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Turbulence on the Lee Side of Mountains
Table 5-2.-Turbulence Reporting Criteria Table

Aerographers Mate 1 & C
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4. He/she should also avoid high lenticular clouds, particularly if their edges are ragged 5. The pressure altimeter may read as much as 1,000 feet lower near the mountain peaks. CLASSIFICATION AND INTENSITY OF TURBULENCE The Airman’s  Information  Manual,   chapter  7, contains a turbulence reporting criteria table, which describes the meteorological characteristics with which the  respective  classes  of  turbulence  are  associated. Terminal  Aerodrome  Forecast  (TAF)  Code, NAVMETOCCOMINST 3143.1, lists code figures for turbulence type and intensity. The Turbulence Reporting Criteria Table, table 5-2, has   been   adopted   as   the   standard. All NAVMETOCCOM  units  must  adopt  these  as  standards as a guide in forecasting turbulence. The  following  is  a  guide  to  the  classification  of turbulence. Extreme Turbulence This   rarely   encountered   condition   is   usually confined to the strongest forms of convection and wind shear, such as the following: .  Mountain  waves  in  or  near  the  rotor  cloud, usually found at low levels, leeward of the mountain ridge when the wind normal to the mountain ridge exceeds 50 knots. . In severe thunderstorms where the production of large hail (three-fourths inch or more) is indicated. It is more frequently encountered in organized squall lines than in isolated thunderstorms. Severe  Turbulence Severe  turbulence  may  also  be  found  in  the following: l In mountain waves: — When the wind normal to the mountain ridge exceeds 50 knots. The turbulence may extend to the tropopause, and at a distance of 150 miles leeward. A reasonable mountain wave turbulence layer is about 5,000 feet thick. — When the wind normal to the mountain ridge is 25 to 50 knots, the turbulence may extend up to 50 miles leeward of the ridge, and from the mountain ridge  up  to  several  thousand  feet  above. .   In   and   near   mature   thunderstorms,   and occasionally  in  towering  cumuliform  clouds. .  Near  jetstreams  within  layers  characterized  by horizontal wind shears greater than 40 knots/150 run, and vertical wind shears in excess of 6 knots/1,000 feet. When such layers exist, favored locations are below and/or above the jet core, and from roughly the vertical axis of the jet core to about 50 to 100 miles toward the cold  side. Moderate Turbulence Moderate turbulence may be found in the following: . In mountain waves: — When the wind normal to the mountain ridge exceeds  50  knots.  Moderate  turbulence  may  be  found from the ridge line to as much as 300 miles leeward. — When the wind normal to the ridge is 25 to 50  knots,  moderate  turbulence  maybe  found  from  the ridge line to as much as 150 miles leeward. .  In,  near,  and  above  thunderstorms,  and  in towering  cumuliform  clouds. . Near jetstreams and in upper level troughs, cold lows, and fronts aloft where vertical wind shears exceed 6 knots/1,000 feet, or horizontal wind shears exceed 10 knots per 100 miles. l At low altitudes, usually below 5,000 feet, when surface winds exceed 25 knots, or the atmosphere is unstable  due  to  strong  insolation  or  cold  advection. Light Turbulence In addition to the situations where more intense classes of turbulence occur, the relatively common class of  light  turbulence  maybe  found: l In mountainous areas, even with light winds. l In and near cumulus clouds. l Near the tropopause. l At low altitudes when winds are under 15 knots, or the air is colder than the underlying surface. 5-36







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