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.
Anything that causes a change in air pressure will therefore require us to climb or descend to keep the altimeter reading constant. Let’s look at how temperature and pressure changes affect the altimeter.
Colder air has a higher density, which means gravity will ‘pull’ the column of air closer to the ground. As we fly towards an area of colder air, the pressure level that we are flying along will in fact get closer to the ground – even though our altimeter is still indicating the same altitude.
So true altitude decreases when flying from a higher temperature area to a lower temperature area.
Much like temperature, pressure changes as we fly from A to B. Typically we use the QNH from the nearest airfield and continually update our altimeter subscale setting as we fly cross country, however this is not always possible. In more remote areas there may be long distances between airfields and we may end up using a somewhat inaccurate pressure setting for a period of time.
True altitude decreases when flying from a high pressure area to a low pressure area.