Index   488 -- Page 489 -- 490

5/26/16
12/10/15                                                                                                                                          AIM


                                                                           FIG 7-1-10
                                                               Evolution of a Microburst



                                                                     WIND SPEED

                                                                             10-20 knots
                                                                               > 20 knots
      HEIGHT (feet)




                               T-5 Min T-2 Min                 T                  T + 5 Min                     T + 10 Min

                      10,000

                      5,000



                                                                           0          1       2       3

                                                                               SCALE (miles)

                                   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                                    3. Visual Signs. Microbursts can be found
a convective rain shaft is seen in FIG 7-1-10. An                                    almost anywhere that there is convective activity.
important consideration for pilots is the fact that the                              They may be embedded in heavy rain associated with
microburst intensifies for about 5 minutes after it                                  a thunderstorm or in light rain in benign appearing
strikes the ground.                                                                  virga. When there is little or no precipitation at the
                                                                                     surface accompanying the microburst, a ring of
  d. Characteristics of microbursts include:
                                                                                     blowing dust may be the only visual clue of its
     1. Size. The microburst downdraft is typically                                  existence.
less than 1 mile in diameter as it descends from the
cloud base to about 1,000-3,000 feet above the
                                                                                          4. Duration. An individual microburst will
ground. In the transition zone near the ground, the
                                                                                     seldom last longer than 15 minutes from the time it
downdraft changes to a horizontal outflow that can
                                                                                     strikes the ground until dissipation. The horizontal
extend to approximately 2 1/2 miles in diameter.
                                                                                     winds continue to increase during the first 5 minutes
     2. Intensity. The downdrafts can be as strong                                   with the maximum intensity winds lasting approxi-
as 6,000 feet per minute. Horizontal winds near the                                  mately 2-4 minutes. Sometimes microbursts are
surface can be as strong as 45 knots resulting in a                                  concentrated into a line structure, and under these
90 knot shear (headwind to tailwind change for a                                     conditions, activity may continue for as long as an
traversing aircraft) across the microburst. These                                    hour. Once microburst activity starts, multiple
strong horizontal winds occur within a few hundred                                   microbursts in the same general area are not
feet of the ground.                                                                  uncommon and should be expected.




Meteorology                                                                                                                                     7-1-45