Turbulence

Turbulence is defined as the irregular movement of air resulting from eddies and vertical currents. It can be caused by various factors, such as surface heating, friction with the terrain, the passage of other aircraft, or sudden changes in wind speed and direction.

Clear Air Turbulence (CAT)

CAT is severe turbulence that occurs in cloudless regions, making it invisible and difficult to detect visually or by weather radar.

• Location: Commonly associated with Jet Streams near the tropopause (above 15,000 ft). It can also occur near thunderstorms (up to 20 NM horizontally or 5,000 ft above a Cb) or due to mountain waves.
• Detection: Because there are no water particles to reflect radar waves, CAT does not appear on weather radar.
• Source of Information: The most reliable source of information on CAT is pilot reports (PIREPs / AIREPs) from aircraft flying ahead. These real-time reports allow other pilots and air traffic control to know the location and severity of the turbulence.

Causes and Types of Turbulence

1. Thermal Turbulence (Convective): Caused by convection currents due to uneven heating of the Earth's surface.
2. Mechanical Turbulence: Caused by wind friction as it flows over irregular terrain or obstacles (buildings, mountains).
3. Wake Turbulence: Generated by wingtip vortices from other aircraft, especially heavy and slow ones.
4. Mountain Waves and Rotors:
   • On the leeward side of mountains, standing waves and rotors can form.
   • Rotor turbulence can be extremely violent and cause loss of aircraft control. It is a critical hazard when flying in mountainous areas with strong winds.
5. Wind Shear:
   • Defined as a sudden change in wind speed and/or direction over a short distance (vertical or horizontal).
   • Although a distinct phenomenon, passing through a wind shear zone feels like severe turbulence.

Intensity Classification (ICAO)

Intensity	Description	Effects on Aircraft and Occupants	Load Factor (G)
Light	Slight and erratic changes in attitude and/or altitude.	Occupants may feel a slight strain against seat belts. Loose objects remain at rest.	< 0.5g
Moderate	Changes in attitude and/or altitude, but the aircraft remains under positive control at all times. Variations in indicated airspeed.	Occupants feel strain against seat belts. Loose objects move. Difficulty walking.	0.5g - 1.0g
Severe	Abrupt changes in attitude and/or altitude. The aircraft may be momentarily out of control. Large variations in airspeed.	Occupants are forced violently against seat belts. Loose objects are tossed about.	> 1.0g
Extreme	The aircraft is violently tossed about and is practically impossible to control.	Structural damage may occur.	-

Turbulence Reporting

It is crucial that pilots report any turbulence encountered (especially moderate or severe) to ATC so that this information can be relayed to other aircraft.

• TURBULENCE MODERATE: Moderate changes, positive control maintained.
• TURBULENCE SEVERE: Abrupt changes, possible momentary loss of control.