"low to high your in the sky"

[suswims]

Can anyone explain why the altimeter reads higher than your actual altitude on a colder than standard day???? You would think if anything, b/c of air density, the altimeter would read lower on a colder day. Anyone out there have a good explanation for this instrument error in the altimeter?

 

[ackattacker]

First, establish firmly in your mind that an altimeter is simply a pressure guage. It does not measure air density.

To visualize what happens, start by imagining what would happen to the atmosphere if you froze the entire planet. The atmosphere would become more dense as all the gas contracted. As it contracts the "top" of the atmosphere gets lower. The whole thing shrinks. So the pressure at a fixed true altitude (top of Mt. Everest say) gets lower. If you kept freezing the planet eventually the top of Mt. Everest would be poking into space.

Another way to visualize it is to imagine that the air pressure you are flying in is being supported by all the air below it. As you cool that air down it shrinks, and the air you are in drops. Hence your indicated altitude is lower than your true altitude.

 

[Smacktard]

First, establish firmly in your mind that an altimeter is simply a pressure guage. It does not measure air density.

To visualize what happens, start by imagining what would happen to the atmosphere if you froze the entire planet. The atmosphere would become more dense as all the gas contracted. As it contracts the "top" of the atmosphere gets lower. The whole thing shrinks. So the pressure at a fixed true altitude (top of Mt. Everest say) gets lower. If you kept freezing the planet eventually the top of Mt. Everest would be poking into space.

Another way to visualize it is to imagine that the air pressure you are flying in is being supported by all the air below it. As you cool that air down it shrinks, and the air you are in drops. Hence your indicated altitude is lower than your true altitude.

And once you get this, you're ready to fly 50 passengers in a jet from coast to coast!

 

[SuperPilot]

I get it! I get it! I'm ready for my shiny new jet now!

 

[ackattacker]

Another way to visualize it is to imagine that the air pressure you are flying in is being supported by all the air below it. As you cool that air down it shrinks, and the air you are in drops. Hence your indicated altitude is lower than your true altitude.

Edit: I wrote this incorrectly. I should have said "your true altitude is lower than your indicated altitude", not the other way around.