Prop swinger, thanks for your reply. The phrase is not just about atmospheric pressure. It is also about temperature. I have seen it quoted in several places but
here is one:
"That is, “high to low look out below” applies to flying from warm air into cold air as it does when flying from high to low pressure."
I understand why flying into lower pressure will mean the pilot needs to lookout below. What I was struggling with was why when flying from high to low temperature they needed to lookout below. Take the example in the attached picture I drew. If the plane flies at a consistent true altitude from 10c to 5c I thought the cold air will be more dense (because cold air is denser than hot air - Charles’s Law). Because density has increased, pressure would increase (Boyle’s Law). On that basis the altimeter would report the plane as being lower than it is so the pilot wouldn't need to look out below. My mistake, I believe was not knowing about Gay-Lussac's law. This states that as the temperature decreases, the pressure also decreases. Cold air at a lower pressure
can be less dense than hot air at a higher pressure (see
here). Therefore the altimeter can end up showing higher than true altitude and the pilot should lookout below. I believe that is the correct explanation?
oggers, in your columns example, surely all 'columns' of air have to be the same height, as they all go to the same height in the atmosphere?
Phiggsbroadband, I fly a c152. I need to know this for my exams. Truth be told, I know how to answer the questions in my exam already, but I struggle to accept facts at face value and am obsessed with understanding them at a deeper level!