Clear-air turbulence

Clear-Air Turbulence (often abbreviated CAT and sometimes colloquially referred to as "air pockets") is the erratic movement of air masses in the absence of any visual cues (such as clouds). Clear-Air Turbulence is caused when bodies of air moving at widely different speeds meet; at high altitudes (7,000-12,000 metres/23,000-39,000 feet) this is frequently encountered around jet streams or sometimes near mountain ranges. Clear-Air Turbulence is impossible to detect either with the naked eye or with radar, meaning that it is difficult to avoid. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers or Doppler LIDARs.

This kind of turbulence creates a hazard for air navigation. Because aircraft move so quickly, they experience sudden unexpected accelerations as they rapidly cross invisible bodies of air which are moving vertically at many different speeds. Cabin crew and passengers on airliners have been injured (and in a small number of cases, killed, as in the case of a United Airlines Flight 826 on December 28, 1997) when thrown around inside an aircraft experiencing turbulence.

Wake turbulence is another dangerous type of CAT. The spinning vortex-pair created by the wings of a large aircraft can deflect or even flip a smaller aircraft.

External links

• Clear Air Turbulence Forecast (USA)