The altimeter in our aircraft measures air pressure and displays this as an altitude above mean sea-level (assuming we have set the correct QNH in the subscale). So if air pressure were to change, our altimeter reading will change. This means that when we fly at a constant altitude on the altimeter, we are in fact flying at a constant air pressure – and we may actually climb or descend slightly in order to stay at the same pressure without knowing it.
This means that anything that changes air pressure will require us to climb or descend to keep the altimeter reading constant. Let’s look at how temperature changes (which cause pressure changes) affect the altimeter.
Imagine you are flying at 2,000ft on a QNH of 1013 and heading towards an area of colder air. Remember that the altimeter can only measure pressure, so the 2,000ft indicated is actually a pressure reading. The colder air you are heading towards has a higher density, which means gravity will ‘pull’ the column of air closer to the ground. In order to keep that 2,000ft indication on the altimeter, you will have to fly at the same air pressure. With the colder air ‘pulled’ closer to the ground, you will have to descend to stay at the same pressure level!
So the temperature error in our altimeter means that our true altitude will be lower when flying below ISA temperatures and true altitude will be higher when flying in air that’s warmer than ISA temperatures.