Work out the standard temperature for any pressure altitude and how far the real air sits above or below it — the starting point for density altitude and true airspeed.
Enter the pressure altitude and the actual outside air temperature (OAT). SkyStudy returns the standard temperature at that level and how far the real air is from it.
Enter a pressure altitude and outside air temperature to see the ISA temperature and the deviation.
ISA+10 means the air is 10 °C warmer than standard at that level. Positive deviations mean thinner air and reduced performance.
Use the ISA deviation here as the input to the density altitude and true airspeed calculators for a full performance picture.
The same lapse-rate maths appears throughout ATPL Meteorology and Performance — practise the mental model, then check it here.
ISA deviation is the difference between the actual outside air temperature and the International Standard Atmosphere temperature for that pressure altitude. ISA+ means warmer than standard, ISA- means colder.
SkyStudy uses the ISA lapse rate of about 1.98 °C per 1000 ft from 15 °C at sea level, held at -56.5 °C at and above the tropopause (around 36,090 ft).
ISA deviation feeds true altitude corrections, density altitude, and true airspeed. Warmer-than-standard air is less dense, so it degrades performance and makes the aircraft true-altitude higher than indicated.
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