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
. 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
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
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
(PER 1,000 FT)