The question of “what wind speed causes turbulence” is based on the misconception that a single number exists. Turbulence is not caused by reaching a fixed speed threshold, but by the disruption of smooth air movement. It is a product of relative speed differences and chaotic air motion, not absolute velocity.
Defining Atmospheric Turbulence
Atmospheric flow is categorized into two states: laminar and turbulent. Laminar flow describes air moving in smooth, parallel layers with little mixing, providing a stable environment for objects passing through it.
Turbulence begins when this stable flow transitions into a chaotic, irregular motion characterized by eddies, swirls, and unpredictable vertical currents. This transition occurs when the inertia of the moving air overcomes its viscosity, causing the flow to break down into vortexes. The air’s velocity is constantly fluctuating in all three dimensions during turbulence.
How Wind Shear and Speed Gradients Create Turbulence
The most common cause of turbulence related to wind speed is wind shear, defined as a significant change in wind speed or direction over a short distance. This effect involves layers of air moving at different velocities right next to each other. When these layers slide past one another too quickly, the friction generates rotational energy, causing the air to tumble.
Wind shear is observed in both horizontal and vertical dimensions. Horizontal shear often occurs near high-altitude jet streams, where a swift, narrow current flows adjacent to slower-moving air masses. For large aircraft, a horizontal change in airspeed of 45 knots is considered significant wind shear. The most intense clear air turbulence (CAT) is often found on the cold-air side of the jet stream.
Vertical shear is experienced as a rapid change in wind speed with altitude, especially noticeable near the Earth’s surface in the atmospheric boundary layer. This near-ground effect, often called frictional turbulence, requires surface winds of 20 knots or higher before significant eddying is generated. For general aviation, a vertical change in speed greater than five knots over a short distance is considered hazardous.
Turbulence Caused by Non-Speed Factors
A single wind speed number is insufficient to predict turbulence because many causes are independent of differential speed. Thermal turbulence (convective turbulence) is driven by vertical air movement rather than horizontal shear. This type occurs when the sun heats the ground, causing warm, buoyant air to rise rapidly, creating updrafts and downdrafts.
Thermal turbulence is most common from late morning through mid-afternoon over land and is classified as light to moderate. Even with minimal horizontal wind, this vertical mixing creates a bumpy ride. The intensity of this convection is dictated by the temperature difference between the rising air and the surrounding atmosphere.
Airflow disruption by physical obstructions creates mechanical turbulence. When air encounters obstacles like mountains, it is forced to move around or over them, creating chaotic eddies on the downwind side. Air flowing over mountain ridges can produce severe mechanical turbulence in the form of mountain waves. These standing waves and strong rotor currents occur when winds of at least 25 knots blow nearly perpendicular to a ridge. Severe turbulence is possible when the perpendicular wind component exceeds 50 knots.
Categorizing Turbulence Intensity and Impact
Since the cause is varied, turbulence is categorized by its measurable effect on an aircraft and its occupants. These standard meteorological categories range from light to extreme.
Light Turbulence
The lightest level of turbulence causes only momentary and slight changes in aircraft attitude. Passengers may feel a slight strain against their seat belts.
Moderate Turbulence
Moderate turbulence introduces more noticeable jolts and requires passengers to feel a definite strain against their restraints. Unsecured items within the cabin will become dislodged, and walking is difficult under these conditions. The aircraft remains under the pilot’s control at all times, though there are moderate changes in altitude or attitude.
Severe and Extreme Turbulence
In severe turbulence, the aircraft experiences large and abrupt changes in its altitude and attitude, and there may be large variations in airspeed. Occupants are forced violently against their seat belts, and the aircraft may be momentarily out of the pilot’s control. Extreme turbulence is the highest classification, violently tossing the aircraft and making control nearly impossible, with the possibility of structural damage.