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8 Answers

From Hot to Cold…

Asked by: 12545 views General Aviation, Student Pilot

When flying from high to low, look out below.  I get this.  If I fail to reset my altimeter, the altimeter will indicate higher than my true altitude.  To main my desired altitude, I will be incline to lower my plane and descend.

The same situation happens when flying hot to cold...indicated will be higher than true.

However, just because there has been a temperature change, does NOT NECESSARILY mean there's been a pressure change.  Therefore, the same adjustments can't be made for temperature error.  I.E. - If no change in pressure exists, the reporting stations will still report the same altimeter (Kollsman window) setting b/c that would not have changed...but the temperature COULD have changed.

There would be increase on my altimeter (if I flew into a colder climate), but it would be SO gradual I would descend to accomdate without realizing it.  But the pressure didn't change and therefore I"d have the same/correct setting.

My questions is, how do you account for this?  I guess when flying into COLD CLIMATES, you would just need to fly at a higher planned altitude.

Also, low pressures makes the altimeter read higher...and cold temperatures make the altimter read higher...However, it's not nessarily accuarate to say therefore that cold temperatures are ALWAYS low pressure, right?  Cold and hot temperatures can exist in both low pressure systems and high pressure systems, right? 

Cold fronts don't HAVE to be TRULY cold cold...they just have to be cooler than the surround air mass...  Same with warm fronts...right?

8 Answers

  1. Best Answer


    Kent Shook on Oct 31, 2010

    I don’t think the temperature changes the barometric pressure as read at sea level (there’s still the same amount of atmosphere above you), but the higher you go, the higher the error due to temperature will be.

    To see how big the error will be, and account for the temperature changes, take a look at Table 7-2-3 in section 7-2-3e of the AIM. I’ve heard that Canadian pilots are required to carry this chart and use it when flying IFR, as they can frequently run into problems with temperature.

    High and low temperatures can and do occur in both high and low barometric pressure systems, and cold fronts are cold relative to the air they’re approaching, so your last two paragraphs/questions, you are right.

    One thing that may be somewhat counterintuitive is that even though cold air is *more dense* than warm air, and low pressure is *less dense* than high pressure, that the “high to low, look out below” works for both of them. The way it was explained to me is that even though cold air is more dense, it causes the atmosphere to “shrink” because of that increased density, so at altitude in cold air, the air will still be less dense than would be expected at that (true) altitude in standard temperatures. That’s also why the error increases with altitude.

    Now, I need a cup of hot chocolate. Brrrrr. 😉

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  2. Brian on Nov 02, 2010

    “However, just because there has been a temperature change, does NOT NECESSARILY mean there’s been a pressure change. ”

    Certainly there are exceptions to the rule, as there are with most rules regarding the physical world. However, when air, in a contained system, is heated, it will expand and ambient pressure will rise; this remains physical fact. The earth isn’t a contained system, hence the exceptions to this rule in this venue.

    That said, we can assume air pressure and temperature to have a direct (they rise and fall together) relationship. After all, Kent & Aviatrix, temperature change has the greatest influence on weather dynamics in part because it changes the airs pressure. The source of this statement is can be found in the first chapter of most books on weather.

    “My questions is, how do you account for this? ”

    VFR Pilot – For the VFR pilot it is quite simple, set your altimeter setting. If you are flying in bad weather, set it more often.

    IFR Pilot – Same as the VFR pilots. However, I believe and IFR pilot should be better read on the intricacies of the subject. This video on [a href=”http://www.robinmaiden.com/2008/01/cold-weather-altimetry/”]Cold Weather Altimetry[/a] provides solid background in my opinion.

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  3. Brian on Nov 02, 2010

    Repost to give a clickable link.
    Cold Weather Altimetry Video – Click here

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  4. Wesley Beard on Nov 09, 2010

    Temperatures affect the altimeter much like pressure does.  The saying holds true for both.  Canadian pilots are required to factor the cold weather temperature into their altitudes they fly.  I would tend to believe they know what they are talking about.  The chart in the AIM is accurate but the US ATC system does not use the chart when assigning altitudes.  It is possible in really cold weather to be only a couple hundred feet above the mountain tops.
     
    Proof:  Look at a GPS approach with LNAV/VNAV minima.  You will see a remark stating the procedure is not authorized above +45 deg C or below -20 deg C indicating the designers of the procedure know the effect temperature has on the approach.  (Those numbers are only an example.)

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  5. Aviatrix on Nov 10, 2010

    Right, but if you have the correct Kollsman window setting (b/c pressure hasn’t changed)…could your altimeter still reflect a change due to cold temperatures? 
    I guess I’m wondering how the altimeter can physically move if there has not been a change in pressure.
       

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  6. Wesley Beard on Nov 10, 2010

    The cold temperature is compensated for when the new altimeter setting is released only at the airport elevation.  The pressure lapse rate changes as the temperature moves farther from the standard (ISA).  We can all agree that the altimeter reports current pressure altitude.
     
    The higher you are the more compensation you will need to ensure adequate terrain separation.  The formula is roughly 4ft per 1000 ft above the airport for each degree colder than standard.  Both Universal and Collins FMS’s have a temperature compensation feature and it adds a considerable more altitude to the FAF altitude (~400 ft) than at DA/MDA (~25 ft).  Here is a useful PPT from the US Air Force on the topic.
    http://44rf.com/misc/USAF_AIS_Cold_WX_Altimeter.ppt

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  7. Pilot143 on Nov 22, 2010

    Kent’s comment:
    “…The way it was explained to me is that even though cold air is more dense, it causes the atmosphere to “shrink” because of that increased density, so at altitude in cold air, the air will still be less dense than would be expected at that (true) altitude in standard temperatures. That’s also why the error increases with altitude.”
    Since the atmosphere shrinks, the plane doesn’t physcially descend along WITH the atmosphere, does it?  It’s not like it’s “riding” on the column of air…at least I don’t think it is.  It’s the altimeter that merely interprets the change and displays an innacurate reading.

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  8. Alex L on Apr 02, 2015

    The answer to this conundrum is in kinetic energy.
    The altimeter is basically a barometer, so it is sensitive to changes in pressure, period.
    Now imagine a balloon filled with standard temperature air. The molecules of air in the balloon dance around, beating against the walls of their container. The force the molecules exert against the walls is called ‘pressure.’
    If you heat the air, the molecules move faster due to increased kinetic energy. They strike the walls with more force = greater pressure.
    If you cool the air, the molecules move slower due to decreased kinetic energy, striking the walls with less force = lower pressure.
    If you fly in hotter than standard air, surrounding air has more kinetic energy, strikes the altimeter’s static ports with more force = higher pressure = indicated altitude is less than true altitude (low to high, clear the sky)
    If you fly in colder than standard air, surrounding air has less kinetic energy, strikes the altimeter’s static ports with less force = lower pressure = indicated altitude is greater than true altitude

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