Turbulence

Light — turbulence that momentarily causes slight, erratic changes in altitude and/or attitude. Occupants may feel a slight strain against seat-belts or shoulder straps.

Moderate — 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. Occupants feel definite strains against seat-belts.

Severe — turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Occupants are forced violently against seat-belts or shoulder straps.
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References

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