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

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

i.e. Lets say the current pressure is 1029 and the temp is minus 4Â°C...

DA = ( PA ) +/- 120Ã„T

DA = (1013-1029 (x 30)) +/- 120Ã„T

DA = ( -480 ) +/- 120Ã„T

You have to work out PA so you can adjust Ã„T (which is 15Â°C if PA is 0 but is lower than 15Â°C if PA is greater than 0 and higher than 15Â°C if PA is less than 0 i.e. negative figures)

So as Ã„T is 15Â°C if PA is 0 it either cools or warms at a rate of 2Â°C per thousand feet (or part therof).

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

Now Ã„T = 16Â°C minus -4Â°C (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 15Â°C and it's minus if the actual temp is less than 15Â°C - 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. Ã„T should read 'Delta T' as in the Greek fourth letter (a triangle) but it wont appear in this post correctly.