Types and severity of clear air turbulence
What makes CAT so dangerous is that it appears very suddenly. Slight turbulence can cause discomfort for passengers, or even injuries if they do not fasten their seat belts. Extreme CAT can inflict structural damage on the airplane.
The following table outlines the connection between the severity of CAT and its consequences for both the aircraft and the people inside it.
| Intensity | Aircraft reaction | Reaction inside aircraft |
| Light not shown in sign. Weather Charts |
Turbulence that causes slight changes in altitude and/or attitude (pich, roll, yaw). IAS (Indicated Air Speed) fluctuates 5 – 15 kt < 0.5g at the aircraft's centre of gravity |
Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered when walking. |
Moderate  |
Turbulence with a greater intensity as light Turbulence. Changes in altitude and/or attitude, but the aircraft remains in positive control all times. IAS fluctuates 15 – 25 kt 0.5 -1.0g at the aircraft's centre of gravity |
Occupants feel definite strain against seat belts or shoulder straps.
Unsecured object are dislodged. Food service and walking are difficult. |
Severe  |
Turbulence that causes large, abrupt changes in altitude and/or attitude.
Aircraft may be momentarily out of control.
IAS fluctuates more as 25 kt. > 1.0g at the aircraft's centre of gravity |
Occupants are forced violently against seat belts or shoulder straps.
Unsecured objects are tossed about. Food service and walking are impossible. |
Table 2.2: Intensity levels of CAT
Example for the effects of severe clear air turbulence:
On 28 December 1997, a Boeing 747-100 was flying from Narita International Airport in Japan to Honolulu International Airport in Hawaii. After two hours of flight, at an altitude of 9400 meters, the plane received reports of severe CAT in this area and the seat belt sign was turned on. Moments later, the aircraft suddenly dropped around 30 meters, seriously injuring many passengers, although there was no damage to the aircraft. The plane turned around and landed safely back in Tokyo, but was not put back in service since it had been scheduled to be retired in early 1998. One passenger died of her injuries after landing in Tokyo.
Summary:
+1.8g dropped to -0.8g
+/- 30 m (100 ft) change in altitude
15 kt horizontal wind shear
1 fatality and 74 injuries