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Altitude difference due to temperature

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Altitude difference due to temperature

Postby legoboyvdlp » Sat Jun 13, 2020 3:26 pm

There is a rather bizarre altitude difference here between /position/altitude-ft and /instrumentation/altimeter/indicated-altitude-ft

Consider what happens if you climb to altitude from Dubai, presently Q0996 and 37 degrees, using live weather and advanced weather.

Image

As you can see there is a difference of some 2200 feet. This does cause some issues at times, which are not the subject of this post.

Setting temperature to 15 degrees (ISA conditions) results in a difference of 300 feet which sounds about right for the pressure difference between Q0996 and Q1013. So it appears, that these altitude deviations are due to temperature.

I just would like some assurance that this is to be expected, because this results in large deviations which are not seen in basic weather.

I would also suggest that if what advanced weather is doing is indeed correct, such offsets due to temperature would be much better to do in the C++ environment code rather than in advanced weather.
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Re: Altitude difference due to temperature

Postby GinGin » Sat Jun 13, 2020 5:22 pm

Hello Lego.

That might be possible.
37 degrees at Dubai corresponds to an ISA + 22 ° at Sea Level.


True Altitude(QNH) = altitude QNH + Delta Altitude temp

Quick rule of thumbs for Delta altitude due to Isa deviation:

( 4 feet per 1000feet of alti layer per 1° of ISA deviation)


If ISA deviation is the same at FL 370
DeltaAltitude would be : DeltaAlti = 4 x 34 x 22 = 2816

Altitude QNH = Pressure alti + (996-1013) x 28 = 33500

True Altitude = 33500 +2816 = 36316 feet, quite close to the true altitude indicated in your datas
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Re: Altitude difference due to temperature

Postby legoboyvdlp » Sat Jun 13, 2020 5:39 pm

I see, thank you for confirming this.
Does this rule of thumb apply to the entire lower atmoshpere (from 0 to 35000 feet)?

Edit - and on a ads b website I see 43,625 feet at present, 1625 feet higher than barometric 41000 feet :mrgreen:

So obviously it does!

I wonder why no other simulator does this...
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Re: Altitude difference due to temperature

Postby Isaak » Sat Jun 13, 2020 6:05 pm

Thanks for the clarification! Indeed sad that no other sim simulates this, I've had the occasional TCAS warning for traffic 2000ft pressure altitude lower than me on Vatsim.
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Re: Altitude difference due to temperature

Postby GinGin » Sat Jun 13, 2020 6:20 pm

legoboyvdlp wrote in Sat Jun 13, 2020 5:39 pm:I see, thank you for confirming this.
Does this rule of thumb apply to the entire lower atmoshpere (from 0 to 35000 feet)?




Yes, quite efficient formula for quick calculation :)
Really interesting for minus ISA actually, where the margin with ground is not the one you think
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Re: Altitude difference due to temperature

Postby Thorsten » Sun Jun 14, 2020 7:16 am

I would also suggest that if what advanced weather is doing is indeed correct, such offsets due to temperature would be much better to do in the C++ environment code rather than in advanced weather.


The modeling of the atmosphere structure with altitude is (and always has been) computed on the C++ side, AW just sets the ground parameters but does not set any pressure or temperature at altitude.

AW does compute the altitude dependence of visibility (or, if so desired winds) as these parameters are not set by the C++ atmosphere routines.

I just would like some assurance that this is to be expected


It's how a hot air ballon works - temperature directly influences the air density at given pressure, which is why the air inside the ballon is lighter than outside. Now, if you heat the whole atmosphere, it makes a difference for the weight of the air column above you (which in essence is the pressure) whether you stack hot or cold air.
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Re: Altitude difference due to temperature

Postby Johan G » Sun Jun 14, 2020 2:15 pm

This effect, a higher temperature causing a less dense air, in turn causing a higher pressure altitude, can be dangerous. Consider for example taking a light aircraft to a remote airstrip in mountainous terrain early in the morning, spending a nice hot summer day day swimming and fishing in some desolate creeks and then in the afternoon taking off for the return trip (with some extra weight).

Some older posts with interesting videos:
Johan G wrote in Thu Aug 06, 2015 3:46 pm:Density altitude and why it matters.

In short, higher temperatures will cause a higher density altitude, which will make the air behave as if your aircraft was at a (sometimes much) higher pressure altitude. This will degrade the performance in many ways. You will have less lift, less efficient propellers and less efficient combustion and in addition to that you will also have a higher stall speed.

[...]

Johan G wrote in Wed Mar 25, 2015 11:51 am:One of the more scary take offs I have seen. High density altitude evening take off by the numbers, but with too little margin (in essence with numbers by the POH//flight manual, but without taking into consideration that those numbers was developed with a factory new aircraft and engine).

[...]
Low-level flying — It's all fun and games till someone looses an engine. (Paraphrased from a YouTube video)
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Some YouTube videos
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Re: Altitude difference due to temperature

Postby V12 » Tue Jul 21, 2020 4:36 am

lego & Isaak :
Other sims can use wind aloft with real tempetature and real ISA dev. In standard, FG do not use wind aloft data and assumes that ISA dev converges to 0 at tropopause (36000 feet).

In my P3D + ActiveSky flights I monitor corelation of the TAT, SAT and ISA dev on mCDU with real SAT from windy.com. Yesterday I observed +22 ISA dev at Dakar and -10 at FL410 over Noadibhu.
When I used FG and Concorde, I never seen ISA dev other than zero above tropopause.
Last edited by V12 on Tue Jul 21, 2020 10:53 am, edited 1 time in total.
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Re: Altitude difference due to temperature

Postby Thorsten » Tue Jul 21, 2020 6:04 am

Lucky you.

(If you buy commercial weather data, you get commercial weather data - that's all there is to the explanation)
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