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In
meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
, 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 a 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 la ...
es in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet. The atmospheric region most susceptible to CAT is the high
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
at altitudes of around as it meets the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
. Here CAT is most frequently encountered in the regions of
jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...
s. At lower altitudes it may also occur near mountain ranges. Thin
cirrus clouds Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...
can also indicate high probability of CAT. CAT can be hazardous to the comfort, and occasionally the safety, of air travelers. CAT in the jet stream is expected to become stronger and more frequent because of
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, with transatlantic wintertime CAT increasing by 59% (light), 94% (moderate), and 149% (severe) by the time of doubling.


Detection

Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with a conventional
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
, 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
scintillometer A scintillometer is a scientific device used to measure turbulent fluctuations of the refractive index of air caused by variations in temperature, humidity, and pressure. It consists of an optical or radio wave transmitter and a receiver at oppos ...
s, Doppler
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
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. Although the altitudes near the tropopause are usually cloudless, thin
cirrus cloud Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...
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.


Factors that increase CAT probability

Detecting and predicting CAT is difficult. 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. 64% of the non-light turbulences (not only CAT) are observed less than away from the core of a jet stream.


Jet stream

A
jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...
alone will rarely be the cause of CAT, although there is horizontal wind shear at its edges and within it, caused by the different relative air speeds of the stream and the surrounding air.
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. They are observed in the atmospheres and ...
caused by this jet stream shear and the Coriolis force cause it to meander.


Temperature gradient

A temperature gradient 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.


Vertical

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.


Horizontal

A horizontal temperature gradient may occur, and hence air density 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.


Wind shear

Wind shear Wind shear (or 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 or horizontal ...
is a difference in relative speed between two adjacent air masses. An excessive wind shear produces vortices, and when the wind shear is of sufficient degree, the air will tend to move chaotically. 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

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.


Horizontal

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

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 is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. Nor ...
at the top of the mountain range


Gravity wave wind shear

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 waves.


Effects on aircraft

In the context of air flight, CAT is sometimes colloquially referred to as "air pockets". Standard airplane radars cannot detect CAT, as CAT is not associated with clouds that show unpredictable movement of the air. Airlines and pilots should be aware of factors that cause or indicate CAT to reduce the probability of meeting turbulence. Aircraft in level flight rely on a constant air density to retain stability. Where air density is significantly different, for instance because of temperature gradient, especially at the tropopause, CAT can occur. Where an aircraft changes its position horizontally from within the jet stream to outside the jet stream, or vice versa, a horizontal temperature gradient may be experienced. Because jet streams meander, such a change of position need not be the result of a change of course by the aircraft. Because the altitude of the tropopause is not constant, an airplane that flies at a constant altitude would traverse it and encounter any associated CAT. On May 1, 2017, Boeing 777 flight SU270 from Moscow to Thailand flew into clear air turbulence. The aircraft suddenly lost altitude and 27 passengers who were not buckled up sustained serious injuries. The pilots were able to stabilize the plane and continue the flight. All passengers who needed medical attention were taken to Bangkok hospital upon arrival. On March 5, 1966,
BOAC Flight 911 BOAC Flight 911 (call sign "Speedbird 911") was a round-the-world flight operated by the British Overseas Airways Corporation (BOAC) that crashed near Mount Fuji in Japan on 5 March 1966, with the loss of all 113 passengers and 11 crew members ...
from Tokyo to Hong Kong, a Boeing 707, broke up in CAT, with the loss of all persons (124) on board. The sequence of failure started with the vertical stabilizer getting ripped off.


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 A pilot report or PIREP is a report of actual flight or ground conditions encountered by an aircraft. Reports commonly include information about atmospheric conditions (like temperature, icing, turbulence) or airport conditions (like runway con ...
(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 (and in a small number of cases, killed, as on
United Airlines Flight 826 On December 28, 1997, United Airlines Flight 826 was operated by a Boeing 747-100 flying from New Tokyo Airport (which changed its name to Narita International Airport in 2004), Japan to Honolulu International Airport, Hawaii. Two hours into t ...
on December 28, 1997).
BOAC Flight 911 BOAC Flight 911 (call sign "Speedbird 911") was a round-the-world flight operated by the British Overseas Airways Corporation (BOAC) that crashed near Mount Fuji in Japan on 5 March 1966, with the loss of all 113 passengers and 11 crew members ...
broke up in flight in 1966 after experiencing severe lee-wave turbulence just downwind of
Mount Fuji , or Fugaku, located on the island of Honshū, is the highest mountain in Japan, with a summit elevation of . It is the second-highest volcano located on an island in Asia (after Mount Kerinci on the island of Sumatra), and seventh-highest p ...
,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
.


Wake turbulence

Wake turbulence Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes variety of elements, the most significant of which are wingtip vortices and jetwash. Jetwash refers to the rapidly moving g ...
is another type of clear-air turbulence, but in this case the causes are quite different from those set out above. In the case of wake turbulence, the rotating vortex-pair created by the wings of a large aircraft as it travels lingers for a significant amount of time after the passage of the aircraft, sometimes more than a minute. When this occurs, the lingering turbulence caused by the wake of the wing tips can deflect or even flip a smaller aircraft on the ground or in the air awaiting landing. This phenomenon can also lead to accidents with large aircraft as well. Delta Air Lines Flight 9570 crashed at the
Greater Southwest International Airport Greater Southwest International Airport , originally Amon Carter Field, was the commercial airport serving Fort Worth, Texas, from 1953 until 1974. Dallas/Fort Worth International Airport opened in 1974 a few miles north to replace Greater Southw ...
in 1972 while landing behind a
DC-10 The McDonnell Douglas DC-10 is an American trijet wide-body aircraft manufactured by McDonnell Douglas. The DC-10 was intended to succeed the DC-8 for long-range flights. It first flew on August 29, 1970; it was introduced on August 5, 19 ...
. This accident led to new rules for minimum following separation time from "heavy" aircraft. American Airlines Flight 587 crashed shortly after takeoff from
John F. Kennedy International Airport John F. Kennedy International Airport (colloquially referred to as JFK Airport, Kennedy Airport, New York-JFK, or simply JFK) is the main international airport serving New York City. The airport is the busiest of the seven airports in the Avia ...
in 2001 due to pilot overreaction to wake turbulence from a
Boeing 747 The Boeing 747 is a large, long-range wide-body airliner designed and manufactured by Boeing Commercial Airplanes in the United States between 1968 and 2022. After introducing the 707 in October 1958, Pan Am wanted a jet times its size, t ...
. Many aircraft are now made with
wingtip device Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...
s to improve both the
lift-to-drag ratio In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under gi ...
and fuel economy – such devices may also marginally reduce the strength of the
wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing ...
. However, such changes are not operationally significant (i.e. do not change the distances or times at which it is safe to follow other aircraft).https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_90-23G.pdf


See also

*
Continuous gusts Continuous gusts or stochastic gusts are winds that vary randomly in space and time. Models of continuous gusts are used to represent atmospheric turbulence, especially clear air turbulence and turbulent winds in storms. The Federal Aviation Admini ...
*
Dryden Wind Turbulence Model The Dryden wind turbulence model, also known as Dryden gusts, is a mathematical model of continuous gusts accepted for use by the United States Department of Defense in certain aircraft design and simulation applications. The Dryden model treats the ...
*
Ellrod index In meteorology the Ellrod index is a technique for forecasting clear-air turbulence (CAT). It is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The deformation ...
*
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 The von Kármán wind turbulence model (also known as von Kármán gusts) is a mathematical model of continuous gusts. It matches observed continuous gusts better than that Dryden Wind Turbulence Model and is the preferred model of the United Sta ...


References


External links


Brace for Turbulence

Clear Air Turbulence Forecast (USA)
* * {{Authority control Aerodynamics Weather hazards to aircraft Meteorological phenomena Turbulence Wind