Turbulence may be defined as irregular and instantaneous motions of the air that are made up of small eddies traveling in the current of air. Atmospheric turbulence is caused by random fluctuations in the flow of wind. Given a wind field with both streamlines and isotachs drawn

smooth, any difference between the actual wind field and the smooth field is attributed to turbulence.

To an aircraft in flight, the atmosphere is considered turbulent when irregular whirls or eddies of air affect the motion of the aircraft, and a series of abrupt jolts or bumps is felt by the pilot. The type of motion felt by the occupants of an aircraft is directly related to the size and intensity of the eddies in the atmosphere. When the eddies have a diameter smaller than the air-craft, a rapid upward and downward fluctuation is felt, similar to what you might feel in a car running over a series of two-by-fours on a road. This is often referred to by pilots as chop. When the eddies have a diameter about the same size or slightly larger than the size of the aircraft, a sharp downward and then upward jolt is felt within the aircraft, similar to that caused by running a vehicle through a pothole just longer than the length of the vehicle. When atmospheric eddies are encountered that are much greater than the size of the aircraft, a sudden dropping or climbing sensation is felt. This may be related to cresting a sharp hill in a rapidly moving car. The differences in intensity of turbulence are caused by the changes in vertical velocity between the ambient air and the air within an eddy, and may be equated to changes in the depth of a pothole in the road. As different vehicles will respond differently to road irregularities based on their weight and size, different size and weight aircraft will respond differently to similar situations in the atmosphere. Keep in mind the following guidelines:

The effect of turbulence is directly proportional to the speed of the aircraft. A faster aircraft will feel turbulence more than a slower aircraft.

The effect is inversely proportional to the weight of the aircraft. A heavier aircraft will feel less turbulence than a lighter aircraft.

For pilots and aircrew, turbulence reporting is subjective at best. Judgment of turbulence severity is influenced by the factors listed above, by the time that the aircraft is in the turbulence, and by the attitude of the person reporting the turbulence (turbulence severity increases directly with the length of the crew day). The established turbulence-reporting criteria in table 6-1-1 are related to the effect of turbulence on the aircraft and its occupants. Many aircraft lack aadaquate equipment to measure rapid airspeed fluctuations or vertical gust velocities, so turbulence reported by the pilot is still an estimate.  

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