Definition
Categories of turbulence intensity used in aviation weather reporting. Severe turbulence causes large, abrupt changes in altitude or attitude, with the aircraft momentarily out of control; occupants are forced violently against seat belts, and unsecured objects are tossed about. Extreme turbulence is more violent still — the aircraft is tossed about, is practically impossible to control, and may cause structural damage.
Plain English
Very rough air. 'Severe' means the plane briefly gets thrown around so hard the pilot can't fully control it and people are slammed against their seat belts. 'Extreme' is even worse — the plane is tossed around, almost impossible to fly, and the airframe itself can be damaged.
Context Anchor
Seen in SIGMETs and other inflight weather information when conditions are dangerous enough that pilots should avoid the affected area if possible.
Derivation
Turbulence comes from a Latin word meaning “confusion” or “disorder.” That fits the aviation meaning: the air is not flowing smoothly, so the aircraft is disturbed by sudden, uneven air movement. Severe means very serious, and extreme means at the far end of intensity.
Why Pilots Care
Severe or extreme turbulence can cause loss of control, passenger and crew injuries, and possible airframe damage.
Grounding Statement
Picture the airplane not just bumping, but being suddenly lifted, dropped, or rolled by air that is moving violently.
Intuition Check
Do not read “severe” or “extreme” as casual words meaning “pretty rough.” In aviation weather, they describe specific dangerous levels of turbulence that can affect control of the aircraft and, at the extreme level, may damage it.
Example Sentence 1
A SIGMET was issued for severe or extreme turbulence over the Rockies, so the crew filed a route well to the south.
Example Sentence 2
During severe or extreme turbulence we secured the cabin and maintained the best possible aircraft control.