Defining standard atmosphere
An ideal understanding of an environment that is anything but ideal.
Air temperature below about 33,000 ft decreases, on average, at a relatively constant rate of about 2°C/1000 ft of increasing altitude. This lapse rate permits us to estimate ISA air pressure and air density at altitudes above sea level. Deviations from this lapse rate mean temperature corrections must be applied to pressure altitudes to interpret aircraft performance accurately.
If you are using meters and degrees Celsius, the temperature multiplier is 36.6, and if using feet and degrees Celsius, use 120 as the multiplier. This estimate won't be exact, but it will be a good approximation that can help you plan.
Remember, if you are not at sea level, ISA temperature is determined by starting with the sea-level ISA temperature—59°F (15°C)—and subtracting 3.6°F for each 1000 ft of pressure altitude (6.5°C/1000 m). Also, if you expect temperatures to heat up a bit before your flight, add a few degrees to the calculation rather than using the current temperature.
The ISA is a comprehensive snapshot of the average worldwide atmospheric temperature, pressure and density. Not only has it allowed aircraft designers to obtain detailed estimates of aircraft performance before the aircraft is ever even constructed—it also lets pilots plan flights in a variety of aircraft because the performance relationships are all based on one set of atmospheric standards.
The limitation, however, is that the ISA is an estimated average—conditions that rarely occur in the real atmosphere. We need to recognize first that these values are estimates based on a standard temperature lapse rate, and, second, that any deviation in temperature from ISA will change the performance characteristics of the aircraft by changing the density altitude.
Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.