Cold Weather Altimetry

[shdw]

If you are like me, you were taught high to low look out below and hot to cold look out below are the same thing. Both are altimeter errors, and both are because you didn't set the kollsman window properly. Well, I discovered today, this is wrong.

Here is an interesting video I think everyone here, even if you know this already, should watch. Makes perfect sense and would be very easy to replicate should you have to teach it: http://www.robinmaiden.com/2008/01/cold-weather-altimetry/

 

[matt152]

Both are altimeter errors, and both are because you didn't set the kollsman window properly. Well, I discovered today, this is wrong.

Interesting video. This should be a refresher for anyone who holds a CFII.

 

[shdw]

Interesting video. This should be a refresher for anyone who holds a CFII.

I am just disappointed I didn't get this information in college in the north east. Maybe I missed the discussion on it, but I got an A in my instrument ground school, so. Anyone else here from DWC specifically remember this being discussed?

We routinely flew in -10 or lower, the graph shows that to be around 100 feet off at 1500 altitude. It would have been nice to know this. Especially on our mission flights into Canada.

 

[matt152]

I am just disappointed I didn't get this information in college in the north east. Maybe I missed the discussion on it, but I got an A in my instrument ground school, so. Anyone else here from DWC specifically remember this being discussed?

I did not go to DWC. But it's available in the Instrument Flying Handbook.

 

[Bernoulli Fan]

If you are like me, you were taught high to low look out below and hot to cold look out below are the same thing. Both are altimeter errors, and both are because you didn't set the kollsman window properly. Well, I discovered today, this is wrong.

Uh, what? How would you set the Kollsman window to a different setting flying into different temperature air? I haven't heard anyone teaching that hot to cold is a Kollsman setting problem. In fact, this guy's video is pretty much how I was taught, although he does draw some nice slopes. Good video anyway.

 

[matt152]

Uh, what? How would you set the Kollsman window to a different setting flying into different temperature air? I haven't heard anyone teaching that hot to cold is a Kollsman setting problem. In fact, this guy's video is pretty much how I was taught, although he does draw some nice slopes. Good video anyway.

:yeahthat:Kollsman is to correct for nonstandard pressure.

 

[shdw]

Uh, what? How would you set the Kollsman window to a different setting flying into different temperature air? I haven't heard anyone teaching that hot to cold is a Kollsman setting problem. In fact, this guy's video is pretty much how I was taught, although he does draw some nice slopes. Good video anyway.

I was told that, since heating and cooling the air changes the airs pressure it would have the same effect as directly raising or lowering the pressure through air masses. Made sense to me since if you heat a quantity of air its pressure increases, vice versa if it is cooled.

Summary: heat changes pressure, pressure depicts the kollsman window setting.

 

[Maurus]

I was told that, since heating and cooling the air changes the airs pressure it would have the same effect as directly raising or lowering the pressure through air masses. Made sense to me since if you heat a quantity of air its pressure increases, vice versa if it is cooled.

Summary: heat changes pressure, pressure depicts the kollsman window setting.

Pressure will not change with temperature unless volume remains constant. Since our atmosphere actually changes size (volume in this case) the volume restriction is not there. As a result, an increase in temperature will only cause an increase in volume (higher atmosphere). A decrease in temperature will cause a decrease in volume(lower atmosphere).

The error being explained is a result of the change of volume.

EDIT: No idea if that is what you are currently believing or just what you were taught in the past. If just the past then disregard.

 

[shdw]

Pressure will not change with temperature unless volume remains constant. Since our atmosphere actually changes size (volume in this case) the volume restriction is not there. As a result, an increase in temperature will only cause an increase in volume (higher atmosphere). A decrease in temperature will cause a decrease in volume(lower atmosphere).

The error being explained is a result of the change of volume.

EDIT: No idea if that is what you are currently believing or just what you were taught in the past. If just the past then disregard.

Past. Well at least 2 days ago past. I knew volume had to remain constant for pressure, but I never even thought about the vertical expansion/contraction. I considered if it would expand horizontally, but disregarded that thought since I figured other pressure systems would contain it. Way to go me.

I am in no way debating this, just posting it here in case others had the same confusion I did. Sharing recently discovered knowledge. Thank you though.

 

[Maurus]

I am in no way debating this, just posting it here in case others had the same confusion I did. Sharing recently discovered knowledge. Thank you though.

I wasn't sure but posted anyway and then added the edit just in case

 

[ppragman]

I didn't realize it, but I've seen this before, flying by mckinley at FL200, I've looked over to the right and seen the peak well above me. Cool.