Lesson 28
5 February

We had a 2.25 hour ground session today.  Among other things, I finally understood something that I've apparently misunderstood most of my life--about the relationship of air temperature to barometric pressure and density.  Here's how I finally got it:

Warm air is less dense than cold air, but the pressure is greater--and vice versa.

I had the pressure part backwards. It makes a big difference in aviation because if you fly from an area of warm air into one of colder air, your barometric altimeter is going to indicate that you're farther from the ground that you really are.  That is, you'll be lower than you think by looking at the altimeter.  Or as the pilot saying goes, "From high to low, watch out below!"  The expanded version doesn't scan, but it accounts for temperature, too:  "From high to low and hot to cold, watch out below!"

It's the "hot to cold" part that I had backwards.  I'd been thinking--erroneously--that because warm air is less dense than cold air, the pressure would be less.  In order to get it straight, I had to think of it as if the air were contained in a rigid container.  In that case, raising the temperature would increase the air pressure inside the container.  But I still don't really understand how it works in free, uncontained air--just that it happens.

What interests me most about this is that it's at least partly a semantic problem--higher is lower.  For example,

I can say that, but I don't feel it in my bones yet.

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