Types of Turbulence
Understanding turbulence causes for better prediction and avoidance
Understanding Turbulence
Turbulence is irregular motion of the air caused by eddies and vertical currents. While it's often uncomfortable, understanding what causes different types of turbulence helps you predict, avoid, or prepare for it.
Each type of turbulence has characteristic causes, locations, and indicators. Knowing these allows you to make informed decisions about routing, altitude, and timing.
Turbulence Intensity Scale
Light
Slight, erratic changes in altitude/attitude. Occupants feel slight strain against seat belts. Unsecured objects may move slightly. No difficulty controlling aircraft.
Moderate
Changes in altitude/attitude occur but aircraft remains in positive control. Occupants feel definite strain against belts. Unsecured objects move. Variations in indicated airspeed. Difficulty maintaining altitude/heading.
Severe
Large, abrupt changes in altitude/attitude. Aircraft may be momentarily out of control. Occupants forced violently against belts. Unsecured objects thrown about. Food service impossible. Walking impossible.
Extreme
Aircraft practically impossible to control. Structural damage may occur. Violent, rapid changes that may exceed structural limits. Exceedingly rare.
Clear Air Turbulence (CAT)
CAT occurs in cloud-free air at high altitude, usually associated with jet streams and upper-level fronts. It cannot be seen or detected by weather radar, making it the most insidious form of turbulence.
Causes
- • Wind shear: Abrupt changes in wind speed/direction between air layers
- • Jet stream: Most common on the polar (cold) side of the jet core
- • Upper fronts: Where air masses meet aloft
- • Troughs: Near upper-level trough lines
Where to Expect CAT
Jet Stream Locations
- • 50-150 miles on cold side of jet core
- • Just below jet core altitude
- • Where jet curves sharply
- • Where jet streams merge
Altitude Range
- • Most common: FL200-450
- • Peak: FL300-400
- • Can occur at any altitude
- • Winter more severe than summer
Avoidance Strategies
- • Check pilot reports (PIREPs) along your route
- • Review jet stream charts—stay away from the cold side
- • Request altitude change (2,000-4,000 ft) if CAT encountered
- • Monitor SIGMET advisories for severe CAT
Mountain Wave Turbulence
Mountain waves form when strong, stable winds flow perpendicular to mountain ridges. The air is deflected upward, then oscillates in waves downwind of the ridge. Can produce severe to extreme turbulence in rotors beneath the waves.
Formation Requirements
- • Wind speed: 25+ knots perpendicular to ridge (within 30°)
- • Stability: Stable air layer at or above ridge height
- • Terrain: Significant ridges or mountain ranges
- • Altitude: Extends from ridge level to well above FL400
Visual Indicators
Lenticular Clouds
Lens-shaped clouds stationary over peaks
Rotor Clouds
Ragged cumulus on lee side of ridge
Cap Cloud
Cloud cap clinging to peak
Rotor Zone Warning
The rotor zone—tumbling air beneath the wave crests on the lee side—produces the most violent turbulence. It can occur from near surface to above ridge height. Severe to extreme turbulence, strong downdrafts (2,000+ fpm), and rapid altitude loss are possible. Avoid the lee side at or below ridge level.
Safe Mountain Flying
- • Cross ridges at 45° angle to allow escape turn
- • Fly at least 50% above ridge height (not just clearing it)
- • Approach ridges from windward side when possible
- • Avoid flying when winds at ridge level exceed 25-30 knots
- • Watch for lenticulars—waves are active even if smooth at your altitude
Convective (Thermal) Turbulence
Thermal turbulence is caused by unequal surface heating creating rising columns of warm air (thermals). Common on sunny afternoons, especially over dark surfaces.
More Turbulent Over
- • Dark plowed fields
- • Asphalt/concrete
- • Rocky, barren terrain
- • Urban areas
- • Dry, sandy areas
Less Turbulent Over
- • Water (lakes, oceans)
- • Forests
- • Irrigated fields
- • Snow cover
- • Clouds shadows
Time of Day Pattern
Mechanical Turbulence
Mechanical turbulence occurs when wind flows over and around surface obstacles— buildings, hangars, tree lines, terrain features. Most significant at low altitudes.
Obstructions
- • Hangars and buildings
- • Tree lines
- • Hills and ridges
- • Cliffs and bluffs
Characteristics
- • Extends 2-3x obstacle height
- • Worst on downwind side
- • Increases with wind speed
- • Common on gusty days
Runway Considerations
Hangars or tree lines near runway thresholds create mechanical turbulence on approach and departure. Be prepared for sudden sink or airspeed changes when crossing these obstacles on final. Add extra approach speed on gusty days.
Wake Turbulence
Wake turbulence is generated by all aircraft producing lift. Wingtip vortices spiral outward and downward, most hazardous behind heavy aircraft at low speeds (takeoff/landing). Can exceed structural limits of light aircraft.
Vortex Behavior
- • Sink rate: 300-500 fpm initially
- • Lateral spread: Move apart at ~5 knots
- • Ground effect: Vortices move laterally near ground
- • Decay: Dissipate within 2-3 minutes (longer in calm air)
Avoidance Procedures
| Situation | Action |
|---|---|
| Takeoff behind larger aircraft | Rotate before their rotation point; climb above their path |
| Landing behind larger aircraft | Stay above their approach path; land beyond their touchdown |
| Intersecting runway | Note where their rotation/touchdown occurred relative to intersection |
| Parallel runway | Beware crosswinds pushing vortices toward your runway |
Required Wait Times
When departing behind a heavy or super aircraft: wait at least 3 minutes. Behind a large aircraft: wait at least 2 minutes. ATC separation does not guarantee wake avoidance—you are responsible for ensuring adequate spacing.
Key Takeaways
- CAT occurs in clear air near jet streams—invisible and undetectable by radar
- Mountain waves produce rotor turbulence—avoid lee side at/below ridge level
- Thermal turbulence peaks mid-afternoon; fly morning/evening for smoother air
- Wake turbulence sinks—stay above larger aircraft's flight path
- File PIREPs when you encounter turbulence—help other pilots