Get the difference between your current QNH & the standard sea level QNH (1013.25) … eg. AD QNH = 1006, means there is a difference of 7 hPa, which equates to a 210 ft difference (calculated by taking the calculated 7 hPa and multiplying it by 30 (30ft difference per hPa)
Once you have 210 as the height difference, you add (if your QNH is less than standard sea level pressure), or subtract it if it’s higher (eg. if the QNH is showing you’re actually less than sea level pressure) it to your current elevation (eg. 3310 ft).
The overall formula can be shown as;
Pressure Height = Elevation + 30 * (1013 – QNH)
Eg. At an elevation of 3310, with QNH of 1006.
- Pressure difference is 1013 – 1006 = 7 hPa difference.
- We then get the pressure height difference in feet … 7 * 30 = 210
- Then add it to our current height … 210 + 3310 = 3520 feet.
Density altitude in feet = pressure altitude in feet + (120 x (OAT – ISA_temperature))
ISA_temperature = the ISA temp at the altitude you are at … calculated by using;
ISA Temperature: Temperature changes at the rate of 2 degrees per thousand ft (gets colder as you go up, and gets warmer as you descend). The standard sea level ISA temp is 15 degrees, so you will need to subtract the temperature difference from 15 degrees to get the ISA temp…
Eg. The aerodrome (AD) is 3310 feet above mean sea level (ASML), and the temp is 28 degrees.
- The ISA temp at 3310 is 3310 / 1000 = 3.31 …. * 2 = 6.62 …. 15 – 6.62 = 8.38 degrees.
- We then need to find out the temperature deviation from the norm = 28 (our temp) – 8.38 (ISA temp at our altitude) = 19.62 … (rounded to the nearest degree gives 20 degrees).
- From here, we need to use the temp deviation to find out our density height. This is done by the following;
ISA Temp * 120 (120 ft per degree) = 20 * 120 = 2400
- Then add 2400 to our pressure height (3520 in this case .. see the above section for workings) which gives us 5,920 ft.