The best source of information on the location and intensity
of turbulence are PIREPs. Therefore, Pilots are encouraged to
familiarize themselves with the turbulence reporting criteria
found in the AIM, which also describes the procedure for
volunteering PIREPs relating to turbulence.

Structural Icing
The very nature of flight in Instrument Meteorological
Conditions means operating in visible moisture such as clouds.
At the right temperatures, this moisture can freeze on the
aircraft, causing increased weight, degraded performance, and
unpredictable aerodynamic characteristics. Understanding,
avoidance and early recognition followed by prompt action
are the keys to avoiding this potentially hazardous situation.

Structural icing refers to the accumulation of ice on the exterior
of the aircraft and is broken down into three classifications:
rime ice, clear ice, and mixed ice. For ice to form, there must
be moisture present in the air, and the air must be cooled to
a temperature of 0° C (32° F) or less. Aerodynamic cooling
can lower the surface temperature of an airfoil and cause ice
to form on the airframe even though the ambient temperature
is slightly above freezing.

Rime ice forms if the droplets are small and freeze immediately
when contacting the aircraft surface. This type of ice usually
forms on areas such as the leading edges of wings or struts.
It has a somewhat rough-looking appearance and a milky
white color.

Clear ice is usually formed from larger water droplets or
freezing rain that can spread over a surface. This is the most
dangerous type of ice since it is clear, hard to see, and can
change the shape of the airfoil.

Mixed ice is a mixture of clear ice and rime ice. It has the
bad characteristics of both types and can form rapidly. Ice
particles become embedded in clear ice, building a very rough
accumulation. The table in Figure 10-15 lists the temperatures
at which the various types of ice will form.

Figure 10-15. Temperature Ranges for Ice Formation.

Structural icing is a condition that can only get worse.
Therefore, during an inadvertent icing encounter, it is
important the pilot act to prevent additional ice accumulation.
Regardless of the level of anti-ice or deice protection offered

by the aircraft, the first course of action should be to leave
the area of visible moisture. This might mean descending
to an altitude below the cloud bases, climbing to an altitude
that is above the cloud tops, or turning to a different course.
If this is not possible, then the pilot must move to an altitude
where the temperature is above freezing. Pilots should report
icing conditions to ATC and request new routing or altitude
if icing will be a hazard. Refer to the AIM for information
on reporting icing intensities.

Fog
Instrument pilots must learn to anticipate conditions leading
to the formation of fog and take appropriate action early in
the progress of the flight. Before a flight, close examination
of current and forecast weather should alert the pilot to the
possibility of fog formation. When fog is a consideration,
pilots should plan adequate fuel reserves and alternate
landing sites. En route, the pilot must stay alert for fog
formation through weather updates from EFAS, ATIS, and
ASOS/AWOS sites.

Two conditions will lead to the formation of fog. Either the
air is cooled to saturation, or sufficient moisture is added
to the air until saturation occurs. In either case, fog can
form when the temperature/dewpoint spread is 5° or less.
Pilots planning to arrive at their destination near dusk with
decreasing temperatures should be particularly concerned
about: the possibility of fog formation.

Volcanic Ash
Volcanic eruptions create volcanic ash clouds containing
an abrasive dust that poses a serous safety threat to flight
operations. Adding to the danger is the fact that these ash
clouds are not easily discernible from ordinary clouds when
encountered at some distance from the volcanic eruption.
When an aircraft enters a volcanic ash cloud, dust particles
and smoke may become evident in the cabin, often along with
the odor of an electrical fire. Inside the volcanic ash cloud,
the aircraft may also experience lightning and St. Elmo's fire
on the windscreen. The abrasive nature of the volcanic ash
can pit the windscreens, thus reducing or eliminating forward
visibility. The pitot-static system may become clogged,
causing instrument failure. Severe engine damage is probable
in both piston and jet-powered aircraft.

Every effort must be made to avoid volcanic ash. Since
volcanic ash clouds are carried by the wind, pilots should plan
their flights to remain upwind of the ash-producing volcano.
Visual detection and airborne radar are not considered
a reliable means of avoiding volcanic ash clouds. Pilots
witnessing volcanic eruptions or encountering volcanic
ash should immediately pass this information along in
the form of a pilot report. The National Weather Service
monitors volcanic eruptions and estimates ash trajectories.
This information is passed along to pilots in the form of
SIGMETs.