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A Question about Altimeter Settings

L

lr172

Well Known Member
I know that air temperature impacts air density and therefore altimeter readings. However, I am curious if temperature is taken into account when an altimeter setting is provided by the FAA/airport.

Is an altimeter setting provided based upon the current observed conditions/temp or is it adjusted to the standard atmospheric temperature of 20C?

I have noticed higher readings when the weather gets cold and also see the warnings in IFR training materials about how cold temps show higher than actual readings. This made me assume that altimeter settings were provided based upon standard temps and not actual.

Thanks,

Larry
 
For IFR purposes, cold temp corrections are based off of height AAE, thus being cold at airport elevation doesn't matter.

Not sure about how the FAA does things (been a zillion years since I flew south of the 49th), but up here, when temperature gets above 15C, the density altitude gets reported in the METAR/ATIS. It can get pretty extreme sometimes - a density altitude of 6000ft for a 1000ft ASL airport isn't unheard of.

As far as how the altimeter setting is corrected for temp, it shouldn't matter.... if an airport is at 1000 ASL, it will always be at 1000ASL and it's pressure altitude will be adjusted accordingly... temperature doesn't matter to the airport, but it does affect your performance values (especially when hot), and how much closer to the ground you really are (extreme cold).
 
Hi Larry,

Altimeter setting is simply the station pressure adjusted based on station elevation in accordance with ISA.

However, you see in a typical metar where at the end it says "...SLPXXX"? That is the station pressure adjusted to the actual temperature to give you a theoretical pressure at sea level. This video I found explains it well:

https://youtu.be/UBHigKBOSS8

Happy Holidays!
 
MK77 said:
Hi Larry,

Altimeter setting is simply the station pressure adjusted based on station elevation in accordance with ISA.

However, you see in a typical metar where at the end it says "...SLPXXX"? That is the station pressure adjusted to the actual temperature to give you a theoretical pressure at sea level. This video I found explains it well:

https://youtu.be/UBHigKBOSS8

Happy Holidays!
Click to expand...

So, my learnings from this is that the station-based pressure observation accounts for current temps, as it is an actual pressure obervation. However, the reduction to SL pressure (i.e. altimeter setting) is based upon std atmo temps. pressure is increased to SL based upon std, but actual altimeter reading/pressure is a reflection of a pressure reduction based upon actual temps. error =standard based increase - actual based decrease. Therefore, the colder it gets, the further above field elevation my altimeter will read. Clearly this error gets larger and larger as you go up in altitude and the spread from std grows.

Larry
 
MK77 said:
Hi Larry,

...SLPXXX"? That is the station pressure adjusted to the actual temperature to give you a theoretical pressure at sea level. This video I found explains it well:
Click to expand...

So, if I convert the Mb based SLP to inHg, I could compare that to the altimeter setting to see exactly how far off my altimeter reading is from field elevation.
 
lr172 said:
So, my learnings from this is that the station-based pressure observation accounts for current temps, as it is an actual pressure obervation. However, the reduction to SL pressure (i.e. altimeter setting) is based upon std atmo temps. pressure is increased to SL based upon std, but actual altimeter reading/pressure is a reflection of a pressure reduction based upon actual temps. error =standard based increase - actual based decrease. Therefore, the colder it gets, the further above field elevation my altimeter will read. Clearly this error gets larger and larger as you go up in altitude and the spread from std grows.

Larry
Click to expand...

Yep. At altitude it's not really an issue because everyone is affected the same just as long as everyone keeps the current baro setting in their altimeter which is why when you are IFR ATC gives you the baro every time you check in on a handoff.

It's way more crucial on approach for obstacle clearance because as you noted the colder it is the greater the error between your indicated altitude and your actual altitude. This is why for some approaches there's notes on cold temp corrections to apply to the published minimums if the temp is at or below what's in the notes.
 
Here's how it really works:
Remember that pressure is just the weight of the air above you (per sq inch).
As it gets hotter, the air expands upward; colder, the air contracts downward. So if you're at sea level, temperature doesn't matter; all the air is above you, regardless of temperature, so the pressure only varies with humidity (water vapor is lighter than nitrogen molecules). You adjust for humidity when you change the Kollsman window). But if the airport is at 5000', then when it's hot, the air has expanded, and there's more air above you than standard. This extra weight produces higher pressure, so the altimeter will think it is at, say, 4000' (lower), if the Kollsman window is set for local humidity (local sea level pressure). BUT that is not how it works. The local reporting station at the airport will give you a 'fake' altimeter setting, higher than the real sea level setting, whatever it takes to make a standard altimeter read 5000' on that day. So on a warm day at South Lake Tahoe (KTVL, elevation 6000'), you will see that the altimeter setting is higher than the SLP number (after converting millibars to inch of Hg). Your altimeter should always read the correct true altitude, when you are at the same altitude as the reporting station.
 
Here's how it really works...

I just push the Barrow button on the SkyView and it sets the altimeter to the nearest ADS-B reporting station.
 
BobTurner said:
Here's how it really works:
Remember that pressure is just the weight of the air above you (per sq inch).
As it gets hotter, the air expands upward; colder, the air contracts downward. So if you're at sea level, temperature doesn't matter; all the air is above you, regardless of temperature, so the pressure only varies with humidity (water vapor is lighter than nitrogen molecules). You adjust for humidity when you change the Kollsman window). But if the airport is at 5000', then when it's hot, the air has expanded, and there's more air above you than standard. This extra weight produces higher pressure, so the altimeter will think it is at, say, 4000' (lower), if the Kollsman window is set for local humidity (local sea level pressure). BUT that is not how it works. The local reporting station at the airport will give you a 'fake' altimeter setting, higher than the real sea level setting, whatever it takes to make a standard altimeter read 5000' on that day. So on a warm day at South Lake Tahoe (KTVL, elevation 6000'), you will see that the altimeter setting is higher than the SLP number (after converting millibars to inch of Hg). Your altimeter should always read the correct true altitude, when you are at the same altitude as the reporting station.
Click to expand...

Thanks for the detailed explanation. This is consistent with my initial thinking on how it worked. Given that the local altimeter setting is calculated to give a correct reading in colder temps, why all the warnings about cold weather showing lower than actual altitude readings. It would seem that if the local station is 50% colder than standard and the temp at 4000' AGL is also 50% colder than standard, I would get an altimeter reading in the vicinity of true altitude. I understand that if the temp is 75% colder than standard at 4000' AGL, I will read low and maybe that's where the warning is coming from. Or maybe there is some algebra in there whereby 50% colder than standard at 4000' AGL will still read lower than true altitude eventhough the local station was 50% colder.

Larry
 
lr172 said:
Thanks for the detailed explanation. This is consistent with my initial thinking on how it worked. Given that the local altimeter setting is calculated to give a correct reading in colder temps, why all the warnings about cold weather showing lower than actual altitude readings. It would seem that if the local station is 50% colder than standard and the temp at 4000' AGL is also 50% colder than standard, I would get an altimeter reading in the vicinity of true altitude. I understand that if the temp is 75% colder than standard at 4000' AGL, I will read low and maybe that's where the warning is coming from. Or maybe there is some algebra in there whereby 50% colder than standard at 4000' AGL will still read lower than true altitude eventhough the local station was 50% colder.

Larry
Click to expand...

The issue is that the altimeter has, built into it, a standard model of the atmosphere. By adjusting the Kollsman window you can force the altimeter to read correctly at one altitude in non-standard conditions, but only one. The internal lapse rate model is unchanged. So if going into ktvl (6000') on a warm day, the altimeter will read correctly as you approach 6000'. And because the warm air has expanded up, you will clear the 8000' hill by, say, 1000', even though on a standard day the clearance is just 500'. The problem comes when it is colder than standard. Your altimeter will read 6000' as you approach the airport, if you have the TVL altimeter setting. But as you approach the 8000' hill on the approach, your altimeter will read, say, 9000' (high). If you correct to the published 8500, you'll actually be at 8000 and can hit the hill. So the issue with cold weather approaches is when there are obstacles substantially higher than the airport - your altimeter won't guarantee clearance.
 
BobTurner said:
By adjusting the Kollsman window you can force the altimeter to read correctly at one altitude in non-standard conditions, but only one. The internal lapse rate model is unchanged.
Click to expand...

Ahhh. Now it makes sense. Thanks for the explanation.

Larry
 
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