Turbulence is a subjective measure, and its measurement is somewhat depending on the size of the aircraft in which it is felt and the aircraft's speed.
Aeronautical Information Manual provides this guide (Chapter 7-1-44):
|Reaction Inside Aircraft
|Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude
(pitch, roll, yaw). Report as Light Turbulence
Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as Light Chop.
|Occupants may feel a slight strain against belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.
|Occasional-Less than 1/3 of the time.
Intermittent-1/3 to 2/3.
Continuous-More than 2/3.
|Turbulence that is similar to Light Turbulence but of greater intensity.
Changes in altitude and/or attitude occur but the aircraft remains in positive control at all
times. It usually causes variation in indicated speed. Report as Moderate Turbulence;
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable change in aircraft or attitude. Report as Moderate Chop.
|Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
1. Pilots should report location(s), time (UTC), intensity, whether in or near clouds, altitude, type of aircraft and, when applicable, duration of turbulence.
2. Duration may be based on time between two locations or over a single location. All locations should be readily identifiable.
a. Over Omaha, 1232Z, Moderate Turbulence, in cloud, Flight Level 310, B707.
b. From 50 miles south of Albuquerque to 30 miles north of Phoenix, 1210Z to 1250Z, occasional Moderate Chop, Flight Level 330, DC8.
|Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as Severe Turbulence.
|Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible.
|Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may causes structural damage. Report as Extreme Turbulence.
|High level turbulence (normally above 15,000 feet ASL) not associated with cumuliform cloudiness, including thunderstorms, should be reported as CAT (clear air turbulence) preceded by the appropriate intensity, or light or moderate chop.
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