FAA Aircraft Dispatcher (ADX)MeteorologyHard
An unsaturated air parcel at the surface has a temperature of 30°C. Using the dry adiabatic lapse rate, what temperature would the parcel have after rising to 5,000 feet, assuming it remains unsaturated throughout the ascent?
- A10°C
- B25°C
- C20°C
- D15°C
Show answer & explanationAnswer & explanation
Correct answer: D. 15°C
The dry adiabatic lapse rate is approximately 3°C per 1,000 feet for an unsaturated rising parcel. Over 5,000 feet, the temperature decrease is 3°C x 5 = 15°C. Subtracting this from the starting temperature: 30°C - 15°C = 15°C.
Why the other options are wrong
- A. This would require a cooling of 20°C, exceeding the dry adiabatic rate applied over 5,000 feet.
- B. This uses only about half the correct cooling amount for 5,000 feet of ascent.
- C. This reflects only about 3,300 feet of cooling at the dry adiabatic rate, not the full 5,000 feet.
Dry Adiabatic Lapse Rate
The rate at which an unsaturated rising (or descending) air parcel cools (or warms) due to expansion (or compression), approximately 3°C per 1,000 feet (5.4°F per 1,000 ft).
- Applies only while the parcel remains unsaturated (below the LCL)
- Once saturated, the parcel cools at the slower saturated (moist) adiabatic rate (~1.1-2.8°C/1000ft)
- Used to estimate parcel temperature changes during vertical motion for stability analysis
Memory trick: Dry air cools 3 degrees Celsius every 1,000 feet — 'three's a nice round number for dry air going up.'