will be no supercooled droplets, but hoarfrost will form
on the aircraft through direct sublimation of water vapor.
. When the temperature curve lies to the right of
the 8D curve in a subfreezing layer, the layer is
subsaturated with respect to both ice and water surface.
No icing will occur in this region.
Modification of the Icing Forecast
The final phase is to modify your icing forecast.
This is essentially a subjective process. The forecaster
should consider the following items: probable
intensification or weakening of synoptic features, such
as low-pressure centers, fronts, and squall lines during
the time interval between the latest analysis and the
forecast; local influences, such as geographic location,
terrain features, and proximity to ocean coastlines or
lake shores, and radar weather observations and pilot
reports of icing. The forecaster should be cautious in
either underforecasting or overforecasting the amount
and intensity of icing.
An overforecast results in a
reduced payload for the aircraft due to increased fuel
load, while an underforecast may result in an operational
R e c a ll
characteristics associated with turbulence.
Determine the four intensities of turbulence.
Forecast surface, in-cloud, and clear air
turbulence (CAT). Recognize the advantages
of the use of Doppler radar in turbulence
Turbulence is of major importance to pilots of all
types of aircraft; therefore, it is also of importance to the
forecaster, whose duty it is to recognize situations where
turbulence may exist, and to forecast both the areas and
intensity of the turbulence. The following text discusses
the classification and intensity of turbulence, the
forecasting of turbulence near the ground, the
forecasting of turbulence in convective clouds, and the
forecasting of clear air turbulence (CAT).
Refer to the AG2 TRAMAN, volume 2, unit 6, for
a discussion of the types and properties of turbulence.
Turbulence may be defined as irregular and
instantaneous motions of air that are made up of a
number of small eddies that travel in the general air
current. Atmospheric turbulence is caused by random
fluctuations in the windflow. Given an analyzed wind
field with both streamlines and isotachs smoothly
drawn, any difference between an actual wind and this
smooth field is attributed to turbulence.
To an aircraft in flight, the atmosphere is considered
turbulent when irregular whirls or eddies of air affect
the motion of the aircraft, and a series of abrupt jolts or
bumps is felt by the pilot. Although a large range of
sizes of eddies exists in the atmosphere, those causing
bumpiness are roughly of the same size as the aircraft
dimensions, and usually occur in an irregular sequence
imparting sharp translation or angular motions to the
aircraft. The intensity of the disturbances to the aircraft
varies not only with the intensity of the irregular motions
of the atmosphere but also with aircraft characteristics,
such as flight speed, weight stability, and size.
Wind Shear and CAT
A relatively tight gradient, either horizontal or
vertical, produces churning motions (eddies), which
result in turbulence. The greater the change of wind
speed and/or direction, the more severe the turbulence.
Turbulent flight conditions are often found in the
vicinity of the jetstream, where large shears in the
horizontal and vertical are found. Since this type of
turbulence may occur without any visual warning, it is
often referred to as CAT.
The term clear air turbulence is misleading because
not all high-level turbulence included in this
classification occurs in clear air. However, the majority
(75 percent) is found in a cloud-free atmosphere. CAT
is not necessarily limited to the vicinity of the jetstream;
it may occur in isolated regions of the atmosphere. Most
frequently, CAT is associated with the jetstream or
mountain waves. However, it may also be associated
with a closed low aloft, a sharp trough aloft, or an
advancing cirrus shield. A narrow zone of wind shear,
with its accompanying turbulence, is sometimes
encountered by aircraft as it climbs or descends through
a temperature inversion. Moderate turbulence may also
be encountered momentarily when passing through the
wake of another aircraft.