Clear-air turbulence

Last updated May 24, 2024

In meteorology, clear-air turbulence (CAT) is the turbulent movement of air masses 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 at altitudes of around 7,000–12,000 m (23,000–39,000 ft ) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT.

CAT can be hazardous to the comfort, and occasionally the safety, of air travelers,^[1] as the aircraft pilots often cannot see and anticipate such turbulences, and a sudden encounter can impart significant stress to the airframe.

Definition

In meteorology, clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.^[2]

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".^[3] It was first noted in the 1940s.^[4]

Detection

Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with a conventional radar,^[5] 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 LIDARs, or N-slit interferometers.^[6]

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 150 nautical miles (280 km) away from the core of a jet stream. Jet stream produces horizontal wind shear 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.^[7]

A strong anticyclone vortex can also lead to CAT.^[8]

Rossby waves caused by this jet stream shear and the Coriolis force cause it to meander.^{[ clarification needed ]}

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 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 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 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 waves.

Effects on aircraft

Pilot rules

When a pilot experiences CAT, a number of rules should be applied:^[9]

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.

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, Japan. The sequence of failure started with the vertical stabilizer getting ripped off.^{[ citation needed ]}
On December 28, 1997 on United Airlines Flight 826, one person died.^[10]
On May 1, 2017, Aeroflot Flight 270 from Moscow to Bangkok, a Boeing 777, flew into clear air turbulence, and 27 passengers who were not buckled up sustained injuries.^[11]
On August 29, 2023, 11 people that were on a Delta Air Lines flight from Milan to Atlanta were sent to the hospital after “severe turbulence”.^[12]
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.^[13]

Related Research Articles

Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of the Earth, Venus, Jupiter, Saturn, Uranus, and Neptune. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.

The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is 18 km in the tropics; 17 km in the middle latitudes; and 6 km in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km.

The stratosphere is the second layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air high in the sky and the cool layers of air in the low sky, close to the planetary surface of the Earth. The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet (UV) radiation by the ozone layer. The temperature inversion is in contrast to the troposphere, and near the Earth's surface, where temperature decreases with altitude.

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 compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.

Cumulonimbus is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively. When occurring as a thunderstorm these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes, hazardous winds, and large hailstones. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell. Cumulonimbus is abbreviated Cb.

Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

<span class="mw-page-title-main">Tropopause</span> The boundary of the atmosphere between the troposphere and stratosphere

The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere, which are the lowest two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer that marks the end of the troposphere, and is approximately 17 kilometres (11 mi) above the equatorial regions, and approximately 9 kilometres (5.6 mi) above the polar regions.

The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth creates pressure, absorbs most meteoroids and ultraviolet solar radiation, warms the surface through heat retention, and reduces temperature extremes between day and night, maintaining conditions allowing life and liquid water to exist on the Earth's surface.

Wind shear, 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 wind shear. Vertical wind shear is a change in wind speed or direction with a change in altitude. Horizontal wind shear is a change in wind speed with a change in lateral position for a given altitude.

In meteorology, lee waves are atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Giant Mountains. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street.

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

Perlan Project Inc. is a 501(c)(3) not-for-profit aeronautical exploration and atmospheric science research organization that utilizes sailplanes (gliders) designed to fly at extremely high altitudes.

A circumpolar vortex, or simply polar vortex, is a large region of cold, rotating air; polar vortices encircle both of Earth's polar regions. Polar vortices also exist on other rotating, low-obliquity planetary bodies. The term polar vortex can be used to describe two distinct phenomena; the stratospheric polar vortex, and the tropospheric polar vortex. The stratospheric and tropospheric polar vortices both rotate in the direction of the Earth's spin, but they are distinct phenomena that have different sizes, structures, seasonal cycles, and impacts on weather.

An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air above the vent of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km (25 mi), penetrating the stratosphere. Stratospheric injection of aerosols by volcanoes is a major cause of short-term climate change.

In atmospheric science, the thermal wind is the vector difference between the geostrophic wind at upper altitudes minus that at lower altitudes in the atmosphere. It is the hypothetical vertical wind shear that would exist if the winds obey geostrophic balance in the horizontal, while pressure obeys hydrostatic balance in the vertical. The combination of these two force balances is called thermal wind balance, a term generalizable also to more complicated horizontal flow balances such as gradient wind balance.

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 or airport conditions. This information is usually relayed by radio to the nearest ground station, but other options also exist in some regions. The message would then be encoded and relayed to other weather offices and air traffic service units.

Atmospheric instability is a condition where the Earth's atmosphere is considered to be unstable and as a result local weather is highly variable through distance and time. Atmospheric stability is a measure of the atmosphere's tendency to discourage vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air. Because it is warmer, it is less dense and is prone to further ascent.

Atmospheric temperature is a measure of temperature at different levels of the Earth's atmosphere. It is governed by many factors, including incoming solar radiation, humidity, and altitude. The abbreviation MAAT is often used for Mean Annual Air Temperature of a geographical location.

Numerous accidents have occurred in the vicinity of thunderstorms due to the density of clouds. It is often said that the turbulence can be extreme enough inside a cumulonimbus to tear an aircraft into pieces, and even strong enough to hold a skydiver. However, this kind of accident is relatively rare. Moreover, the turbulence under a thunderstorm can be non-existent and is usually no more than moderate. Most thunderstorm-related crashes occur due to a stall close to the ground when the pilot gets caught by surprise by a thunderstorm-induced wind shift. Moreover, aircraft damage caused by thunderstorms is rarely in the form of structural failure due to turbulence but is typically less severe and the consequence of secondary effects of thunderstorms.

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

↑ Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 93. ISBN 9780850451634.
↑ Stull, B. R., 1988 An introduction to Boundary Layer Meteorology, Kluwert Academic Publishers 666 pp.
↑ Chambers, E., 1955: Clear air turbulence and civil jet operation. J. Roy. Aeronaut. Soc.,59, 613–628.
↑ Baughman, E. E., Jr., 1946: Turbulence with a stable lapse rate. Bull. Amer. Meteor. Soc.,27, 459–462.
↑ John J. Hicks, Isadore Katz, Claude R. Landry, and Kenneth R. Hardy, "Clear-Air Turbulence: Simultaneous Observations by Radar and Aircraft" Science. 18 August 1967: Vol. 157. no. 3790, pp. 808–809
↑ F. J. Duarte, T. S. Taylor, A. B. Clark, and W. E. Davenport, "The N-slit interferometer: an extended configuration", J. Opt.12, 015705 (2010).
↑ Binding, A. A. "Association of clear-air turbulence with 300 mb contour patterns". The Meteorological Magazine94 (1965): 11–19.
↑ Knox, John A. (1997-06-01). "Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows". Monthly Weather Review. 125 (6): 1251–1259. Bibcode:1997MWRv..125.1251K. doi: 10.1175/1520-0493(1997)125<1251:PMOCAT>2.0.CO;2 . ISSN 1520-0493.
↑ Lankford, Terry T. (2001). Controlling Pilot Error:Weather. New York: McGraw-Hill. pp. 49–53. ISBN 978-0-07-137328-9.
↑ "Killer turbulence hits flight". BBC News .
↑ Ross, Alice (1 May 2017). "Severe turbulence on Aeroflot flight to Bangkok leaves 27 people injured". The Guardian . Retrieved 30 June 2018.
↑ Jeong, Andrew (30 August 2023). "11 injured in 'severe turbulence' on Delta flight from Milan to Atlanta". The Washington Post . Retrieved 30 August 2023.
↑ Reals, Tucker (May 21, 2024). "Severe turbulence on Singapore Airlines flight 321 from London kills two, injures others, airline says". CBS News.

External links

Brace for Turbulence
Clear Air Turbulence Forecast (USA)
Sharman, R. D.; J.D. Doyle; M.A. Shapiro (Jan 2012). "An Investigation of a Commercial Aircraft Encounter with Severe Clear-Air Turbulence over Western Greenland". J. Appl. Meteorol. Climatol. 51 (1): 42–53. Bibcode:2012JApMC..51...42S. doi: 10.1175/JAMC-D-11-044.1 .
Williams, P. D.; M. Joshi (2013). "Intensification of winter transatlantic aviation turbulence in response to climate change". Nature Climate Change. 3 (7): 644–648. Bibcode:2013NatCC...3..644W. doi:10.1038/nclimate1866.

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[1] Wragg, David W. (1973). A Dictionary of Aviation (first ed.). Osprey. p. 93. ISBN 9780850451634.

[2] Stull, B. R., 1988 An introduction to Boundary Layer Meteorology, Kluwert Academic Publishers 666 pp.

[3] Chambers, E., 1955: Clear air turbulence and civil jet operation. J. Roy. Aeronaut. Soc.,59, 613–628.

[4] Baughman, E. E., Jr., 1946: Turbulence with a stable lapse rate. Bull. Amer. Meteor. Soc.,27, 459–462.

[5] John J. Hicks, Isadore Katz, Claude R. Landry, and Kenneth R. Hardy, "Clear-Air Turbulence: Simultaneous Observations by Radar and Aircraft" Science. 18 August 1967: Vol. 157. no. 3790, pp. 808–809

[6] F. J. Duarte, T. S. Taylor, A. B. Clark, and W. E. Davenport, "The N-slit interferometer: an extended configuration", J. Opt.12, 015705 (2010).

[7] Binding, A. A. "Association of clear-air turbulence with 300 mb contour patterns". The Meteorological Magazine94 (1965): 11–19.

[8] Knox, John A. (1997-06-01). "Possible Mechanisms of Clear-Air Turbulence in Strongly Anticyclonic Flows". Monthly Weather Review. 125 (6): 1251–1259. Bibcode:1997MWRv..125.1251K. doi: 10.1175/1520-0493(1997)125<1251:PMOCAT>2.0.CO;2 . ISSN 1520-0493.

[9] Lankford, Terry T. (2001). Controlling Pilot Error:Weather. New York: McGraw-Hill. pp. 49–53. ISBN 978-0-07-137328-9.

[BBC-10] "Killer turbulence hits flight". BBC News .

[11] Ross, Alice (1 May 2017). "Severe turbulence on Aeroflot flight to Bangkok leaves 27 people injured". The Guardian . Retrieved 30 June 2018.

[12] Jeong, Andrew (30 August 2023). "11 injured in 'severe turbulence' on Delta flight from Milan to Atlanta". The Washington Post . Retrieved 30 August 2023.

[13] Reals, Tucker (May 21, 2024). "Severe turbulence on Singapore Airlines flight 321 from London kills two, injures others, airline says". CBS News.

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