Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types
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
Usually found beneath thunderstorms and visible rain or virga
Usually 1 to 2 miles in diameter; wind speeds can exceed 100 knots (10,000 Feet Per Minute (FPM)) and be accompanied by rain or other obscuring phenomena; usually lasts less than 10 minutes [Figure 1]
Intense horizontal outflows at low altitudes result in extreme headwind to tailwind differentials that have been recorded in excess of 200 KIAS
An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground
Experience has shown that microbursts are not isolated, but usually occur in groups
Microburst Characteristics:
Aeronautical Information Manual, Evolution of a Microburst
Microbursts are small scale, intense local 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
Microbursts may be either wet, referring to their presence from a thunderstorm, or dry, referring to their presence in clear air
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
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' 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
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 [Figure 2]
The aircraft may encounter a headwind (performance increasing) followed by a downdraft and tailwind (both performance decreasing), possibly resulting in terrain impact
Intensity:
The downdrafts can be as strong as 6,000 FPM
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
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
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
Indications of Microbursts:
Blowing dust, dust devils, and gust fronts (down bursts will occasionally generate distinctive circular dust patterns)
Thunderstorms in vicinity with visible areas of intense downdrafts indicated by rain or virga
Sudden and unexplained increase in airspeed as noted on airspeed indicator accompanied by increased AoA, indicative or rolling outflow
Sudden increase in rate of descent accompanied by a lower AoA, indicative of entry into microburst core
Extreme variations in wind velocity and direction in short time
Significant differences between winds at 1,500' to 2,000' AGL and the surface winds
Ground stations, radar, PIREPS and LLWAS help in identification
Aeronautical Information Manual, Microburst Encounter During Takeoff
Recovery:
If you find yourself caught in a microburst, immediately go around and execute a missed approach
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