Pressure altitude and Density altitude are critical factors in determining the performance of an aircraft at a given location. Determining both of these values affects overall aircraft safety and health of the crew (meaning that if the aircraft crashes due to poor pilot estimations of aircraft performance they will no longer be healthy) onboard as well as the airworthiness of a specific airframe.

A “Standard Day” is defined as a situation where an aircraft at sea level (where temperature is 59°F) and a barometric altimeter setting of 29.92, the altimeter will read 0 feet.

Pressure Altitude can be found using the tables and method below:

Pressure Altitude in Standard Units


 Altitude      Pressure   Temperature
  (Feet)       (in. Hg)    (Deg. F)
         
        0        29.92       59.0       
    1,000        28.86       55.4        
    2,000        27.82       51.9        
    3,000        26.82       48.3        
    4,000        25.84       44.7        
    5,000        24.89       41.2        
    6,000        23.98       37.6        
    7,000        23.09       34.0        
    8,000        22.22       30.5        
    9,000        21.38       26.9        
   10,000        20.57       23.3        
   11,000        19.79       19.8        
   12,000        19.02       16.2        
   13,000        18.29       12.6        
   14,000        17.57        9.1        
   15,000        16.88        5.5        
   16,000        16.21        1.9        
   17,000        15.56       -1.6        
   18,000        14.94       -5.2        
   19,000        14.33       -8.8        
   20,000        13.74      -12.3        
   25,000        11.10      -30.15        
   30,000         8.89      -47.98
   35,000         7.04      -68.72
   40,000         5.54      -69.70
   45,000         4.35      -69.70
   50,000         3.43      -69.70
   55,000         2.69      -69.70
   60,000         2.12      -69.70
   65,000         1.67      -69.70
   70,000         1.31      -69.70
   75,000         1.03      -69.70
   80,000         0.81      -69.70
   85,000         0.64      -64.80
   90,000         0.50      -56.57
   95,000         0.40      -48.34
  100,000         0.32      -40.11

Pressure Altitude in Metric Units


 Height  Temperature     Pressure  
(Meters)  (Deg. C)    (Millibars-mb)
0000        15.0           1013
1000         8.5            900
2000         2.0            800
3000        -4.5            700
4000       -11.0            620
5000       -17.5            540
6000       -24.0            470
7000       -30.5            410
8000       -37.0            360
9000       -43.5            310
10000      -50.0            260
11000      -56.5            230
12000      -56.5            190
13000      -56.5            170	
14000      -56.5            140
15000      -56.5            120
16000      -56.5            100
17000      -56.5             90	
18000      -56.5             75
19000      -56.5             65
20000      -56.5             55
21000      -55.5             47
22000      -54.5             40
23000      -53.5             34
24000      -52.5             29
25000      -51.5             25
26000      -50.5             22
27000      -49.5             18
28000      -48.5             16
29000      -47.5             14
30000      -46.5             12
31000      -45.5             10
32000      -44.5             8.7
33000      -41.7             7.5
34000      -38.9             6.5
35000      -36.1             5.6

Manually Calculating Pressure Altitude
Field altitude is defined as the height mean sea level of a given airfield. Pressure altitude (PA) can be determined through use of the field level and local barometric pressure. When local pressure is above 29.92 (standard day pressure,) then PA is less than field elevation. The opposite is of correct also, meaning that when local pressure is below 29.92, then PA is greater than Field altitude. For instance:

Field Elevation = 3447
Barometric Pressure = 30.02

Where each .01 inch of Mercury equals 10 feet,
 
   (30.02-29.92) x 100 = 100 

   Pressure Altitude = 3347

         -or-

Field Elevation = 8723
Barometric Pressure = 28.75

   (29.92-28.75) x 100 = 117

   Pressure Altitude = 8840

Pressure Altitude and Your Weather
Using the tables above you should be able to determine the theoretical average barometric pressure for where you live on a non-standard day. For instance, how would you know that there is a low pressure front over Denver?

This is where things become a little bizarre. Looking at the weather for the date that this node was written, Denver's current barometric pressure is 30.12. Why is that when Denver is at an elevation of roughly 5,000 feet above sea level? Shouldn't this reading be somewhere closer to say, 24.89, which is the result provided by the chart above for 5,000 ASL?
The figure that is provided is the corrected Station Pressure which takes the local value (in this case somewhere in the ballpark of 24.89) and then resets it to 29.92, which we figured out earlier was 0 feet on an altimeter at sea level with 59°F.

With this in mind, you can now determine what your barometric pressure is doing relative to our benchmark of 29.92. From that is becomes clear that there is no low pressure front over Denver, but what appears to be a slight rise in barometric pressure.

Sources:
www.lineclear.com/pdf-files/unitedperf.pdf
virtualskies.arc.nasa.gov/weather/ tutorial/tutorial2f.html
www.nw.faa.gov/ats/zdvartcc/high_mountain/density.html
A1-SRRPV-NFM-500 Pilot’s Checklist for the RQ-2 Pioneer Unmanned Aerial Vehicle.
(Note 1: Yes…I used my PCLagain.)
(Note 2:m_turner- I am NOT turning this into a clearinghouse of weather updates on Denver for your benefit. Just this once.)

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