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Altimeters use slightly evacuated (compared to atmospheric pressure at sea level) metal aneroid wafers. The wafers have a reference pressure outside the aircraft referred to as the "static port". Altimeters are designed to show the correct reading at standard temperature (15C at sea level), but at nonstandard temperature will show a slightly inaccurate indication.

Specifically when the outside air is warmer the altimeter will read a lower indication than true altitude, and when outside air is cooler the altimeter will read higher than true altitude.

I'm wondering why this is the case, since in my head it seems like the air inside the wafers would be subject to the same changes in behavior as the air outside the altimeter, assuming the altimeter is the same temperature as the outside air.

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  • $\begingroup$ for a definitive answer, try posting this on the aviation stack exchange. $\endgroup$
    – niels nielsen
    Commented Feb 28, 2018 at 23:35
  • $\begingroup$ @nielsnielsen pilots mostly don't really know, that is why I posted here. Most pilots just quote the Pilot's Handbook of Aeronautical Knowledge, which is inaccurate in its section on altimeters. $\endgroup$
    – OneChillDude
    Commented Mar 9, 2018 at 15:08
  • $\begingroup$ Huh. I have generally found the aviation experts a better source on stuff like this. But I favor whatever works best for you. $\endgroup$
    – niels nielsen
    Commented Mar 9, 2018 at 19:01

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The altimeter is based on difference in pressure. However, the temperature of the air also affects the altimeter. This is why it is calibrated at 15 degrees, sea level. If the temperature is not held constant, it will affect the accuracy of the altimeter.
Since it is calibrated at 15 degrees, a higher temperature will give a lower than true reading (warm air is thinner), while colder temperature will give a higher than true reading (cold air is denser).

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The altimeter works by measuring the deformation of the metal aneroid wafers. This deformation is temperature dependent since the wafer expands and contracts with temperature so the instrument must be recalibrated for large temperature changes.

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