FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesEasy
A pilot studying the International Standard Atmosphere (ISA) needs to recall the reference values used as the baseline for altimeter and performance calculations. What are the standard sea-level temperature, pressure, and lapse rate?
- A59°F, 29.92 in. Hg, and 2°C increase per 1,000 feet
- B15°C, 29.92 in. Hg, and 2°C decrease per 1,000 feet
- C0°C, 29.92 in. Hg, and 3.5°C decrease per 1,000 feet
- D15°C, 30.00 in. Hg, and 1°C decrease per 1,000 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 15°C, 29.92 in. Hg, and 2°C decrease per 1,000 feet
The International Standard Atmosphere defines sea-level standard conditions as 15°C (59°F) and 29.92 in. Hg, with temperature decreasing at approximately 2°C per 1,000 feet up to the tropopause.
Why the other options are wrong
- A. Temperature decreases, not increases, with altitude in the standard atmosphere.
- C. 0°C is not the standard sea-level temperature, and 3.5°C/1,000 ft is not the standard lapse rate.
- D. 30.00 in. Hg and 1°C/1,000 ft are not standard ISA values.
International Standard Atmosphere (ISA)
A theoretical model of atmospheric conditions used as a baseline reference for aircraft instruments and performance calculations.
- Sea-level standard temperature: 15°C (59°F)
- Sea-level standard pressure: 29.92 in. Hg (1013.25 hPa)
- Standard lapse rate: approximately 2°C decrease per 1,000 ft
Memory trick: 15 and 29.92 — the sky's baseline to compare to