Turbulence causes
Turbulence = rough air: irregular aircraft motion, especially rapid up-and-down, from rapid variation in atmospheric wind velocities. Effect varies with aircraft size (FAA AWH 19.1). Classify by cause:
Mechanical: wind over rough ground, hills, or buildings.
- Stronger with stronger wind, rougher ground.
- Windward side of mountain: air moves smoothly up.
- Leeward slope: air tumbles down, turbulence increases, tends to push aircraft into terrain.
- Valley and canyon downdrafts can be severe.
Thermal (convective): rising thermals from daytime surface heating.
- Worst on hot afternoon over dry ground. Fades after sunset.
- Air too dry for cumulus: invisible dry convection, no visual warning.
- Cumulus present: cloud top marks top of convective current. Turbulence at and below cloud, smooth air above.
- Every updraft has a matching, broader, slower downdraft.
| Surface | Heating | Gives |
|---|---|---|
| Pavement, rock, barren ground | Fast (and cool fast) | Updrafts |
| Water, vegetation | Slowly | Downdrafts |
Orographic: wind flow forced over mountains.
- At cruise above FL180 near mountains: moderate or greater, rapid onset, risk of structural damage (FAA AWH 12.4.1, 19.2.5.1).
Landing over uneven terrain:
- Rising air off barren or paved ground: can balloon aircraft on final, cause overshoot.
- Sinking air off water or thick vegetation: can cause landing short.
Frontal: lifting of warm air plus wind shear at the frontal surface.
- Worst at a fast cold front.
Wind shear: change in wind speed and/or direction over a short distance. Underlies much of the above.
- Gives violent updrafts and downdrafts, abrupt changes to horizontal motion (TC AIM GEN, wind shear; FAA AWH 19.2.4).
Trap: expecting thermal turbulence only where clouds show; dry convection is invisible.
References
- FAA Aviation Weather Handbook (FAA-H-8083-28)