ISA temperature by altitude
The International Standard Atmosphere, usually shortened to ISA, gives pilots and aircraft manufacturers a common reference for temperature, pressure, and density. At mean sea level the standard temperature is 15°C. Through the lower atmosphere, standard temperature decreases at about 1.98°C per 1,000 feet, commonly rounded to 2°C per 1,000 feet for quick pilot math.
Quick ISA temperature rule
A useful estimate is ISA temperature = 15°C minus 2°C for every 1,000 feet of altitude. At 5,000 feet, the quick estimate is about 5°C. At 10,000 feet, it is about -5°C. The exact standard-atmosphere relationship is slightly more precise, but the shortcut is useful for mental checks.
What ISA deviation means
ISA deviation compares actual outside air temperature with the standard temperature expected at that altitude. If ISA temperature is 5°C and the reported temperature is 25°C, the atmosphere is ISA +20. If the actual temperature is -5°C, conditions are ISA -10.
Why pilots care
Temperature affects air density, engine performance, propeller performance, true airspeed, and many aircraft performance charts. A warmer-than-standard day generally raises density altitude. That can increase takeoff distance and reduce climb performance. Turbine performance information also commonly uses ISA deviation.
ISA and pressure altitude
For performance work, altitude references matter. Pressure altitude is the pressure surface used for standard-atmosphere comparisons, while indicated or field elevation may differ depending on the altimeter setting. When a POH or AFM asks for pressure altitude and temperature, use those exact inputs rather than substituting another altitude.
ISA +20 at 7,000 feet
At 7,000 feet, the quick ISA estimate is about 1°C. If outside air temperature is 21°C, that is approximately ISA +20. That does not directly tell you takeoff distance or climb rate, but it immediately tells you the atmosphere is much warmer than standard and performance deserves careful attention.
Related PilotDesk tools
Density altitude calculator · Pressure altitude calculator · Airport weather
ISA temperature questions
What is ISA temperature at 5,000 feet?
Using the common 2°C-per-1,000-foot training estimate, about 5°C.
What does ISA +20 mean at 8,000 feet?
ISA at 8,000 feet is about −1°C using the quick estimate, so ISA +20 is about 19°C.
Do I use field elevation or pressure altitude for ISA comparisons?
Use the altitude reference required by the performance chart or method. Aircraft performance charts commonly use pressure altitude, so do not silently substitute field elevation.