High to Low Look Out Below

Today we are going to take a quick look at what happens when you fly your plane from an area of high pressure to an area of low pressure and when flying from hot air to cold air.

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.

6 Comments on “High to Low Look Out Below”

  1. 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?

    1. 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.

  2. “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?

    1. 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.

  3. 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!

    1. 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

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