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PPL · EASA 050 · The atmosphere
The ISA temperature at an altitude
In the International Standard Atmosphere (ISA), what is the temperature at 7 000 ft?
Explanation
The ISA starts at +15 °C at mean sea level, and up to the tropopause its temperature falls by 1.98 °C per 1 000 ft, which exams round to 2 °C. Take the drop over the altitude away from +15 °C. In the formula, T_ISA is the ISA temperature in °C and h the altitude in feet.
Step by step
- At sea level the ISA temperature is +15 °C.
- Drop over 7 000 ft: 7 × 2 = 14 °C.
- 15 − 14 = +1 °C.
Why not the other answers
- −6 °C uses 3 °C per 1 000 ft. That is the dry adiabatic lapse rate, the cooling of a parcel of unsaturated air as it rises, not the ISA lapse rate of 2 °C per 1 000 ft.
- +29 °C adds the drop instead of taking it away. In the troposphere the temperature falls as you climb.
- −14 °C is only the drop. It leaves out the +15 °C the ISA starts with at sea level.
Tip. Quick check: the ISA passes 0 °C at about 7 500 ft, so anything higher is below zero in the standard atmosphere.
Original questions written by FlightDesk Aero and aligned with the EASA syllabus. They are not questions from the official EASA exam bank.