The International Standard Atmosphere is the shared yardstick behind almost every altitude, speed and performance figure a pilot uses, and knowing what it actually says makes the day's corrections far easier to reason about.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
A model, not the weather
The International Standard Atmosphere (ISA) is a defined, idealised model of how pressure, temperature and density change with height. It is not a forecast and not an average of any real day. It exists so that altimeters, airspeed indicators, performance charts and engineering data all use one agreed reference, instead of every manufacturer and authority choosing their own.
ICAO publishes it as Doc 7488, and the same model underpins the performance reference conditions in ICAO Annex 8. Because it is a model, the real atmosphere almost never matches it exactly, and the difference between the two is the whole reason corrections exist.
The sea-level values
The ISA fixes mean sea level at three reference values, described in the FAA Pilot's Handbook of Aeronautical Knowledge and the Aviation Weather Handbook:
- Temperature: 15 degrees Celsius (59 degrees Fahrenheit).
- Pressure: 1013.25 hectopascals (29.92 inches of mercury).
- Density: about 1.225 kilograms per cubic metre.
These are the numbers behind a standard altimeter setting of 1013 hPa or 29.92 inHg, and behind the "standard day" assumption printed on most performance charts.
The lapse rate and the tropopause
From sea level upward, the standard temperature falls at a steady lapse rate of about 1.98 degrees Celsius per 1000 feet, which almost everyone rounds to 2 degrees per 1000 feet for mental maths. That holds up to the standard tropopause at about 36 090 feet (11 km), where the standard temperature reaches about minus 56.5 degrees Celsius.
Above the tropopause, in the standard lower stratosphere, the model holds the temperature constant at that minus 56.5 degrees rather than continuing to fall. So the simple "2 degrees per 1000 feet" rule only applies in the troposphere, below roughly 36 000 feet.
ISA deviation: the number that actually matters
On a real day the air is rarely standard, and the useful quantity is the ISA deviation: the actual temperature minus the standard temperature at the same level. It is written as ISA plus or ISA minus a number of degrees.
A worked example, using the rounded lapse rate:
- At 10 000 feet, the standard temperature is 15 minus (10 times 2), or minus 5 degrees Celsius.
- If the actual outside air temperature is minus 15, you are ISA minus 10 (10 degrees colder than standard).
- If it is plus 5, you are ISA plus 10 (10 degrees warmer than standard).
That single figure feeds straight into the corrections pilots care about. Warm air (ISA positive) is less dense, which raises density altitude and stretches takeoff and climb performance. Cold air (ISA negative) makes the altimeter over-read true altitude, which is why cold-temperature altitude corrections exist. ISA deviation also appears in true airspeed and Mach calculations, because the speed of sound depends on temperature.
Common pitfalls
- The 2-degrees rule stops at the tropopause. Above about 36 090 feet the standard temperature is held constant, so do not keep subtracting.
- ISA is not an average day. It is a fixed reference. Treating it as "normal weather" leads to under-correcting on genuinely hot, cold or high days.
- Mind the sign. ISA deviation is actual minus standard. Warmer than standard is ISA plus; colder is ISA minus.
In Pilot EFB
Pilot EFB includes the unit and altitude conversions and the density-altitude and cold-temperature corrections that all lean on the standard atmosphere, with the working shown so you can follow each step rather than trust a single output. The figures are for your own planning and reference; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so verify against your aircraft's approved data and current official sources before you act on them.