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24.1 Clear air turbulence (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 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. See TBL GEN 3.5.11, Turbulence Reporting Criteria Table.
25. Microbursts
25.1 Relatively recent meteorological studies have confirmed the existence of microburst phenomena. 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 precipita-tion, microbursts are not easily detectable using conventional weather radar or wind shear alert systems.
25.2 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.appear-ing convective cells that have little or no precipitation reaching the ground.
25.3 The life cycle of a microburst as it descends in a convective rain shaft is seen in FIG GEN 3.5.9, Evolution of a Microburst. An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground.
25.4 Characteristics of microbursts include:
25.4.1 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.
25.4.2 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.
25.4.3 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.
FIG GEN 3.5.9
Evolution of a Microburst
WIND SPEEDWIND SPEED
10-20 knots10-20 knots
T-5 MinT-5 Min T-2 MinT-2 Min T
T + 5 MinT + 5 Min > 20 knots> 20 knots T + 10 MinT + 10 Min
0
123
SCALE (miles)
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.
25.4.4 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 1 hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected.
FIG GEN 3.5.10
Microburst Encounter During Takeoff
NOTE.
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.
25.5 Microburst wind shear may create a severe experience of penetrating one is characterized in hazard for aircraft within 1,000 feet of the ground, FIG GEN 3.5.10. The aircraft may encounter a particularly during the approach to landing and headwind (performance increasing), followed by a landing and take.off phases. The impact of a downdraft and a tailwind (both performance microburst on aircraft which have the unfortunate decreasing), possibly resulting in terrain impact.
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