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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.
REFERENCE−
AIM, PIREPs Relating to Turbulence, Paragraph 7−1−24.
7−1−27. 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 2/17/05
7−1−46 Meteorology
FIG 7−1−14
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 knotts
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 7−1−14. 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
surface can be as strong as 45 knots resulting in a
90 knot shear (headwind to tailwind change for a
traversing aircraft) across the microburst. These
strong horizontal winds occur within a few hundred
feet of the ground.
3. Visual Signs. Microbursts can be found
almost anywhere that there is convective activity.
They may be embedded in heavy rain associated with
a thunderstorm or in light rain in benign appearing
virga. When there is little or no precipitation at the
surface accompanying the microburst, a ring of
blowing dust may be the only visual clue of its
existence.
4. Duration. An individual microburst will
seldom last longer than 15 minutes from the time it
strikes the ground until dissipation. The horizontal
winds continue to increase during the first 5 minutes
with the maximum intensity winds lasting approximately
2−4 minutes. Sometimes microbursts are
concentrated into a line structure, and under these
conditions, activity may continue for as long as an
hour. Once microburst activity starts, multiple
microbursts in the same general area are not
uncommon and should be expected.
2/17/05 AIM
Meteorology 7−1−47
FIG 7−1−15
Microburst Encounter During Takeoff
A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing
performance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft
(3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane.
Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWG
Associates, Inc., Tullahoma, Tennessee.
e. Microburst wind shear may create a severe
hazard for aircraft within 1,000 feet of the ground,
particularly during the approach to landing and
landing and take-off phases. The impact of a
　
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