Clear-air turbulence (CAT) is a form of atmospheric turbulence that occurs in cloud-free, precipitation-free regions, typically at commercial aviation cruising altitudes (9,000-12,000 m). It is especially dangerous because it is invisible: it cannot be detected by conventional weather radar since it contains no hydrometeors.
Main causes
CAT is primarily associated with wind shear near the jet stream, where abrupt changes in wind speed and direction generate Kelvin-Helmholtz waves that break into turbulence. It can also occur above mountain systems (mountain waves propagating turbulence into the stratosphere) and near fronts.
Intensity ranges from light (slight bumps, seatbelts recommended) to severe or extreme (unsecured objects and people thrown against the ceiling, possible structural damage to the aircraft). Serious incidents injure dozens of people per year in global aviation. Unbuckled passengers are the most vulnerable.
Recent studies link climate change to an increase in CAT: differential warming of the atmosphere intensifies jet stream shear. Models predict a 100-300 % increase in severe CAT over the North Atlantic by end of century. See also: jet stream, wind shear.