clear-air turbulence

n. (Abbr. CAT)
Atmospheric turbulence that occurs under tranquil and cloudless conditions and subjects aircraft to strong updrafts and downdrafts.

Turbulence above the boundary layer but not associated with cumulus convection. The atmosphere is a fluid in turbulent motion. That turbulence of a scale sensed by humans in aircraft is primarily associated with the boundary layer within a kilometer or so of the Earth, where it is induced by the surface roughness, or in regions of deep convection such as cumulus cloud development or thunderstorms. However, aircraft occasionally encounter turbulence when flying at altitudes well above the surface and far from convective clouds. This phenomenon has been given the rather unsatisfactory name of clear-air turbulence (CAT).

What is primarily sensed in CAT by the human is vertical acceleration. This acceleration will depend on the person's location in the plane, the speed of flight relative to the air, and the response characteristics of the airframe. A plane with a wing that generates aerodynamic lift more efficiently or an air-frame with less weight per unit wing area will respond more strongly to a given gust magnitude.

CAT is encountered in the atmosphere with a probability depending on flight altitude, geographical location, season of the year, and meteorological conditions. Given this variability and the small scale of the phenomenon, it is difficult to establish reliable statistics on the frequency of its occurrence. Although CAT may be encountered in unexpected meteorological contexts, there are highly favored locations for its occurrence. One is in the vicinity of the jet stream, particularly in ridges and troughs where the wind direction is turning sharply. A second and even more common location of occurrence is in the lee of a mountain range when a strong air flow is distorted by being forced over the range. In this situation a gravity lee wave is generated, which propagates to stratospheric heights. At various altitudes and distances from the mountain, this wave may break, and as many as a dozen or more CAT patches, light to severe, may be formed. Despite knowledge of these favored meteorological areas, it is not possible to forecast with confidence the precise location of a CAT patch. Warning forecasts for substantial portions of routes are typically given to pilots when CAT conditions prevail. See also Jet stream.

Aside from the practical implications of CAT for air transport, this phenomenon plays a role of undetermined magnitude in the dissipation of the kinetic energy of the atmosphere.

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The noun has one meaning:

Meaning #1: strong turbulence in an otherwise cloudless region that subjects aircraft to violent updrafts or downdrafts

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