Intensity of Turbulence
There are four basic turbulence intensities that are used by aviation community.
1. Extreme turbulence:
Manifest by violent bumping of the aircraft, occasionally resulting in significant structural damage and loss of control. Fortunately, extreme turbulence is a relatively rare occurrence, generally confined to the lee of mountain ranges at relatively low levels in the vicinity of a rotor cloud (see photo), when the cross-barrier wind component is greater than 50 knots, or in severe thunderstorms associated with squall lines.
2. Severe Turbulence:
When an aircraft encounters severe turbulence the result is sudden large changes in altitude, airspeed and attitude, occasionally leading to temporary loss of control. Severe turbulence may be found with mountain waves wind the cross-barrier wind component is 25-50 knots, whereupon the turbulence may occur from ridgetop level to a few thousand feet above, for up to 50 miles to the lee of the range. If the cross-barrier component exceeds 50 knots, the severe turbulence layer be as deep as 5000 feet, extending up to 150 miles to the lee of the range. Severe turbulence is also observed near thunderstorms and in strong horizontal and vertical wind shear environments extending above and below, as well as up to 100 miles poleward of the core of the upper-level jet stream.
3. Moderate Turbulence:
Characterized by sudden, frequent bumpiness but little or no variations in aircraft altitude or attitude.
4. Light turbulence:
Even lighter bumpiness that is little more than a nuisance to aircraft operators.