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Barometric altimetry relies on atmospheric pressure dropping at a steady rate with altitude, starting from a measured ground-level pressure. This, in turn, relies on atmospheric density dropping at a constant, steady rate with altitude, which it does not always do:

  • For instance, if the air close to the ground is much more humid (and, thus, less dense) than the air higher up, then the atmospheric density will drop at a slower-than-standard rate with altitude as the increasing rarefaction of the air is partially compensated by its decreasing humidity, causing the atmospheric pressure to fall off more slowly with altitude than it “should”, and causing an uncorrected altimeter to read falsely low.
  • Conversely, in a industrial area, with heavily-polluted low-level air containing high concentrations of dense gasses such as carbon dioxide, sulfur oxides, nitrogen dioxide, hydrogen chloride, sulfuric and nitric acids, and heavy hydrocarbons, plus lots of soot and tar particulates which raise the air’s mass significantly while adding very little additional volume, atmospheric density falls off faster with altitude than in a standard atmosphere, as one rises out of the pollution-densified lower layers; this causes the atmospheric pressure to lapse at a higher-than-standard rate, which would cause an uncorrected altimeter to read falsely high.

How do barometric altimeters compensate for nonstandard pressure lapse rates resulting from nonstandard atmospheric density profiles stemming from variations in atmospheric composition with altitude?


This is different from How does an altimeter deal with the non-linear pressure gradient?. That question is about how an altimeter is designed to match the non-linear ISA model; my question is about how one deals with situations where the ambient air isn't the ISA.

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    $\begingroup$ They don't, and they don't have to. As long as all airplanes use the same type of altimeter, they will all have the same error, thus the relative vertical position of aircraft is constant, even if the actual altitude is not. $\endgroup$ – J. Hougaard Feb 17 at 6:42
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    $\begingroup$ @Ben I don't think it is a duplicate. That question is about the non-linearity, which is accounted for in the ISA and calibrated. This question asks about anomalies which aren't accounted for in the ISA. $\endgroup$ – Dan Hulme Feb 17 at 11:09
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    $\begingroup$ This is not a duplicate of the linked question. The link asks about compensating for the non-linear nature of the mathematical pressure model. This question asked about non standard atmospheric conditions. And, I would like to answer it. $\endgroup$ – Max Power Feb 18 at 3:49
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    $\begingroup$ @DanHulme: I was about to explain why it isn't a duplicate, but you nailed it right off the bat. $\endgroup$ – Sean Feb 18 at 3:49
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    $\begingroup$ i just flagged it for review to get it re-opened. I like some things about SE but the points based moderator system has issues with arbitrary actions. $\endgroup$ – Max Power Feb 18 at 4:07
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They don't.

Since all aircraft in an area are (or at least should be) having roughly the same error, the errors cancel out and there is no practical effect on vertical separation.

Falsely low readings result in more terrain or obstacle clearance than needed, so that's not a problem either.

Falsely high readings do potentially create a problem, but the minimum clearance is deliberately large to account for the accepted range of altimetry error plus some additional buffer. For example, it doesn't matter if you're 100ft lower than you think when you're still 900ft (rather than 1000ft) above any hazards.

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