Clear air turbulence weather, 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.
TURBULENCE
Turbulence causes the formation of eddies of many different length scales. Most of the kinetic energy of the turbulent motion is contained in the large scale structures. The energy "cascades" from these large scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues, creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place.
