FAA Aircraft Dispatcher (ADX)MeteorologyHard
A dispatcher uses a Skew-T log(P) diagram to lift a surface air parcel dry adiabatically starting at a temperature of 25°C and a dew point of 13°C. Using a dry adiabatic lapse rate of 3°C per 1,000 feet and a dew point lapse rate of 0.5°C per 1,000 feet, at approximately what height will the lifted condensation level (LCL) occur?
- A3,600 feet
- B4,800 feet
- C6,000 feet
- D2,400 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 4,800 feet
The temperature-dew point spread is 25°C − 13°C = 12°C. As the parcel rises, temperature falls 3°C/1,000 ft while dew point falls 0.5°C/1,000 ft, closing the spread at a net rate of 2.5°C/1,000 ft. The LCL height equals 12°C ÷ 2.5°C per 1,000 ft = 4.8, or approximately 4,800 feet, where temperature and dew point converge and saturation occurs.
Why the other options are wrong
- A. This value would result from using an incorrect spread or convergence rate.
- C. This overestimates the height needed for temperature and dew point to converge.
- D. This underestimates the convergence height using an incorrect combined lapse rate.
Lifted Condensation Level (LCL)
The altitude at which a rising, unsaturated air parcel cools to its dew point, forming the base of a cumulus cloud.
- Temperature falls at dry adiabatic rate (~3°C/1,000 ft)
- Dew point falls more slowly (~0.5°C/1,000 ft)
- LCL height = spread ÷ convergence rate ×1,000 ft
Memory trick: Spread ÷ 2.5 ×1000 = where the cloud base begins.