FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A pilot report describes moderate turbulence encountered at 3,000 feet AGL while flying over hilly terrain during a period of strong, gusty surface winds. No convective activity is present in the area. What is the most likely cause of this turbulence?
- AClear air turbulence associated with jet stream wind shear
- BMechanical turbulence caused by wind flow disrupted by irregular terrain
- CWake turbulence generated by a preceding aircraft
- DThermal turbulence caused by strong surface heating
Show answer & explanationAnswer & explanation
Correct answer: B. Mechanical turbulence caused by wind flow disrupted by irregular terrain
When strong winds flow over rough or irregular terrain such as hills, ridges, or buildings, the airflow becomes disrupted into eddies, producing mechanical turbulence at low altitudes. This is distinct from thermal turbulence (convective heating) or clear air turbulence (high-altitude jet stream shear).
Why the other options are wrong
- A. CAT occurs at high altitudes near the jet stream, not at 3,000 feet AGL over terrain.
- C. Wake turbulence is aircraft-generated and would not be described as related to terrain and wind.
- D. Thermal turbulence requires surface heating/convection, which is absent here.
Mechanical Turbulence
Turbulence caused by the disruption of smooth airflow as wind passes over irregular terrain, buildings, or other obstacles, most common at low altitudes with strong winds.
- Requires both strong wind and rough/irregular terrain
- Produces low-level eddies and gusty, choppy turbulence
- Common in mountainous or hilly terrain with strong surface winds
Memory trick: Mechanical = wind Mashed by terrain into eddies