Type
Cause
Mechanical
Caused by horizontal and vertical wind shear, it is the result of
pressure gradient differences, terrain obstructions, or frontal zone
shear.
Two major types of mechanical turbulence are:
Clear Air Turbulence (CAT) includes all turbulence not
associated with visible convective activity, includes high-level
frontal and jet stream turbulence, and may occur in high-level,
non-convective clouds.
Mountain Wave (MW) often occurs in clear air in a stationary
wave downwind of a prominent mountain range and is caused
by the disturbance of wind by the mountain range.
Wake
Results when a smaller aircraft encounters vortices from a larger
aircraft. A vortex is formed at an aircrafts wingtip as air circulates
outward, upward, and around the wingtip. The size of the vortex
varies with the size and weight of the aircraft. Vortices sink at 400-500
ft/minute and stabilize 900 ft below the flight path where they begin to
dissipate.
Thermal
This type of turbulence is associated with surface heating. As solar
radiation heats the surface, the air above it is warmed through
conduction. Thermal convection within the boundary layer transports
the warm air upward in an uneven and irregular motion, creating
eddies and gusts (i.e. turbulence). It is particularly hazardous for
takeoff/landings when it happens over a short duration, affects
airspeed variations of >10kts, and is within 2000 feet of the ground.
Low-Level
This is a severe change in either wind speed and/or direction over a
Wind Shear
short period of time, resulting in an increase or decrease of air across
(LLWS)
a wing and thus a sudden increase or loss of lift for the aircraft. It is
particularly hazardous for takeoffs/landings when it happens over a
short duration, effects airspeed variations of >10 knots, and is within
2,000 ft of the ground.
Table 1-13 -- Four causes of turbulence
Degrees of Turbulence
As stated earlier, turbulence is classified and reported in degrees or intensities. Table 1-
14 gives a brief description of these intensities to clarify.
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