Cold Air Altimetery

[Alamanach]

...to just ordinary tripple 7 captains and DPE's like me... the "compacting of the atmosphere" in cold air is a much simpler and yet a totally correct answer...

As coincidence would have it, you were my DPE about ten years ago.

OK, I think we've wrapped up another one.

 

[FL420]

Alamanach, you know your stuff and that's good. OTOH, your explanations make my head hurt; but I guess that's my problem, not yours.

 

[UndauntedFlyer]

As coincidence would have it, you were my DPE about ten years ago.

That's all interesting. I hope I was a good guy and passed you?

 

[Alamanach]

That's all interesting. I hope I was a good guy and passed you?

You did. Your exact words were, "You're an excellent pilot."

 

[A Squared]

First of all, the Ideal Gas Law most definitely works for standard atmosphere. In fact, the fit is so good, I suspect the Ideal Gas Law may have gone into the definition of standard atmosphere.

Yes, of course the standard atmosphere is based on the ideal gas law. I've never said the ideal gas law was invalid. That is something you read into my words, repeatedly. What I did say is the the ideal gas law does not completely explain all that is going on. remember we're not talking about the standard atmosphere, we're talking deviations from the standard atmosphere.

If the ideal gas law is the "only the only correct way to understand this phenomenon", explain why barometric pressure doesn't vary in perfect proportion with temperature, every time, all the time?

Why are exceptionally cold temperatures usually associated with exceptionallyt high barometric pressure.

Example, the lowest barometric pressure ever recorded on the north american continent was 31.85" hg, at a time (jan 1989) when all of alaska was experiencing record low temps.

According to the ideal gas law, exceptionally cold temps are always, without exception, accompanied with low barometric temps, yet the reverse is usually true.

Explain this using *only* the ideal gas law.

 

[Alamanach]

According to the ideal gas law, exceptionally cold temps are always, without exception, accompanied with low barometric temps...

I assume you meant to say barometric pressures-- and it is not the case. By our simplified form of P=DT, we see that low temperatures would accompany high pressure if the density is higher than the temperature is low. Those would be some pretty exceptional conditions, to be sure, but still within a range that makes Ideal Gas Law error negligible.

Why are exceptionally cold temperatures usually associated with exceptionallyt high barometric pressure.

If this is true-- and we get back into something you would know about more than I-- then why are we worried about cold air causing our altimeters to read high? Is this something different from the usual cold air stuff we've been discussing?

 

[Alamanach]

A Squared, I have just now come across something interesting-- Part 2-4 of Chapter 7 of the AIM. It says:

Cold, dry air masses produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels. When the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates.

It goes on from there. The word dry is crucial. If we remove water vapor from the air, then the composition is not what it was, and we are not dealing with quite the same substance. This means that the r of PV=nrT will take on a different value. (I believe I mentioned in my second post in this thread that r is specific to a substance.) The arctic air is certainly going to be a lot dryer than what we see here in the temperate zones (and by the way, it will be absolute humidity that matters here), and that could account for these extraordinary densities.

So, humidity plays an important part in how we can apply the Ideal Gas Law. I confess, I hadn't really paid any attention to that in my earlier posts.