In
meteorology
Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
, clear-air turbulence (CAT) is the
turbulent
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
movement of
air mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...
es in the absence of any visual clues such as
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s, and is caused when bodies of air moving at widely different speeds meet.
The
atmospheric
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...
region most susceptible to CAT is the high
troposphere
The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...
at altitudes of around as it meets the
tropopause
The tropopause is the atmospheric boundary that demarcates the lowest two layers of the atmosphere of Earth – the troposphere and stratosphere – which occurs approximately above the equatorial regions, and approximately above the polar regi ...
. Here CAT is most frequently encountered in the regions of
jet stream
Jet streams are fast flowing, narrow thermal wind, air currents in the Earth's Atmosphere of Earth, atmosphere.
The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the gl ...
s. At lower altitudes it may also occur near
mountain range
A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have aris ...
s. Thin
cirrus clouds can also indicate high probability of CAT.
CAT can be hazardous to the comfort, and occasionally the safety, of
air travelers, as the aircraft pilots often cannot see and anticipate such turbulences, and a sudden encounter can impart significant
stress to the
airframe
The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system.
Airframe design is a field of aeros ...
.
CAT in the jet stream is expected to become stronger and more frequent because of
climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
, with transatlantic wintertime CAT increasing by 60% (light), 95% (moderate), and 150% (severe) by the time of
doubling.
Definition
In
meteorology
Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
, clear-air turbulence (CAT) is the
turbulent
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
movement of
air mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...
es in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.
In aviation, CAT is defined as "the detection by aircraft of high-altitude inflight bumps in patchy regions devoid of significant cloudiness or nearby thunderstorm activity". It was first noted in the 1940s.
Detection
Clear-air turbulence is not possible to detect with the naked eye and very difficult to detect with a conventional
radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
, with the result that it is difficult for aircraft pilots to detect and avoid it. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as
scintillometers,
Doppler LIDAR
Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...
s, or
N-slit interferometer
The ''N''-slit interferometer is an extension of the double-slit experiment, double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of ''N''-slit arrays in optics was illustrated by Isaac Newton, Ne ...
s.
Factors
At typical heights where it occurs, the intensity and location cannot be determined precisely. However, because this turbulence affects long range aircraft that fly near the tropopause, CAT has been intensely studied. Several factors affect the likelihood of CAT. Often more than one factor is present.
As of 1965 it had been noted that 64% of the non-light turbulences (not only CAT) were observed less than away from the core of a
jet stream
Jet streams are fast flowing, narrow thermal wind, air currents in the Earth's Atmosphere of Earth, atmosphere.
The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the gl ...
. Jet stream produces horizontal
wind shear
Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...
at its edges, caused by the different relative air speeds of the stream and the surrounding air. Wind shear, a difference in relative speed between two adjacent air masses, can produce vortices, and when of sufficient degree, the air will tend to move chaotically.
A strong
anticyclone
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
vortex can also lead to CAT.
Rossby waves
Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby in the Earth's atmosphere in 1939. They ar ...
caused by this jet stream shear and the
Coriolis force
In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...
cause it to meander.
Although the altitudes near the tropopause are usually cloudless, thin
cirrus cloud can form where there are abrupt changes of air velocity, for example associated with jet streams. Lines of cirrus perpendicular to the jet stream indicate possible CAT, especially if the ends of the cirrus are dispersed, in which case the direction of dispersal can indicate if the CAT is stronger at the left or at the right of the jet stream.
A
temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with Dimensional analysis, ...
is the change of temperature over a distance in some given direction. Where the temperature of a gas changes, so does its density and where the density changes CAT can appear.
From the ground upwards through the troposphere temperature decreases with height; from the tropopause upwards through the stratosphere temperature increases with height. Such variations are examples of temperature gradients.
A horizontal temperature gradient may occur, and hence
air density
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere at a given point and time. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmosph ...
variations, where air velocity changes. An example: the speed of the jet stream is not constant along its length; additionally air temperature and hence density will vary between the air within the jet stream and the air outside.

As is explained elsewhere in this article, temperature decreases and wind velocity increase with height in the troposphere, and the reverse is true within the stratosphere. These differences cause changes in air density, and hence viscosity. The viscosity of the air thus presents both inertias and accelerations which cannot be determined in advance.
Vertical wind shear above the jet stream (i.e., in the stratosphere) is sharper when it is moving upwards, because wind speed decreases with height in the stratosphere. This is the reason CAT can be generated above the tropopause, despite the stratosphere otherwise being a region which is vertically stable. On the other hand, vertical wind shear moving downwards within the stratosphere is more moderate (i.e., because downwards wind shear within the stratosphere is effectively moving against the manner in which wind speed changes within the stratosphere) and CAT is never produced in the stratosphere. Similar considerations apply to the troposphere but in reverse.
When strong wind deviates, the change of wind direction implies a change in the wind speed. A stream of wind can change its direction by differences of pressure. CAT appears more frequently when the wind is surrounding a low pressure region, especially with sharp troughs that change the wind direction more than 100°. Extreme CAT has been reported without any other factor than this.
Mountain waves are formed when four requirements are met. When these factors coincide with jet streams, CAT can occur:
* A mountain range, not an isolated mountain
* Strong perpendicular wind
* Wind direction maintained with altitude
*
Temperature inversion
In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...
at the top of the mountain range
The tropopause is a layer which separates two very different types of air. Beneath it, the air gets colder and the wind gets faster with height. Above it, the air warms and wind velocity decreases with height. These changes in temperature and velocity can produce fluctuation in the altitude of the tropopause, called
gravity wave
In fluid dynamics, gravity waves are waves in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the oc ...
s.
Effects on aircraft
Pilot rules
When a pilot experiences CAT, a number of rules should be applied:
* The aircraft must sustain the recommended velocity for turbulence.
* When following the jet stream to escape from the CAT, the aircraft must change altitude and/or heading.
* When the CAT arrives from one side of the airplane, the pilot must observe the thermometer to determine whether the aircraft is above or below the jet stream and then move away from the tropopause.
* When the CAT is associated with a sharp trough, the plane must go through the low-pressure region instead of around it.
* The pilot may issue a
Pilot Report (PIREP), communicating position, altitude and severity of the turbulence to warn other aircraft entering the region.
Cases
Because aircraft move so quickly, they can experience sudden unexpected accelerations or 'bumps' from turbulence, including CAT – as the aircraft rapidly cross invisible bodies of air which are moving vertically at many different speeds. Although the vast majority of cases of turbulence are harmless, in rare cases cabin crew and passengers on aircraft have been injured when tossed around inside an aircraft cabin during extreme turbulence. In a small number of cases, people have been killed and at least two aircraft disintegrated mid-air.
*On July 19, 1961,
Aerolíneas Argentinas Flight 644 from Buenos Aires to Comodoro Rivadavia, a Douglas DC-6, disintegrated mid-air after entering an area of severe CAT that caused stress exceeding the structural limitations of the aircraft. The failure started at the aircraft's
wing
A wing is a type of fin that produces both Lift (force), lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform (aeronautics), planform. Wing efficiency is expressed as lift-to-d ...
.
*On March 5, 1966,
BOAC Flight 911 from Tokyo to Hong Kong, a Boeing 707, broke up in CAT, with the loss of all persons (124) on board after experiencing severe
lee-wave turbulence just downwind of
Mount Fuji
is an active stratovolcano located on the Japanese island of Honshu, with a summit elevation of . It is the highest mountain in Japan, the second-highest volcano on any Asian island (after Mount Kerinci on the Indonesian island of Sumatra), a ...
,
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. The sequence of failure started with the vertical stabilizer getting ripped off.
*On December 28, 1997, on
United Airlines Flight 826, one person died and 17 others were seriously injured in a CAT event.
*On May 21, 2024, one passenger died and dozens were injured on
Singapore Airlines Flight 321 from London to Singapore, causing the plane to divert to Bangkok.
See also
*
Continuous gusts
*
Dryden Wind Turbulence Model
*
Ellrod index
*
N-slit interferometer
The ''N''-slit interferometer is an extension of the double-slit experiment, double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of ''N''-slit arrays in optics was illustrated by Isaac Newton, Ne ...
*
von Kármán Wind Turbulence Model
*
Wake turbulence
References
External links
Brace for TurbulenceClear Air Turbulence Forecast (USA)*
*
{{Authority control
Aerodynamics
Weather hazards to aircraft
Meteorological phenomena
Turbulence
Wind