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

Right. Ambient temp doesn't change air pressure. BUT going from cold air to hot air is an indicator that you've probably moved to a higher pressure weather system, and visa versa.
 
Nicely done.
 
WDD said:
Right. Ambient temp doesn't change air pressure. BUT going from cold air to hot air is an indicator that you've probably moved to a higher pressure weather system, and visa versa.
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Nope. As the guys says, cold air contracts so that the column of air is shortened, leaving less air above you so that the altimeter reads high, fooling you into descending.

Moving from a high pressure to a low will also make the altimeter read high. The "high to low, look out below" applies to both air temp and local atmospheric pressure.

The changes in the freezer and oven are due to thermal contraction and expansion of the air inside the sealed aneroid inside the altimeter. Cabin temps in the airplane can do that too, but the changes are tiny.

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What about that the pressure reading after low temp and high temp goes the wrong way, AND the actual air pressure above the altimeter hot, normal, or cold is the same inside his house?

also, if the same molecules are packed more densely, the weight ought to be the same above, since the column supposedly then shortens. It’s the same weight, packed in for a shorter column according to this. Should weigh the same?
 
Dan Thomas said:
As the guys says, cold air contracts so that the column of air is shortened
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This is testing my high school science background a bit, but this explanation doesn't make sense to me. I am fully versed in the Noble Gas Law (PV=nRT), and I understand that dropping the temperature results in less pressure. But the explanation for why the pressure goes down as being cause by a reduction in the height of the air column makes no sense to me. 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?

It's always these kinds of questions that trip me up trying to apply my limited chemistry knowledge to weather systems.
 
LongRoadBob said:
What about that the pressure reading after low temp and high temp goes the wrong way, AND the actual air pressure above the altimeter hot, normal, or cold is the same inside his house?

also, if the same molecules are packed more densely, the weight ought to be the same above, since the column supposedly then shortens. It’s the same weight, packed in for a shorter column according to this. Should weigh the same?
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It's because of that sealed aneroid inside the altimeter. It has a fixed amount of air in it, and so when it gets cold in the freezer that air contracts and shrinks the aneroid. Higher ambient air pressure introduced to the altimeter's case also shrinks that aneroid, making the altimeter read lower. When the air in the aneroid gets hot it expands the aneroid, and an altimeter usually reads an expanding aneroid as an altitude increase as the case pressure falls.

Flying level into cold air from warm puts you above more of the air while your actual altitude didn't change. It contracted as it got cold, so now there's more under you and less above, so the altimeter sees less pressure (less air above) and it over-reads.

Fooling with a whiz-wheel and calculating true altitude can make it clear.

Whiz Wheel.png
 
PPC1052 said:
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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That's how pressure highs and lows come about. Solar heating and movement of cold and warm air masses.
 
LongRoadBob said:
if the same molecules are packed more densely, the weight ought to be the same above, since the column supposedly then shortens. It’s the same weight, packed in for a shorter column according to this. Should weigh the same?
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At the bottom it does. But, let's say you are at a whole bunch of feet / meters AGL geometric altitude (not pressure altitude) - about 10% up a warm air column, but then it gets real cold and the column gets shorter so you are 12% up the column so there is less air above you so less weight / pressure from above. Your altimeter would read lower at the same geometric altitude.
 
I'll have to nope your nope - at least on the ground. Cold air contracts and leaves a shorter column above you. You still have the same amount of molecules of air above you. If you went up 20,000 AGL - OK, then the shortness of the column comes into play, as you'll have less a % of the air column above you. How much of the altitude discrepancy is due to that vs the actual low pressure system signified by colder air, I don't know.
 
The weight of the atmosphere does not change, so ideally pressure at sea level does not change due to temperature in a fundamental way.
But the height of the atmosphere will change with temperature. So what changes is the pressure lapse rate.
Hot to cold, lookout below is still correct. This is not because cold means higher pressure, but because cold means a higher pressure lapse rate.
 
Many people have a GPS now, so it is also easy to compare the altimeter to it...yes GPS has a small altitude error also, but it gives you a good ballpark idea.
 
Some airports are cold weather restricted. Depending on the approach mins have to be adjusted up for cold weather. All of this with the correct barometer set. So... cold weather does affect altimeters. It’s not because you are flying into a different pressure environment. It’s because it’s cold and the altimeter is indicating incorrectly.
 
Tarheelpilot said:
Some airports are cold weather restricted. Depending on the approach mins have to be adjusted up for cold weather. All of this with the correct barometer set. So... cold weather does affect altimeters. It’s not because you are flying into a different pressure environment. It’s because it’s cold and the altimeter is indicating incorrectly.
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And just when I thought this was getting clear....... I have no idea why temperature can change the altimeter when the air pressure does not change.
 
FlightInsight said:

I had a hard time figuring out why the altimeter does what it does in cold weather. Hopefully this video clarifies things.
Dan, CFI
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Do it again but don't leave it in the freezer for 30 minutes. Just put in in there where the air is cold. Maybe close the door for a few seconds. What temperature the instrument itself gets to is not pertinent. It ain't outside in the cold, it's in the cockpit. What is pertinent is the pressure at the Static Port and how temperature affects pressure. I'm wondering if maybe freezing the instrument itself may have some effect. Like cold causes things to shrink a little. Like the linkages and the bellows inside the altimeter.
 
Hopefully I can better understand something here.

If I have the correct altimeter setting for the airport why does cold temp affect the altimeter?

Im not flying into different weather temps or pressures but it still reads incorrectly.
 
Tarheelpilot said:
Hopefully I can better understand something here.

If I have the correct altimeter setting for the airport why does cold temp affect the altimeter?

Im not flying into different weather temps or pressures but it still reads incorrectly.
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Because the pressure lapse rate changes with the temperature. At Sea level it will be correct. The higher you go, the more error there will be because the lapse rate has changed from the lapse rate the altimeter is calibrated to.
 
I have the idea that of the two, pressure variation is the more prevalent factor for inaccurate readings. that of the two, temperature will give less variation/error, or am I wrong?

Also, could you have where pressure goes from high to low, but temperature offsets because it is higher? Guess that ties into the first statement above.

I’m thinking because lapse rate for temp is less per unit than lapse rate for pressure.
 
LongRoadBob said:
I have the idea that of the two, pressure variation is the more prevalent factor for inaccurate readings. that of the two, temperature will give less variation/error, or am I wrong?

Also, could you have where pressure goes from high to low, but temperature offsets because it is higher? Guess that ties into the first statement above.

I’m thinking because lapse rate for temp is less per unit than lapse rate for pressure.
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It depends how high you are. See my last post.
 
Tarheelpilot said:
Hopefully I can better understand something here.

If I have the correct altimeter setting for the airport why does cold temp affect the altimeter?

Im not flying into different weather temps or pressures but it still reads incorrectly.
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Your altimeter is supposed to be accurate on the ground when set to the altimeter setting, it should not be reading incorrectly. The altimeter setting takes temperature into account, until you change altitude relative to field elevation, in which case the pressure lapse rates come in to play.
 
Salty said:
Because the pressure lapse rate changes with the temperature. At Sea level it will be correct. The higher you go, the more error there will be because the lapse rate has changed from the lapse rate the altimeter is calibrated to.
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Because it’s cold. So... like I said. If it’s cold the altimeter doesn’t indicate altitude correctly. It has to be adjusted based on temperature. Just as a friendly reminder the airport doesn’t have to be a high altitude airport for it to be significant. Plenty of places in the eastern seaboard require minimum adjustments in the winter. Places that rocky mountain pilots would consider being effectively at sea level.
 
Tarheelpilot said:
Because it’s cold. So... like I said. If it’s cold the altimeter doesn’t indicate altitude correctly. It has to be adjusted based on temperature. Just as a friendly reminder the airport doesn’t have to be a high altitude airport for it to be significant. Plenty of places in the eastern seaboard require minimum adjustments in the winter. Places that rocky mountain pilots would consider being effectively at sea level.
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You don't adjust an altimeter for temperature. That is taken into account in the altimeter setting. The altimeter is supposed to read the correct field elevation when set to the altimeter setting.
 
dmspilot said:
You don't adjust an altimeter for temperature. That is taken into account in the altimeter setting. The altimeter is supposed to read the correct field elevation when set to the altimeter setting.
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Im not saying you correct the altimeter. I’m saying you adjust the approach minimums. As an example the published minimum is 250’. With the cold temp adjustment you change the minimum by adding 50’ to the publish of 250 so now you go around at 300’.

you don’t do anything to the altimeter other than set the correct barometer and read the altitude...

you’ll still be going missed at the same actual altitude it’s just incorrect on the altimeter
 
Tarheelpilot said:
Im not saying you correct the altimeter. I’m saying you adjust the approach minimums. As an example the published minimum is 250’. With the cold temp adjustment you change the minimum by adding 50’ to the publish of 250 so now you go around at 300’.

you don’t do anything to the altimeter other than set the correct barometer and read the altitude...

you’ll still be going missed at the same actual altitude it’s just incorrect on the altimeter
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First, you don't set the barometer in an altimeter. Second, approach minimums are adjusted because of how cold weather effects the atmosphere, not how cold affects the altimeter.
 
Tarheelpilot said:
Hopefully I can better understand something here.

If I have the correct altimeter setting for the airport why does cold temp affect the altimeter?

Im not flying into different weather temps or pressures but it still reads incorrectly.
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On the airport it doesn't. The Altimeter 'setting' is corrected for temperature. So it will read correctly at airport elevation. As you ascend you are flying into different temps and pressures. Some of the pressure change is you are getting higher into thinner air. If the temperature is 'standard' and remains that way as you climb, the altimeters indicated altitude will closely match actual altitude. But as the temperature starts getting farther away from standard, then temperatures affect on pressure becomes pronounced and indicated altitude starts straying from actual altitude. The higher you go, the more pronounced. Temperature inversions notwithstanding. It's that column of air thang
 
Salty said:
Because the pressure lapse rate changes with the temperature. At Sea level it will be correct. The higher you go, the more error there will be because the lapse rate has changed from the lapse rate the altimeter is calibrated to.
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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.

Then there is the problem of "how high is sea level here?" :)

 
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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Very true.
 
dmspilot said:
First, you don't set the barometer in an altimeter. Second, approach minimums are adjusted because of how cold weather effects the atmosphere, not how cold affects the altimeter.
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Yup. The cold doesn't affect the altimeter. It affects the altitude the altimeter is telling you that it thinks you are at, indicated altitude. It tells you that by reading the atmosphere, air pressure. Pressure is affected by temperature. Not as much as changes in actual altitude but change nonetheless. And the changes can be significant when it gets very very cold. Enough that the difference between indicated and actual altitude can be dangerous.
 
luvflyin said:
On the airport it doesn't. The Altimeter 'setting' is corrected for temperature. So it will read correctly at airport elevation. As you ascend you are flying into different temps and pressures. Some of the pressure change is you are getting higher into thinner air. If the temperature is 'standard' and remains that way as you climb, the altimeters indicated altitude will closely match actual altitude. But as the temperature starts getting farther away from standard, then temperatures affect on pressure becomes pronounced and indicated altitude starts straying from actual altitude. The higher you go, the more pronounced. Temperature inversions notwithstanding. It's that column of air thang
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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.

edit:

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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dmspilot said:
First, you don't set the barometer in an altimeter. Second, approach minimums are adjusted because of how cold weather effects the atmosphere, not how cold affects the altimeter.
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Barometer: obvious typo. Thanks for the heads up.

cold weather changes lapse rate. Altimeter is not able to adjust for the actual lapse rate. Result ? Altimeter is only accurate for one altitude. Conclusion... in extremely cold weather the altimeter is not accurate and has operational impact on safety during approaches.
How can you honestly say that cold weather does not affect the altimeter?
 
Tarheelpilot said:
pedant
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You posted an incorrect statement ("it's not the pressure environment") followed by an overly simplistic misleading statement ("It’s because it’s cold and the altimeter is incorrect"). I don't think taking issue with that deserves namecalling.
 
dmspilot said:
You posted an incorrect statement ("it's not the pressure environment") followed by an overly simplistic misleading statement ("It’s because it’s cold and the altimeter is incorrect"). I don't think taking issue with that deserves namecalling.
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I found the general position of many to be misleading and simplified it intentionally.

I find what you and others have said to be pedantic from the beginning. I should have just started there and saved us all a lot of time. I was trying to get that point made without just saying folks were being pedantic but did a really bad job.
Thanks for taking the time to engage in the chat.
 
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