Welcome Guest. Sign in or Signup

3 Answers

True altitude vs. Indicated nonstandard temperature

Asked by: 1864 views
Aircraft Systems, Weather

So, I understand that in colder-than-standard temperatures, your true altitude is lower than your indicated and vice versa for warmer-than-standard. I want to know why.

Doesn't warmer air decrease air density due to the molecules moving around much more and thus causing the altimeter to read a higher altitude? Looking at the sealed aneroid wafers located in the altimeter, when there are less molecules of air in the instrument the air in the wafers want to seek balance and will expand to meet the lower air density in the case... if this were to happen wouldn't the altimeter read higher?

Warmer air also decreases aircraft performance if I am not mistaken and thus how would this allow our aircraft to be at a higher true altitude than on a colder day with more dense air giving us better performance?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers

  1. Best Answer


    John D Collins on Dec 11, 2022

    The altimeter measures air pressure calibrated to read the altitude above sea level based on a standard atmosphere with standard pressure and temperature lapse rates. The lower the pressure, the higher the altimeter indication. A higher temperature increases the air pressure. So if the temperature rises above the standard value for the atmosphere at a given height above sea level, the higher pressure will cause the altimeter to read a lower value, in other words, the true altitude will be higher than the indicated altitude. The opposite is true for a temperature lower than the standard value at a given height above sea level. The colder temperature will reduce the air pressure, so the altimeter will indicate a higher altitude.

    The barometric altimeter is not corrected for non standard temperatures. On very cold days the altimeter will read higher than your true altitude and this can lead to being closer to the ground on an instrument approach, with greater risk of colliding with terrain or obstacles. Some airports in the US designated as Cold Temperature airports require the pilot who is conducting approaches at these airports to adjust the minimums to account for the temperature.

    A altitude determined by GPS is not affected by temperature. During the winter you will notice your barometric altimeter tends to indicate above the GPS altitude and during the hot months of summer, tends to indicate below the GPS altitude. The two tend to agree when on the ground at the airport, but as you climb the barometric altimeter introduces more and more error due to the temperature and the GPS error remains the same, so the two indications diverge.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  2. KDS on Dec 11, 2022

    Look at Chapter 5 in your Aviation Weather book (AC 00-6B).

    You can find that AC online here:

    https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_00-6B.pdf

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  3. Bryan on Dec 11, 2022

    You’re giving the altimeter too much credit. You can adjust your altimeter, right? For what? Non-standard pressure. But there’s no adjustment for non-standard temperature, right? Why? Because the instrument can’t account for it. As clever as the altimeter is, it doesn’t know it can’t correct for non-standard temperatures. It just measures the pressure like it was told to. It doesn’t even know it might be lying to you.

    If you’re flying around in a standard atmosphere (29.92 and 15C), your altimeter is bang on. If you keep the 15C but change the pressure, you can adjust the altimeter to stay accurate. But the further you wander away from 15C, the greater error the altimeter will DISPLAY. So while you know that -10C outside means the air is more dense than the same amount of pressure at 15C, your altimeter doesn’t. It can only show you what the altitude would be at the pressure it observes if the temperature were 15C. As you noted, an air mass of the same pressure but warmer temperature is less dense so the altimeter shows a higher altitude than is accurate. That’s why your true altitude is lower than your indicated altitude in colder than standard temperatures. Flip the whole thing around for warmer than standard–but you don’t hear about that as much because it doesn’t have the tendency to kill you. =)

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.