What temperature effect can cause an altimeter to under read when flying?

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When considering how temperature affects altimeter readings, the key concept to understand is that altimeters are calibrated to the International Standard Atmosphere (ISA) conditions, which assume a specific atmospheric model with standard temperature and pressure at various altitudes.

When the air is warmer than ISA, the actual air density decreases compared to the standard conditions. As the altimeter measures atmospheric pressure to determine altitude, less dense air results in a lower pressure reading at a given altitude. Consequently, if the altimeter is set with the expectation of standard temperature conditions, it will interpret the lower pressure as a higher altitude than the aircraft is actually flying. This phenomenon leads to the altimeter under reading.

In contrast, when the air is colder than ISA, the higher density causes the altimeter to over-read, indicating a lower altitude than the aircraft is truly at. Normal temperature conditions or temperature variations at the equator do not present a distinct altimeter reading issue in the same way that an ISA discrepancy does, making the consideration of warmer air a critical factor.

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