The criteria for each type of CAT areas follows: . Mountain wave CAT. Winds 25 knots or greater, normal  to  terrain  barriers,  and  significant  surface pressure  differences  across  such  barriers. . Trough CAT. That portion of a trough that has horizontal shear on the order of 25 knots, or more, in 90 nautical miles. . Closed low aloft CAT. If the flow is merging or splitting, moderate or severe CAT maybe encountered. Also,  to  the  northeast  of  a  cutoff  low  aloft,  significant CAT may be experienced. As with the jetstream CAT, the intensity of this type of turbulence is related to the strength of the shear. . Wind shear CAT. Those zones in space in which wind speeds are 60 knots or greater, and both horizontal and vertical shear exists, as indicated in table 5-1. No provision is made for light CAT because light turbulence serves only as a flight nuisance. Any of the above situations can produce moderate to severe CAT. However, the combination of two or more of the above conditions  is  almost  certain  to  produce  severe  or  even extreme CAT. A jetstream may be combined with a mountain wave or be associated with a merging or splitting  low. Turbulence on the Lee Side of Mountains When   strong   winds   blow   approximately perpendicular  to  a  mountain  range,  the  resulting turbulence may be quite severe. Associated areas of steady updrafts and downdrafts may extend to heights from 2 to 20 times the height of the mountain peaks. Under these conditions when the air is stable, large waves tend to form on the lee-side of the mountains, and may  extend  150  to  300  miles  downwind.  They  are referred  to  as  mountain  waves.   Some  pilots  have reported  that  flow  in  these  waves  is  often  remarkably smooth, while others have reported severe turbulence. The structure and characteristics of the  mountain  wave were presented in volume 2 of the AG2 TRAMAN. Refer to figure 6-1-5 in volume 2 for an illustration of a mountain wave. The  windflow  normal  to  the  mountain  produces  a primary  wave,  and,  generally  less  intense,  additional waves  farther  downwind.  The  characteristic  cloud patterns may or may not be present to identify the wave. The  pilot,  for  the  most  part,  is  concerned  with  the primary wave because of its more intense action and proximity  to  the  high  mountainous  terrain.  Severe turbulence frequently can be found 150 to 300 miles downwind, when the winds are greater than 50 knots at the mountaintop level. When winds are less than 50 knots  at  the  mountaintop  level,  a  lesser  degree  of turbulence may be experienced. Some  of  the  most  dangerous  features  of  the mountain wave are the turbulence in and below the roll cloud, the downdrafts just to the lee side of the mountain peaks, and to the lee side of the roll clouds. The cap cloud must always be avoided because of turbulence and concealed mountain peaks. The following five rules have been suggested for flights over mountain ranges where waves exist: 1. The pilot should, if possible, fly around the area when  wave  conditions  exist.  If  this  is  not  feasible, he/she should fly at a level that is at least 50 percent higher than the height of the mountain range. 2.  The  pilot  should  avoid  the  roll  clouds,  since these are the areas with the most intense turbulence. 3. The pilot should avoid the strong downdrafts on the lee side of the mountain. Table 5-1.-Wind Sheer CAT with Wind Speed 60 Knots or Greater HORIZONTAL VERTICAL CAT SHEAR SHEAR INTENSITY (NAUT/MI) (PER 1,000 FT) 25k/90 9-12k Moderate 25k/90 12-15k Moderate, at  times severe 25k/90 above  15k Severe 5-35