FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesEasy
A pilot studying atmospheric properties recalls that the International Standard Atmosphere defines an average rate at which temperature decreases with altitude throughout the troposphere. What is this standard (average) lapse rate?
- A2°C per 1,000 feet
- B3°C per 1,000 feet
- C1°C per 1,000 feet
- D3.5°C per 1,000 feet
Show answer & explanationAnswer & explanation
Correct answer: A. 2°C per 1,000 feet
The ISA standard lapse rate is approximately 2°C (3.5°F) per 1,000 feet, equivalent to 6.5°C per kilometer, used as the reference for pressure altitude and temperature calculations.
Why the other options are wrong
- B. 3°C/1000ft overstates the standard rate; this exceeds even the dry adiabatic rate for some layers.
- C. 1°C/1000ft is too shallow; this is roughly the dry adiabatic lapse rate divided incorrectly.
- D. 3.5°C/1000ft is the Fahrenheit equivalent value, not the Celsius standard lapse rate.
Standard (ISA) Lapse Rate
The average decrease in temperature with altitude in the troposphere under the International Standard Atmosphere model.
- Approximately 2°C (3.5°F) per 1,000 feet
- Equivalent to 6.5°C per kilometer
- Basis for pressure altitude and altimeter calibration
Memory trick: 'Two degrees per thousand' — think of climbing two flights and losing two degrees.