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Meteorology 7-1-45
7-1-24. Wind Shear PIREPs
a. Because unexpected changes in wind speed and
direction can be hazardous to aircraft operations at
low altitudes on approach to and departing from
airports, pilots are urged to promptly volunteer
reports to controllers of wind shear conditions they
encounter. An advance warning of this information
will assist other pilots in avoiding or coping with a
wind shear on approach or departure.
b. When describing conditions, use of the terms
“negative” or “positive” wind shear should be
avoided. PIREPs of “negative wind shear on final,”
intended to describe loss of airspeed and lift, have
been interpreted to mean that no wind shear was
encountered. The recommended method for wind
shear reporting is to state the loss or gain of airspeed
and the altitudes at which it was encountered.
EXAMPLE-
1. Denver Tower, Cessna 1234 encountered wind shear,
loss of 20 knots at 400.
2. Tulsa Tower, American 721 encountered wind shear on
final, gained 25 knots between 600 and 400 feet followed
by loss of 40 knots between 400 feet and surface.
1. Pilots who are not able to report wind shear in
these specific terms are encouraged to make reports
in terms of the effect upon their aircraft.
EXAMPLEMiami
Tower, Gulfstream 403 Charlie encountered an
abrupt wind shear at 800 feet on final, max thrust required.
2. Pilots using Inertial Navigation Systems
(INSs) should report the wind and altitude both above
and below the shear level.
7-1-25. Clear Air Turbulence (CAT) PIREPs
CAT has become a very serious operational factor to
flight operations at all levels and especially to jet
traffic flying in excess of 15,000 feet. The best
available information on this phenomenon must
come from pilots via the PIREP reporting procedures.
All pilots encountering CAT conditions are urgently
requested to report time, location, and intensity (light,
moderate, severe, or extreme) of the element to the
FAA facility with which they are maintaining radio
contact. If time and conditions permit, elements
should be reported according to the standards for
other PIREPs and position reports.
REFERENCEAIM,
PIREPs Relating to Turbulence, Paragraph 7-1-23.
7-1-26. Microbursts
a. Relatively recent meteorological studies have
confirmed the existence of microburst phenomenon.
Microbursts are small scale intense downdrafts
which, on reaching the surface, spread outward in all
directions from the downdraft center. This causes the
presence of both vertical and horizontal wind shears
that can be extremely hazardous to all types and
categories of aircraft, especially at low altitudes. Due
to their small size, short life span, and the fact that
they can occur over areas without surface precipitation,
microbursts are not easily detectable using
conventional weather radar or wind shear alert
systems.
b. Parent clouds producing microburst activity
can be any of the low or middle layer convective
cloud types. Note, however, that microbursts
commonly occur within the heavy rain portion of
thunderstorms, and in much weaker, benign
appearing convective cells that have little or no
precipitation reaching the ground.
AIM 2/14/08
7-1-46 Meteorology
FIG 7-1-13
Evolution of a Microburst
T-5 Min T-2 Min T T 5 Min T 10 Min
HEIGHT (feet)
10,000
5,000
WIND SPEED
10-20 knots
> 20 knots
SCALE (miles)
0 1 2 3
Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence at
the surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984,
Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAA
Report No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp.
c. The life cycle of a microburst as it descends in
a convective rain shaft is seen in FIG 7-1-13. An
important consideration for pilots is the fact that the
microburst intensifies for about 5 minutes after it
strikes the ground.
d. Characteristics of microbursts include:
1. Size. The microburst downdraft is typically
less than 1 mile in diameter as it descends from the
cloud base to about 1,000-3,000 feet above the
ground. In the transition zone near the ground, the
downdraft changes to a horizontal outflow that can
extend to approximately 2 1/2 miles in diameter.
2. Intensity. The downdrafts can be as strong
as 6,000 feet per minute. Horizontal winds near the
　
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