Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual cues such as clouds, and is caused when bodies of air moving at widely different speeds meet.

CAT is never produced in the stratosphere. Similar considerations apply to the troposphere but in reverse. So  I suggest that planes should study new approach to fly and avoid the Alaska and other range of danger unpredictable CAT. 

The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 meters (23,000–39,000 ft) as it meets the troopopause . I also blame  HAARP IN Alaska for this. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus cloud can also indicate high probability of CAT.

Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with conventional radar, 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. So the companies lies to you when they say they can not detected , it is false, They can not see it but there are ways to detect and companies must implement different technology and protocols to save people lifes.

Detecting and predicting CAT is hard for meteorologists because it is at such heights that even when caused by factors that can be measured, 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 turbulence (not only CAT) are observed less than 150 nautical miles (280 km) away from the core of a jet stream. Jet stream alone will rarely be the cause of CAT.

Temperature gradientA temperature gradiant 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.

Wind

Wind shears 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 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.

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

         tropopause, called gravity waves. 

CAT  OR AIR POCKET, Effects on aircraft

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.

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.

 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).

BOAC FLIGHT 911  broke up in flight in 1966 after experiencing severe turbulence just downwind of  mount Fuji , Japan. 

Wake turbulence is another dangerous 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. This phenomenon can also lead to accidents with large aircraft as well.

Delta air lines flight 9570 crashed at the GS -SW International airport in 1972 while landing behind a DC -10 . This accident led to new rules for minimum following separation time from “heavy” aircraft. AAF flight 587 crashed shortly after takeoff from JFK International Airport. in 2001 due to pilot overreaction to wake turbulence from a Boeing 747. 

Thank you for reading .Good luck for those who are witting the Ultrasound and Doppler physics  SPI /ARDMS this year.

           Steve Ramsey.  Canada.