AIM
4/3/14
7−1−59
Meteorology
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.
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.
Current weather radar systems are able to
objectively determine precipitation intensity. These
precipitation intensity areas are described as “light,”
“moderate,” “heavy,” and “extreme.”
REFERENCE
−
Pilot/Controller Glossary, Precipitation Radar Weather Descriptions.
EXAMPLE
−
1. Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between
ten o’clock and two o’clock, one five miles. Precipitation
area is two five miles in diameter.
2. Alert provided by an FSS:
(aircraft identification) EXTREME precipitation two zero
miles west of Atlanta V
−O−R, two five miles wide, moving
east at two zero knots, tops flight level three niner zero.
7
−
1
−
29. Thunderstorm Flying
a.
Thunderstorm Avoidance. Never regard any
thunderstorm lightly, even when radar 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.
2.
Don’t attempt to fly under a thunderstorm
even if you can see through to the other side.
Turbulence and wind shear under the storm could be
hazardous.
3.
Don’t attempt to fly under the anvil of a
thunderstorm. There is a potential for severe and
extreme clear air turbulence.
4.
Don’t fly without airborne radar into a cloud
mass containing scattered embedded thunderstorms.
Scattered thunderstorms not embedded usually can
be visually circumnavigated.
5.
Don’t trust the visual appearance to be a
reliable indicator of the turbulence inside a
thunderstorm.
6.
Don’t assume that ATC will offer radar
navigation guidance or deviations around thunder-
storms.
7.
Don’t use data-linked weather next genera-
tion weather radar (NEXRAD) mosaic imagery as the
sole means for negotiating a path through a
thunderstorm area (tactical maneuvering).
8.
Do remember that the data-linked NEXRAD
mosaic imagery shows where the weather was, not
where the weather is. The weather conditions may be
15 to 20 minutes older than the age indicated on the
display.
9.
Do listen to chatter on the ATC frequency for
Pilot Weather Reports (PIREP) and other aircraft
requesting to deviate or divert.
10.
Do ask ATC for radar navigation guidance
or to approve deviations around thunderstorms, if
needed.