FAA Instrument Flight Instructor (FII)Instruments and SystemsMedium
Under standard atmospheric conditions, sea-level temperature is 15°C. Using the standard lapse rate, what is the expected standard temperature at 18,000 feet MSL?
- A-27°C
- B-24°C
- C-18°C
- D-21°C
Show answer & explanationAnswer & explanation
Correct answer: D. -21°C
The standard atmosphere lapse rate is approximately 2°C per 1,000 feet up to the tropopause. At 18,000 feet: 18 × 2°C = 36°C decrease from the sea-level standard of 15°C, giving 15 − 36 = −21°C.
Why the other options are wrong
- A. -27°C would require a lapse rate of about 2.33°C per 1,000 feet, which is incorrect.
- B. -24°C overstates the lapse rate beyond the standard 2°C per 1,000 feet.
- C. -18°C would result from a lapse rate closer to 1.83°C/1000 ft, not the standard 2°C/1000 ft.
Standard Atmosphere Lapse Rate
In the International Standard Atmosphere, temperature decreases at approximately 2°C per 1,000 feet from a sea-level standard of 15°C, up to the tropopause at about 36,000 feet where standard temperature is −56.5°C.
- Sea-level standard: 15°C and 29.92 inHg
- Lapse rate: approximately 2°C per 1,000 feet
- Tropopause standard temperature: approximately -56.5°C at ~36,000 ft
Memory trick: Two degrees colder every thousand feet, like climbing into a freezer