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
