FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesHard
A meteorologist reports the surface temperature is 25°C, and a radiosonde reading at 5,000 feet shows a temperature of 5°C. Using a dry adiabatic lapse rate of 3°C per 1,000 feet, what is the existing atmospheric stability?
- AUnstable, because the actual lapse rate exceeds the dry adiabatic rate
- BNeutral, because the actual lapse rate exactly equals the dry adiabatic rate
- CStable, because the actual lapse rate is less than the dry adiabatic rate
- DIsothermal, because temperature does not change with altitude
Show answer & explanationAnswer & explanation
Correct answer: A. Unstable, because the actual lapse rate exceeds the dry adiabatic rate
The actual (existing) lapse rate is (25°C - 5°C) / 5,000 ft = 20°C / 5 = 4°C per 1,000 feet. Since this exceeds the dry adiabatic lapse rate of 3°C per 1,000 feet, a parcel of air lifted will remain warmer (less dense) than its surroundings at every altitude, causing it to continue rising—indicating an unstable atmosphere.
Why the other options are wrong
- B. The rates are not equal (4°C/1,000ft vs. 3°C/1,000ft), so this is not neutral.
- C. The actual lapse rate (4°C/1,000ft) is greater, not less, than the dry adiabatic rate.
- D. Temperature clearly decreases with altitude here, so it is not isothermal.
Atmospheric Stability via Lapse Rate Comparison
Atmospheric stability is determined by comparing the actual (existing) lapse rate to the dry adiabatic lapse rate (3°C/1,000 ft); a steeper actual rate indicates instability.
- Dry adiabatic lapse rate = 3°C per 1,000 feet
- Actual lapse rate steeper than DALR = unstable
- Actual lapse rate less steep than DALR = stable
Memory trick: Steeper drop than 3°C/1,000ft = air keeps climbing like a rocket (unstable).