Critical Wind Values, Situations and Associated Turbulence
Category II Aircraft |
||
| Intensity | Horizontal Wind Shear | Vertical Wind Shear |
| Light | < 25 kt / 90 nm | 3 - 5 kt / 1,000 ft |
| Moderate | 25 - 49 kt / 90 nm | 6 - 9 kt / 1,000 ft |
| Severe | 50 - 89 kt / 90 nm | 10 - 15 kt / 1,000 ft |
| Extreme | >= 90 kt / 90 nm | > 15 kt / 1,000 ft |
| Light Turbulence | |
| Light Turbulence Characteristics | Light Turbulence Situations |
| - May cause slight, erratic changes in attitude
and/or altitude. - Usually, produces a slight variation in airspeed (5-15 knots). - Seat belts may be required and occupants may feel a gentle strain against restraints. - Loose objects in the aircraft may be displaced slightly. - Little or no difficulty is encountered while walking. - The vertical gust velocity is 5-20 feet/second. |
- In mountainous areas even with light
winds. - In and near cumulus clouds. - Near the tropopause. - At low altitudes when the winds exceed 15 knots. |
| Moderate Turbulence | |
| Moderate Turbulence Characteristics | Moderate Turbulence Situations |
| - Moderate changes experienced in
aircraft attitude or altitude, but the aircraft remains in positive control at all times. - Usually, small variations in air speed (15-25 knots). - Occupants feel strain against seat belts or have difficulty walking and loose objects move about. - The vertical gust velocity is 20-35 feet/second. |
- In mountain waves as far as 300 miles leeward of
the ridge when the wind normal to the ridge exceeds 50 knots. - In mountain waves as far as 150 miles leeward of the ridge when wind normal to the ridge is 25-50 knots. - In thunderstorms and towering cumuliform clouds. - Near the jet stream on the cold side. - At low altitude when the surface winds exceed 25 knots. |
| Severe Turbulence | |
| Severe Turbulence Characteristics | Severe Turbulence Situations |
| - Abrupt changes in aircraft attitude or altitude
are experienced. - Aircraft may be out of control for short periods. - Usually, large variations in airspeed (25 knots). - Occupants are forced violently against seat belts and loose objects are tossed about. - The vertical gust velocity is 35-50 feet/second. |
- Up to 150 miles leeward
of the ridge when a mountain wave exists and winds normal to the ridge exceed 50 knots. - Up to 50 miles leeward of the ridge when a mountain wave exists and winds normal to the ridge are from 25-50 knots. - In and near mature thunderstorms. - Near jet stream altitude and about 50 to 100 miles on the cold side. |
| Extreme Turbulence | |
| Extreme Turbulence Characteristics | Extreme Turbulence Situations |
| - The aircraft is violently tossed about and
practically impossible to control. - Structural damage may occur. - Rapid fluctuations in airspeed are > 25 knots. - The vertical gust velocity is > 50 feet/second. |
This turbulence is rarely encountered. It is usually
found in the strongest forms of convection and wind shear. The two most frequent locations
of extreme turbulence are: - In mountain waves in or near the rotor cloud. - In severe thunderstorms, especially in organized squall lines. |
Updated: February 5, 1999