Clear-air turbulence

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Clear air turbulence weather (CAT), 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 conventional 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 or 'bumps' 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 tossed around inside an aircraft cabin during extreme turbulence. BOAC flight 911 broke-up in flight in 1966 after experiencing severe lee wave turbulence just downwind of Mount Fuji, Japan.

Wake turbulence is another dangerous type of CAT. The rotating vortex-pair created by the wings of a large aircraft can deflect or even flip a smaller aircraft on the ground. It can also lead to accidents in large aircraft as well. Delta Air Lines Flight 9570 crashed at the Greater Southwest International Airport in 1972 while landing behind a DC-10, leading to new rules for minimum following separation from "heavy" aircraft. American Airlines Flight 587 crashed shortly after takeoff from John F. Kennedy International Airport in 2001 due to pilot overreaction to wake turbulence from a Boeing 747 that caused separation of the vertical stabilizer.

Inflight turbulence often accounts for nervous passengers. In a recent poll 35% of fearful fliers cite turbulence as a major concern.^[1]