What happens to an altimeter in cold weather? I put mine in the freezer to find out

Capt. Geoffrey Thorpe said:
At the airport elevation it will be correct. If you dug a big hole to get down to sea level, the reading would be off there. The barometer setting given by the airport is the "correction" to get the altimeter to read airport elevation at the airport.

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This is an interesting idea. If you dug a hole at the airport down to sea level and put a barometer at the bottom, what would it read? For us sea level pilots, this may be tough to answer, but let's imagine we're in Denver (the mile high city).

If the altimeter setting is the same as the atmospheric pressure at the airport, we'd expect altimeter setting at Denver to be about 5 inches less than a sea level setting. Check your METARs, but here is what mine say at current writing:

KCGS (College Park MD ~ sea level) - 30.05
KDEN (Denver, CO - over 5000 feet) - 29.67

Denver's a little lower on the pressure, but not by five inches. Why? Because the altimeter setting is not barometric pressure. It's barometric pressure, corrected as if it were at sea level. So if you dug a hole in Denver and put your barometer down to sea level, it should read around 29.67.

So, at sea level, temp doesn't affect altitude. Because, as others have said, the reported altimeter setting takes all this into account. But as you climb, the altimeter setting account for temperature effects at higher altitudes since we're assuming sea level pressures.

If I was in Colorado I'd do a quick vid showing errors at low temps, but the biggest mountain in my neighborhood, Sugarloaf, is barely 1,000 feet
 
FlightInsight said:
It's barometric pressure, corrected as if it were at sea level. So if you dug a hole in Denver and put your barometer down to sea level, it should read around 29.67.
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That is not correct either. The altimeter setting is not equal to sea level pressure — in fact, METARs also have a separate Sea Level Pressure section in the remarks. In the case of KDEN this morning, "SLP017" which means 1001.7mb or 29.58"Hg.
 
dmspilot said:
That is not correct either. The altimeter setting is not equal to sea level pressure — in fact, METARs also have a separate Sea Level Pressure section in the remarks. In the case of KDEN this morning, "SLP017" which means 1001.7mb or 29.58"Hg.
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Great point. This is where I disclaim that I'm not a meterologist, but a pilot. I do know that altimeter setting, and SLP, are derived two different ways using formulas to determine what that "bottom of the hole" barometer would read. I understand SLP takes temp effects into consideration better. However, the larger point is that what ever setting you're using on the altimeter, it still can only "assume" the state of the atmosphere above sea level, and so errors get compounded as you go up.
 
Tarheelpilot said:
If this is true then explain why minimums for approaches have to be adjusted in cold weather. If the altimeter is correct at field elevation why would it be wrong at 200 ft above field elevation to the point of being so wrong in that 200 ft altitude change that it requires increasing the minima to be safe on the approach.
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It's not so much the field elevation that's the threat unless you're doing a CAT III approach. It's the obstacles on the approach. If you're flying into an airport surrounded by mountains and you have the approach that passes fairly close over one of them, maybe 1500 or 2000 feet above airport elevation, and you didn't apply any correction on a really cold day, you will follow that altimeter right into the mountain on your IFR approach. You only have to hit it a little bit to kill yourself.
Edit: you can get errors even at low altitude, as reflected in the temp correction charts:

kbtv101w.jpg
 
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Dan Thomas said:
It's not the field elevation that's the threat. It's the obstacles on the approach. If you're flying into an airport surrounded by mountains and you have the approach that passes fairly close over one of them, maybe 1500 or 2000 feet above airport elevation, and you didn't apply any correction on a really cold day, you will follow that altimeter right into the mountain on your IFR approach. You only have to hit it a little bit to kill yourself.

A 200 foot altitude wouldn't show much error.
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Well my goodness. I am failing miserably.

yes. The hard inanimate things on the ground are the issue. That’s what I’ve been getting at from the beginning.

I don’t make the stuff up.

jeppesen provides the data and corrections to approach minimums.

yes 200’ is enough to matter.
 
Tarheelpilot said:
pedantic
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That's pilots and POA.

Regular person: "How deep can killer whales dive?"
Richard, the Pedant: "Don't you mean 'Orca'?"
Regular person: "Thanks Dick, doesn't answer my question though"
 
Tarheelpilot said:
If this is true then explain why minimums for approaches have to be adjusted in cold weather. If the altimeter is correct at field elevation why would it be wrong at 200 ft above field elevation to the point of being so wrong in that 200 ft altitude change that it requires increasing the minima to be safe on the approach.

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I understand why this is happening... I’m just trying to wrap my head around how you can be so damn pedantic to say that cold weather does not affect an altimeter when it clearly does. If it didn’t there would be no reason to adjust to local “corrected” setting to get it to indicate correctly and there would be no reason to adjust minimums in approaches. The non standard pressure lapse you talk about is because it’s cold. Saying the stuff isn’t happening because cold but because pressure is just ... special. In my opinion. Think what you want. But you better remember to adjust mins when needed because of cold temps or you might hit something.
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Yeah, it gets into detail. In my post #32 I did say “the cold doesn’t affect the altimeter.” Yes, ultimately it does but not directly. Cold affects the pressure. It does it in a predictable, measurable manner. It’s the pressure that changes the indicated altitude the altimeter reads. The altimeter reads pressure, not temperature. I can see that as coming across as pedantic but that was not my intent. Altimetry gets complicated, evidenced by this and other threads on the subject. I think the details are important to help understand it. I meant nothing personal by doing it as a reply to a reply to one of your posts. I was just getting it out there. I hope we’re good.
 
luvflyin said:
Yeah, it gets into detail. In my post #32 I did say “the cold doesn’t affect the altimeter.” Yes, ultimately it does but not directly. Cold affects the pressure. It does it in a predictable, measurable manner. It’s the pressure that changes the indicated altitude the altimeter reads. The altimeter reads pressure, not temperature. I can see that as coming across as pedantic but that was not my intent. Altimetry gets complicated, evidenced by this and other threads on the subject. I think the details are important to help understand it. I meant nothing personal by doing it as a reply to a reply to one of your posts. I was just getting it out there. I hope we’re good.
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Of course we are good.
Thanks for the reply. I try not to take anything in here personally. The communication method is way to limited.
 
PPC1052 said:
. The relevant column of air would be the entire column pushing down due to gravity, which would be all the way out to the outer boundary layer of our atmosphere. If that column of air were to shrink in height due to a change in temperature, would that not create a void at the top of that column into which more air from the surrounding area would be pulled by gravity, thereby filling that void and bringing the height (and weight) of the column back up up, thus generating more pressure?
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This sort of problem always puzzled me as well for basically the same reasons.

The answer is that the atmosphere is not in an equilibrium situation. I like to think of it like waves on the ocean. If you are underwater laying on the ground there as waves roll into shore, the height of the water above you does indeed vary. The same thing happens on the synoptic scale in the atmosphere, as both the height of the air column (measured to a certain pressure level) and it’s temperature (and therefore density) vary.

This happens because of friction and the viscosity of air, so the air doesn’t instantly flow in to fill in the gaps. Also relevant is the finite speed of transfer of heat through the air. So it is a non equilibrium situation.
 
PeterNSteinmetz said:
This happens because of friction and the viscosity of air, so the air doesn’t instantly flow in to fill in the gaps.
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And weight. Air is heavier than many realize. 0.07681 lb/cubic foot, if my memory is right. In a 10 x 12 bedroom with an 8-foot ceiling, the air weighs about 74 pounds. The air in the whole house probably weighs more than the whole family. On the scale of a cyclonic flow, let's say, (a low-pressure area in the northern hemisphere) that's hundreds of miles across and however many miles deep, the weight of air is absolutely massive and so its inertia is also massive. And the circular flow generates some centrifugal force that helps keep the low there.

A study of Coriolis Force and how it contributes to generating the various systems is interesting stuff. In CPL groundschool a lot of students had trouble with it. I found it fascinating.
 
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