Altitude - Density Altitude

[Konstantinos]

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.

Edited April 13, 2013 by Konstantinos

[pwagen]

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?

 

From my reading of it, almost. But it's actually a bit simpler than what you seem to think here. While the formula uses both sea level temperature and sea level pressure, both of these are the "standard" values (I'm guessing average or mean values). So both are constant, which makes DA vary only due to your current air pressure and temperature.