression that you are at the same altitude where in fact you are lower.
HOT TO COLD DON’T BE BOLD
Hot to Cold don’t be Bold
I hope this simple explanation has helped. Please leave any comments below if you don’t understand or have any points to add for others benefit.
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Thanks for this article! I understand how changing the altimeter setting fixes the high pressure to low pressure scenario. And I understand that hot air expands/cold air retracts. But does updating the altimeter setting fix the hot air to cold air scenario, keeping you at the correct altitude MSL? and how?
Hi Jessica, sorry for the delay in reply, my internet has been down.. Things I have to deal with in the jungle;)
So to answer your question: Yes, changing the altimeter setting will correct the altitude error. This works because the hot to cold are essentially changes in pressure. In a closed system, when the temperature of a mass of air is increased it will expand, resulting in an increase of pressure. Therefore you can assume hot is has a higher pressure than cold air. So when you look at HOT to COLD you can replace it with HIGH to LOW.
This is a general rule and works in a closed system, however the earth is not a closed system and there may be exceptions.
I hope this helps you understand a little better.
PV=nRT is Boyle’s gas law. If the temperature is increased, and the volume is fixed, like in the pot example below, then the pressure has to increase. On the other hand, if the temperature is increased, and the volume is allowed to change so the pressure stays constant, the volume will increase as temperature increases. This is the example of a balloon expanding the one that heats up, or contracting if pork in the refrigerator. The volume is allowed to change.
When you say, “when we fly from a hot area to a cold area our aircraft will be lower than what it indicates if we do not change the sub-scale setting on our altimeter.” This doesn’t make sense to me, because when you move from hot(less dense) to cold(more dense) air, the altimeter will turn counterclockwise(down) therefore indicating a lower altitude which will cause you to climb, i.e. “low to high, look to the sky”.
“If you had an altimeter in the elevator it will still be reading the same”
This is where I fell off the elevator!
In all seriousness, I understand the expanding and shrinking… but why would the elevator float at a different level but the Altimeter stay the same? If there is shrinking/contraction and expansion, wouldn’t that do the same to the meter as is happening to the elevator?
I see where you are coming from, and it is not always easy to understand. But let me try using the same analogy. Lets say for example that you are sitting at the very top of a column of air, and only had the vacuum of space above you (no air above you). In this case there would be no pressure on you, because all the air is underneath you. Now lets say the air cools down, this in turn will cause the air to shrink. With the shrinking of air the column will get short and therefore you will be at a lower altitude. But this does not change the fact that you are still at the top of that column of air, and therefore the pressure above you will not be any different. If you look at the height of the atmosphere around the earth you will notice it changes depending on where it is, around the poles (colder) it is lower compared to the equator (hotter), this ties into the reasoning here.
I hope this makes it a little easier, but if not, let me know and will try my best to give another explanation.
Hi i am having some confusion with reading different things online regarding temperature and pressure. It seem to be contradictory and i was hoping you could help?
A source from Quora basically says, when molecules expand , density decreases. See Below.
“When pressure increases molecules come closer to each other which means increase in density and when pressure drops molecules of gases become free to expand and get away from each other which means density decrease. Since in case of gases pressure and temperature are proportional to each other for a given volume so in this way temperature also causes to the change in the density.”
And on this website you explain:
“when the temperature of a mass of air is increased it will expand, resulting in an increase of pressure”
Thanks!
Hi Elliot,
I think what is confusing you is the wording of each of these explanations. But essentially we are both saying the same thing, although it may not look like it.
Keep in mind that the temperature to pressure relationship is proportional in a closed system.
I think the main thing here is when I say “when the temperature of a mass of air is increased it will expand, resulting in an increase of pressure”, by this I mean it will try to expand, it won’t necessarily be able to.
The other thing you may misunderstand from the Quora text is that when they say “pressure drops molecules of gases become free to expand” doesn’t mean that the volume that they occupy increases, it just means each molecule has more space to move around.
Think about it this way – if air is in a pot, you put it on a stove and heat it up you will have air trying to escape the pot (trying to expand). But if there is no way for the air to escape its pressure will increase, and with that increase of pressure you will get an increase in density.
I am not sure if this explains it properly, if you still don’t understand please let me know and I will try explaining it a different way.
Cheers,
Mike
This is the best thing I’ve found on this and I’ve been reading everything. I’m a student pilot struggling with the different variations on the FAA questions for this. Most of the other explanations are too complicated and technical. Meanwhile, it’s simple physics. Good writers are rare!
Cheers, I am glad you enjoyed and learnt from it:)
hi dear admin.
I want to insure that in cold air pressure will increases but because of retracting of the air mass the related pressure at the ground level will decreases so when we are saying “flying from high to low” we mean that the air pressure at the ground level decreases or temperature decreases.
other thing that I want to ensure is that how much is the maximum range of ground pressure changes in a point with considering the temperature changes (max range of changes in height).
thank you.
Hi, I am not 100% sure what you are asking, but I will try answer. “Flying high to low” would be either flying High Pressure to Low Pressure or flying High Temp to Low Temp, therefore if you fly High Pressure to Low pressure you will be flying lower than what your altimeter reads and same for Temp.
I am not sure what the range would be, but I can’t imagine it being too extreme.
Good afternoon.
I’m also slightly confused. I’ve read in several books that;
As the earth warms the mass of air directly above it through condensation, the warmer, less dense air will begin to rise. Resulting in a low surface pressure?
Likewise, with a cold denser air cooling and sinking, increasing the surface pressure?
So why and how does the above statements come into “from high (cold sinking) to low (warm rising) look out below”?
This doesn’t make sense to me. I’ve read travelling from warm air to cold air will result in your altimeter over reading (meaning you’re lower than indicated). This is therefore back to from in regards to the surface pressure statements I’ve stated above.
Thank you
*through conduction. Will begin the rise.
Hi Liam,
Sorry for the late reply.
So in the ideal world this would be a closed system (which in reality it is not, but for this to make sense it needs to be). When air heats up it expands and therefore the pressure will increase and when it cools down it will decrease.
Boyles Law
p(pressure).v(volume) = k(tempreature) (closed system)
This is why when we fly at flight levels we all fly on 1013, this just keeps the spacing even.
I hope this helps a little.
Mike
Hello, I am currently in cfi training and I believe that from high to low lookout below is 100%completely accurate but from hot to cold lookout below, it is indeed inaccurate. This is my reason why, when in colder temps, the wafer will contract rather than expand which will result in an indication of a lower altitude making you want to climb to a higher altitude. It should instead be from cold to hot lookout below. I have not been proved wrong by any CFI that I have talked about this question with but if you have the explanation, please explain