I have found out that an ISA atmosphere is:

  • 15 °C temperature
  • 5 °C dewpoint
  • 1013.25 hPa pressure

What are the conditions of an ISA +10, ISA -15 and ISA -30? I know the temperature, but what are pressure and dewpoint?

I need to create those conditions to do some flight testing in a simulator.


2 Answers 2


First of all:

I have found out that an ISA atmosphere is: 15 degC temperature 05 degC dewpoint 1013.25 pressure

Not entirely correct. The temperature and pressure are ISA values at mean sea level. These alter with altitude.
Dewpoint is not defined in ISA, as ISA does not contain dust, moisture and water vapor.
ISA-30 atmosphere means temperature deviation from ISA temperature. This means that on a given altitude, temperature is 30 degrees colder than in ISA. Respectively ISA+10 states 10 degrees warmer.

Below is a table of ISA values. enter image description here

For more physics for calculations, here is a very nice pdf on the matter.


ISA+X is the ISA atmosphere with X°C of variation.

It´s used to determine how diferent (in terms of Temperature) of the Standard the actual condictions are and adjust the calculations.

ISA as you said is:

+15°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft

So, ISA +5 is:

+20°C / 0ft / 1013.2 hpa / LAT 45° / Term. Grad. -2°C/1000ft

The pratical use is like the following:

If you are at FL075 and the OAT is -15°C whats your difference to ISA?

ISA Air Temp. @ FL075 is:
ISAt = 15 - 2 * alt/1000
ISAt = 15 - 2 * 7500/1000
ISAt = 15 - 2 * 7.5
ISAt = 15 - 15
ISAt = 0°C

So you are 15°C below the ISA temp at your FL or simply ISA-15.

  • 3
    $\begingroup$ To be exact, lapse rate is 1,98 °C / 1000ft. $\endgroup$
    – Sami
    Apr 5, 2017 at 13:58
  • $\begingroup$ So only temperature varies and not sealevel pressure? $\endgroup$
    – Invariant
    Apr 5, 2017 at 14:17
  • 2
    $\begingroup$ @Invariant the ISA+X is the deviation of temperature to the ISA Ceteris Paribus. This is because while Flying QNE you "simulate" flying ISA assuming that the Pressure is 1013.25hpa $\endgroup$ Apr 5, 2017 at 15:05
  • 1
    $\begingroup$ @Invariant The sea level pressure in ISA is always 1013,25hPa. When the atmosphere temperature changes (ie. ISA+25) also the pressure on other altitudes changes. Check the pdf on my answer for the formula. $\endgroup$
    – Sami
    Apr 5, 2017 at 15:47
  • 1
    $\begingroup$ @AndreasAlmeida QNE is not an altimeter setting. QNE means "threshold altitude using standard pressure setting". $\endgroup$
    – Sami
    Apr 5, 2017 at 16:01

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