High to low, I’m going to blow
Nothing else might get me more tripped up, frustrated and ready to blow a gasket than trying to make sense of altimeter errors! I’m talking about your altimeter’s pressure and temperature errors.
It's confusing and difficult to illustrate as made obvious in the hundreds of reddit and pilotsofamerica.com forum posts - the semantics are maddening. This sent me on an hours long quest so I can understand it and better articulate it.
To this end, here's what I believe to be the simplest explanation of the topic:
Your altimeter is a fancy barometer
Start with the basics, if you’re unclear how altitude, humidity and temperature affect air pressure and create altimeter errors begin with this video:
Think like the altimeter
The saying goes, high to low, look out below.
The job of the altimeter is that of a barometer. It senses how much pressure is being exerted on it.
When we move an altimeter / barometer from a high pressure to a low pressure we ask, "what will the altimeter think?" It's now in air that is exerting less force, so it will *think* it is at a higher altitude (because air exerts less pressure as you move upwards in the atmosphere).
So, if we fly from a high to a low our altimeter's hands will indicate a climb. Our reaction will naturally be to push the nose over, to descend to maintain our desired altitude. This will in turn put us closer to the ground. And the principle works in reverse, low to high, clear the sky.
Much of my explanation comes from the video below and culled from this article.
Hot to cold, look out below
Where I get tripped up with hot to cold is applying the same thinking of pressure differences to temperature, but that doesn't work. You have to think about it like this:
Hot air expands. Cold air condenses. A chosen pressure altitude in a column of hot air will be higher than standard because the column of air is taller. The same chosen pressure altitude in a column of cold air will be lower than standard because the column of air is shorter.
This one is hard to understand in words, the video from the always excellent Cyndy Hollman is the best and simplest way to visualize it.
What did I miss or get wrong? Let me know in the comments below!