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several times each year. Flying into a volcanic ash
cloud can be extremely dangerous. At least two
B747s have lost all power in all four engines after
such an encounter. Regardless of the type aircraft,
some damage is almost certain to ensue after an
encounter with a volcanic ash cloud.
b. While some volcanoes in the U.S. are
monitored, many in remote areas are not. These
unmonitored volcanoes may erupt without prior
warning to the aviation community. A pilot observing
a volcanic eruption who has not had previous
notification of it may be the only witness to the
eruption. Pilots are strongly encouraged to transmit a
PIREP regarding volcanic eruptions and any
observed volcanic ash clouds.
c. Pilots should submit PIREPs regarding volcanic
activity using the Volcanic Activity Reporting (VAR)
form as illustrated in Appendix 2. If a VAR form is not
immediately available, relay enough information to
identify the position and type of volcanic activity.
d. Pilots should verbally transmit the data required
in items 1 through 8 of the VAR as soon as possible.
The data required in items 9 through 16 of the VAR
should be relayed after landing if possible.
7−1−29. Thunderstorms
a. Turbulence, hail, rain, snow, lightning, sustained
updrafts and downdrafts, icing conditions−all
are present in thunderstorms. While there is some
evidence that maximum turbulence exists at the
middle level of a thunderstorm, recent studies show
little variation of turbulence intensity with altitude.
b. There is no useful correlation between the
external visual appearance of thunderstorms and the
severity or amount of turbulence or hail within them.
The visible thunderstorm cloud is only a portion of a
turbulent system whose updrafts and downdrafts
often extend far beyond the visible storm cloud.
Severe turbulence can be expected up to 20 miles
from severe thunderstorms. This distance decreases
to about 10 miles in less severe storms.
AIM 2/17/05
7−1−56 Meteorology
c. Weather radar, airborne or ground based, will
normally reflect the areas of moderate to heavy
precipitation (radar does not detect turbulence). The
frequency and severity of turbulence generally
increases with the radar reflectivity which is closely
associated with the areas of highest liquid water
content of the storm. NO FLIGHT PATH THROUGH
AN AREA OF STRONG OR VERY STRONG
RADAR ECHOES SEPARATED BY 20−30 MILES
OR LESS MAY BE CONSIDERED FREE OF
SEVERE TURBULENCE.
d. Turbulence beneath a thunderstorm should not
be minimized. This is especially true when the
relative humidity is low in any layer between the
surface and 15,000 feet. Then the lower altitudes may
be characterized by strong out flowing winds and
severe turbulence.
e. The probability of lightning strikes occurring to
aircraft is greatest when operating at altitudes where
temperatures are between minus 5 degrees Celsius
and plus 5 degrees Celsius. Lightning can strike
aircraft flying in the clear in the vicinity of a
thunderstorm.
f. METAR reports do not include a descriptor for
severe thunderstorms. However, by understanding
severe thunderstorm criteria, i.e., 50 knot winds or
3/4 inch hail, the information is available in the report
to know that one is occurring.
g. NWS radar systems are able to objectively
determine radar weather echo intensity levels by use
of Video Integrator Processor (VIP) equipment.
These thunderstorm intensity levels are on a scale of
one to six.
REFERENCE−
Pilot/Controller Glossary, Radar Weather Echo Intensity Levels.
EXAMPLE−
1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) level five intense weather echo
between ten o’clock and two o’clock, one zero miles,
moving east at two zero knots, tops Flight Level three nine
zero.
2. Alert provided by an AFSS/FSS:
(aircraft identification) level five intense weather echo, two
zero miles west of Atlanta V−O−R, two five miles wide,
moving east at two zero knots, tops Flight Level three nine
zero.
7−1−30. Thunderstorm Flying
a. Above all, remember this: never regard any
thunderstorm “lightly” even when radar observers
report the echoes are of light intensity. Avoiding
thunderstorms is the best policy. Following are some
Do’s and Don’ts of thunderstorm avoidance:
1. Don’t land or takeoff in the face of an
approaching thunderstorm. A sudden gust front of
low level turbulence could cause loss of control.
　
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