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This winter I was on a flight in Europe during a huge winter storm. That's why the pilot announced for the flight that we will have some medium turbulence during the flight. However, it wasn't really shaking more than normal.

On the other hand, on my last flight over the Atlantic the pilot announced only small turbulence, but I'm not sure if I would have remained in my seat without my seatbelt.

Obviously, the forecast could just been wrong, but nevertheless I'm interested if there is any official classification of turbulence. So what is medium turbulence? What is severe turbulence?

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2 Answers 2

up vote 13 down vote accepted

The FAA has guidelines for pilot weather reports which cite a "U.S. Standard Turbulence Criteria Table" (which I could not find online):

  1. Light. Loose objects in aircraft remain at rest.

  2. Moderate. Unsecured objects are dislodged. Occupants feel definite strains against seat belts and shoulder straps.

  3. Severe. Occupants thrown violently against seat belts. Momentary loss of aircraft control. Unsecured objects tossed about.

  4. Extreme. Aircraft is tossed violently about, impossible to control. May cause structural damage.

But note that these are somewhat subjective reports by pilots of what happened in their plane whereas a forecast says what could happen. Turbulences are by nature chaotic and unpredictable. Even in an area where heavy turbulences exist, any specific plane may get lucky and avoid them, or unlucky and hit the worst spots.

Additionally, the size of the plane makes a big differences. A turbulence that throws a little Cessna completely out of control may be barely felt at all in a large airliner.

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Pilots also extensively use reports from other planes in the area at similar altitude to "forecast" what is coming. Air Traffic Control will share reports between flight crew so that they know what others in the area have experienced, giving the chance to either change altitude/course to try and avoid turbulence, or to warn passengers/cabin crew and get them seated. –  Doc Sep 14 '12 at 16:11

In addition to Michael's answer, there is another classification of turbulence:

  • Anticipated turbulence: This is the good type. The captain will know about the turbulence in advance and will try his best to avoid it. In case the turbulence is not avoidable then the captain will notify the cabin crew. The cabin crew will announce that to the passengers and prepare the cabin for the worst. In this kind of turbulence things are always fine because people and the cabin are prepared for the turbulence.. seat belts are buckled up and cabin items are secured. One more thing, this kind of turbulence most likely to be safe because the captain would not risk entering the turbulent area unless the airplane can handle it safely.

  • Unanticipated turbulence: This is the evil type. The forecast and the airplane's radar will show clear weather. The cockpit crew, cabin crew and passengers are not notified in advance and the cabin items are most likely to be loose (carts, trays...etc). This type is really dangerous and most likely to be severe and causes injuries specially to the people who are standing and due to flying unsecured items. That's why in the seat belt off announcement the announcer will say something like (The seat belt sign has been turned off, for your safety keep your belt fastened at all times unless you need to move). So it is highly advised to keep your seat belt fastened at all times unless you are going to toilet or so.

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