Altitude - Density Altitude 4

[hmmy]

Hello all. I am trying to understand the difference between altitude and density altitude. The only difference is that density altitude depends on my position's current air temperature and altitude does not?

If my position's current air temperature is increased, then the density altitude is also increased, but the altitude remains fixed. If my position's current air temperature is decreased, then the density altitude is also decreased, but the altitude remains fixed. I am right?

According barometric formula of wikipedia (

http://en.wikipedia.org/wiki/Atmospheric_pressure#Altitude_atmospheric_pressure_variation),

the altitude depends on my position's current atmoshperic pressure, current seal level atmospheric pressure and current sea level temperature.

According wikipedia (

http://en.wikipedia.org/wiki/Density_altitude#Calculation),

the density altitude depends on my position's current atmoshperic pressure, current seal level atmospheric pressure, current sea level temperature AND MY POSITION'S CURRENT AIR TEMPERATURE.

Have I understood well?

Also, I have created two scripts on matlab which calculate altitude and density altitude. Here are the codes and two simple running of them. Tell me your opinion:

Altitude:

fprintf ( 1,'Hello, I will calculate your altitude.\n' );
cslt=input('Please, give me the sea level temperature (degrees C):');
fprintf ( 1, 'Thank you.\n' );
cslap=input('Please, give me the sea level atmospheric pressure (mb):');
fprintf ( 1, 'Thank you.\n' );
cpsap=input('Please, give me the atmospheric pressure of your position (mb):');
fprintf ( 1, 'Thank you.\n' );
alt=8.31447*(cslt+273.15)*log(cslap/cpsap)/(9.80665*0.0289644);
altf= alt*3.2808399;
fprintf('Your altitude is %d m or %d feet\n', alt, altf);

>> altitude
Hello, I will calculate your altitude.
Please, give me the sea level temperature (degrees C):17
Thank you.
Please, give me the sea level atmospheric pressure (mb):1017.9
Thank you.
Please, give me the atmospheric pressure of your position (mb):999.9
Thank you.
Your altitude is 1.515332e+002 m or 4.971562e+002 feet
>>

Density Altitude:

fprintf ( 1,'Hello, I will calculate your density altitude.\n' );
ts=input('Please, give me the sea level temperature (degrees C):');
fprintf ( 1, 'Thank you.\n' );
ps=input('Please, give me the sea level atmospheric pressure (mb):');
fprintf ( 1, 'Thank you.\n' );
tm=input('Please, give me the temperature of your position (degrees C):');
fprintf ( 1, 'Thank you.\n' )
pm=input('Please, give me the atmospheric pressure of your position (mb):');
fprintf ( 1, 'Thank you.\n' );
da=145442.156*(1-((pm/ps)/((tm+273.15)/(ts+273.15)))^0.234969);
dam= da/3.2808399;
fprintf('Your density altitude is %d m or %d feet\n', dam, da);

>> Density_Altitude
Hello, I will calculate your density altitude.
Please, give me the sea level temperature (degrees C):17
Thank you.
Please, give me the sea level atmospheric pressure (mb):1017.9
Thank you.
Please, give me the temperature of your position (degrees C):23.5
Thank you.
Please, give me the atmospheric pressure of your position (mb):999.9
Thank you.
Your density altitude is 4.146678e+002 m or 1.360459e+003 feet
>>

Soon, I will buy a weather multimeter. Its instructions say: The DENSITY ALTITUDE screen is calculated from the absolute values of station pressure, relative humidity and temperature. And my mind is confused. Why it depends on relative humidity? What formula it uses?

Thank you in advance.

Are you ready for the future?

 

[rb1957]

relative humidity affects local air density

Quando Omni Flunkus Moritati

 

[IRstuff]

"altitude and density altitude" how is this confusing?

altitude is the physical vertical position. density altitude is an estimation of altitude based on a canonical model of the atmosphere as a function of altitude. There are a multitude of different models of the atmosphere; last I checked, there were at least 4, each with slight variations in parameters. Since these are all models, they cannot account for all possible atmospheric conditions, so the estimated altitude can be quite different than reality. There is an apocryphal story that a bunch of F-111 Wild Weasels crashed during the Vietnam War because their barometric altimeters were grossly different than reported by their radar altimeters.

total pressure is a function of the partial pressures of all the constituent gases, including water vapor:

http://en.wikipedia.org/wiki/Partial_pressure

TTFN
faq731-376

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[rb1957]

there's a standard atmosphere that has a "standard" density vs altitude curve. this allows us engineers to compare the loads on a plane flying at altitude with one flying at SL.

"density altitude" is saying (IMHO) that the density of the local atmosphere is equal to such and such altitude given a "standard" atmosphere (and not height above the ground).

altimeters work on local air density. when altitude is critical (eg landing and takeing off) the altimeters are tuned to the local current density (as reported by the control tower).

of course radio altimeters don't need this correction.

personally, i wonder about code that says "Hello, I will calculate your density altitude." ... doesn't sound very professional (IMHO).

Quando Omni Flunkus Moritati

 

[hmmy]

Thank you both for your answers. I think now my mind is better. rb1957 the code is for private use, not for business.

But I want to focus on three subjects:

1) If density altitude depends on humidity, then why wikipedia's formula doesn't use this?

http://en.wikipedia.org/wiki/Density_altitude#Calculation?

Maybe because it is an approximate formula?

2) What is the relation between oxygen and density altitude? Does density altitude give us information about the percentage existence of oxygen in our position? Or it is only depends on altitude?

3) What similarities I will feel if I am in 2.000 m altitude and 2.000 m density altitude?

Are you ready for the future?

 

[MintJulep]

1) Because Wikipedia articles can be written by anyone, regardless of the author's knowledge or qualifications, and therefore should never be thought of as a reliable source of technical information. Note however in the article at hand all of the statements of "assuming dry air".

2) Depending on what you think you mean by "percentage existence of oxygen", yes or no. Since all of the components of air will closely enough follow ideal gas laws over any aerodynamic aviation range of altitudes the percentage of oxygen relative to other components by volume will not change with altitude. That is to say, air is about 78% Nitrogen, 21% Oxygen, 1% Argon, 0.3% Carbon Dioxide and the rest is "other stuff" at any altitude.

3) It entirely depends on how the 2000 meter density altitude is achieved. Look at the "Density altitude calculation chart" in your wiki article to understand why.

 

[IRstuff]

You seem to think there is some sort of "correct" answer; there is none. These equations are all empirical, as demonstrated by the fact that the Wiki article cites two, similar, yet different equations. See:

https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56396/FAA P-8740-02 DensityAltitude

[hi-res]%20branded.pdf Note the rather simplistic treatment of humidity, and this is from the FAA itself.

This is repeated in the Pilot's Handbook:

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/

TTFN
faq731-376

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[rb1957]

1) good grief (that's the short form of the rant intended). though how humidity affects density would be interesting to investigate ... i mean a water molecule weighs less than an oxygen, or nitrogen; maybe it increases the specific heat of the air (so it feels different) ?

3) sounds like you're missing the point ... at 2000m physical altitude the density altitude can be quite different maybe +-200m depending mostly on the local temperature, and the local weather (high or low pressure systems), and to a lesser extent on a bunch of other factors.

Quando Omni Flunkus Moritati

 

[kontiki99]

Going back to OP, altitude is on one hand knowing high the aircraft is above the ground, density altitude is important because aircraft performance varies with changes in density altitude.

There are 5 or 6 altitude definitions, look in FAA-H-8083-3A Airplane Flying Handbook and FAA-H-8083-25A Pilots Handbook of Aeronautical Knowledge and it will explain it all (available on the

http://www).

In theory, there is no difference between theory and practice; experience suggests that in practice, there is.

 

[hmmy]

Thank you all for your helps.

Are you ready for the future?