When flying in air which is colder than ISA (International Standard Altitude), how will the altimeter read compared to true altitude?

My manual says the altimeter will over read - why?

  • 1
    $\begingroup$ Are we assuming that the density of the air is the same as what ISA specifies? (Or maybe we don't need to assume anything—does the density of atmospheric layers generally stay the same regardless of temperature?) $\endgroup$ Sep 5 '17 at 21:25
  • $\begingroup$ "In winter the mountains become taller" $\endgroup$
    – Jan
    Dec 20 '19 at 17:48

Air expands as the temperature increases and it compresses when the temperature decreases.

enter image description here

image source: aviationweather.ws

When it is colder than ISA, the air column is compressed and therefore you are flying lower than the altimeter is indicating. In other words: the altimeter will over read in cold air.

So to fly safely over an obstacle at a cold day, you have to add extra margin to the obstacle's height, because your altimeter will show a higher altitude than your true altitude.

  • $\begingroup$ I don't really understand what you're saying here. You're saying that air becomes less dense as the temperature increases and less dense when the temperature decreases. When it is colder than ISA, the air column becomes more dense. But how is that relevant? Altimeters don't measure density, do they? $\endgroup$ Sep 5 '17 at 21:29
  • 2
    $\begingroup$ @TannerSwett Altimeters measure pressure and convert that to an altitude based on the assumption that the pressure distribution is according to ISA. Pressure becomes lower as you get higher, because the weight of the atmosphere above you gets lower as you get higher (i.e. there is less atmosphere above you when you get higher). If the density is higher (e.g. because of cold) the gradient at which the pressure drops is steeper. So the pressure that is normally (in ISA) found at 10 000ft will on a cold day be found at e.g. 9500 ft. $\endgroup$
    – DeltaLima
    Sep 5 '17 at 21:46
  • $\begingroup$ Ah, I understand now. I was thinking of compression locally (in a given patch of air, the molecules get closer together), whereas you were talking about compression on a large scale (in a given column of air extending from the ground up to space, the "pressure layers" get closer together). Right? $\endgroup$ Sep 5 '17 at 22:19
  • $\begingroup$ @TannerSwett exactly. $\endgroup$
    – DeltaLima
    Sep 6 '17 at 8:47
  • 1
    $\begingroup$ I'm studying for my PPL and this concept has always given me fits. Your picture is the best illustration by far I have seen. $\endgroup$
    – PJNoes
    Jan 31 '18 at 18:29

DeltaLima has it right, though I still always find it counter-intuitive. I mean, wouldn't denser air mean your altimeter will read lower?
But--the thing to remember is that it's not the weight of the air above you, it's the weight of the air below you. The altimeter is calibrated to the pressure reported by the ground station. Therefore, it IS accurate on the ground. But colder, denser air, as DeltaLima points out, will have a steeper pressure gradient--so as you climb above ground, the altimeter indicates altitude rising faster than the true height.

The moniker is: "High to low, look out below": as temperature falls, you are lower than you think!


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