# Pressure Altitude <-> Desnsity Altitude Conversion

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### Pressure Altitude <-> Desnsity Altitude Conversion

The equation is DA = PA +/- 120 횆T (횆T is ICAO Temp corrected for a particular PA compared to the ICAO standard atmosphere - which is approximately 1013mb and 15째C. ICAO air also cools or warms at 2째C per thousand feet and 1mb pressure change is equal to 30 feet ascent or descent)

If the actual outside air temp is 15째C and the pressure is 1013mb then PA = 0ft

i.e. Lets say the current pressure is 1029 and the temp is minus 4째C...

DA = ( PA ) +/- 120횆T
DA = (1013-1029 (x 30)) +/- 120횆T
DA = ( -480 ) +/- 120횆T

You have to work out PA so you can adjust 횆T (which is 15째C if PA is 0 but is lower than 15째C if PA is greater than 0 and higher than 15째C if PA is less than 0 i.e. negative figures)

So as 횆T is 15째C if PA is 0 it either cools or warms at a rate of 2째C per thousand feet (or part therof).

In our example we are at -480ft PA which means that (approximately!) it will be warmer by 1째C than ICAO therefore = 16째C (I'm sure you can make this more accurate with your converter equation!!)

Now 횆T = 16째C minus -4째C (actual air temp) = 20

Therefore:
DA = ( -480 ) +/- 120 x 20

Is it plus or minus 120 x 20? - It's Plus if the actual temp is greater than 15째C and it's minus if the actual temp is less than 15째C - so in this case we minus it:

DA = ( -480 ) - 120 x 20
DA = -480 - 2400
DA = -2800ft

Voila!

Hope this helps the equation writing.....

PPI Zulu

P.S. 횆T should read 'Delta T' as in the Greek fourth letter (a triangle) but it wont appear in this post correctly.
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