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point of impact. A stationary thunderstorm produces a microburst that
spreads in a 360-degree circle from the initial point of impact (left image,
figure 5-19). However, most thunderstorms are in motion and cause the
microburst to move in the direction of the storm (right image, figure
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MultiScan™ Radar Microbursts And Windshear
5-19). This movement distorts the circular air flow of the microburst so
that it becomes elliptical. In these cases, the edge of the ellipse that is
at the front of the storm produces the strongest winds and the trailing
edge has somewhat weaker winds.
Figure 5-19 Stationary and Moving Microburst Impact Patterns
Microburst are classified as either “wet” or “dry”. Wet microbursts occur
when a thunderstorm draws in dry air from the surrounding atmosphere.
The dry air evaporates rain within the thunderstorm, which causes the
air to cool. This cooler, heavier air then plunges towards the ground. In
this scenario, heavy rain from the thunderstorm may completely mask
the actual microburst.
Dry microbursts occur when the air below the thunderstorm is dry.
In this case, rain from the thunderstorm evaporates as it descends
towards the surface. This evaporation causes the air to cool. This
cooler, heavier air then plunges towards the ground. In this situation,
the only evidence of a microburst may be dust kicked up by the wind
when it impacts with the ground.
Rain that evaporates before it reaches the ground is known as virga.
An observer on the surface would see this as wisps of rain coming from
the cloud but disappearing before they reached the ground. Similarly,
as an aircraft descends, virga is sometimes evident on the radar. Notice
how the rain shower in figure 5-21 dissipates as the aircraft descends.
Dry air below the thunderstorm is causing the rain to evaporate. When
the radar displays rain at higher altitudes but the rain disappears as
the aircraft descends, virga is present and the conditions are right for
the formation of a dry microburst.
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Figure 5-20 shows a weather radar display for an aircraft flying at 5,000
feet with a display range of 40 NM. Thunderstorms are visible in the
lower left corner, and a significant rain shower is visible in the middle
right.
Figure 5-20 Weather Radar Rainfall Display
Figure 5-21 shows a sequence of weather radar displays as the aircraft
descends to 2,500 feet over a period of approximately 2 1/2 minutes
with the display range set to 20 NM. Note how the display shows
rainfall evaporating as the aircraft descends. This sequence of displays
indicates conditions are right for the formation of a dry microburst event.
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MultiScan™ Radar Microbursts And Windshear
Figure 5-21 Weather Radar Virga Display
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WINDSHEAR
Although the downdraft from a microburst can be dangerous to an
aircraft, the greatest threat comes from the change in wind direction
— commonly called windshear — near the center of the microburst
which may result in a corresponding loss of airspeed. The following
paragraphs and associated figures provide an example of how a
windshear event can affect an aircraft landing.
Consider the following example of a microburst that produces a 40-knot
horizontal windshear. In this example (figure 5-22), the aircraft is on
approach with an air speed of 130 knots, 300 pounds thrust (power) and
an initial altitude of 400 meters.
Figure 5-22 Windshear Example — Approach Conditions
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MultiScan™ Radar Microbursts And Windshear
When the aircraft enters the leading edge of the windshear zone, the
indicated air speed jumps from 130 to 170 knots due to the additional
airflow (headwind) over the wing (figure 5-23). An unexpected increase
in aircraft performance is, thus, often the first clue that a windshear
event has been encountered.
Figure 5-23 Windshear Example — Headwind Zone
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The increased approach speed will tend to make the aircraft float above
the approach course. Because the aircraft is high and fast, the natural
　
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