background image

AIM

10/12/17

7

−1−47

Meteorology

FIG 7

−1−12

Evolution of a Microburst

T-5 Min

T-5 Min T-2 Min

T-2 Min

T

T

T + 5 Min

T + 5 Min

T + 10 Min

T + 10 Min

HEIGHT (feet)HE

I

G

HT

 (

f

e

e

t)

10,000

10,000

5,000

5,000

WIND SPEED

WIND SPEED

10-20 knots

10-20 knots

> 20 knots

> 20 knots

SCALE (miles)

SCALE (miles)

0

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

−1−12. 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 approxi-
mately 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.