FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A dispatcher is briefing a crew on conditions favorable for persistent condensation trails (contrails) forming behind an aircraft at cruise altitude. Under which conditions are persistent contrails most likely?
- ACold temperatures and high relative humidity in the upper troposphere
- BWarm, dry air at low altitude with low relative humidity
- CDry adiabatic subsidence near the tropopause
- DUnstable air with strong convective activity below 18,000 feet
Show answer & explanationAnswer & explanation
Correct answer: A. Cold temperatures and high relative humidity in the upper troposphere
Persistent contrails form when engine exhaust adds water vapor and heat to air that is already very cold and near saturation with respect to ice; if the ambient air is sufficiently humid, ice crystals persist and spread rather than evaporating quickly. Warm, dry, or subsiding air does not support persistence.
Why the other options are wrong
- B. Warm, dry, low-altitude air is too warm and too dry to support ice crystal persistence.
- C. Subsiding, drying air causes contrails to evaporate quickly rather than persist.
- D. Convective activity is unrelated to jet-engine contrail formation.
Contrail Formation
Condensation trails form when hot, moist engine exhaust mixes with cold ambient air; persistence requires the surrounding air to already be near ice saturation.
- Requires cold temperatures typically below -40°C
- Persistent contrails indicate high relative humidity aloft
- Non-persistent (short) contrails indicate dry air at altitude
Memory trick: Cold + wet sky = contrail stays; warm + dry sky = contrail fades fast.