Lightning-Warning Systems for Use by Airports (2008) / Chapter Skim
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Pages 7-24
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From page 7... ...
This charge imbalance creates an atmospheric electric field (roughly 100 V/m near the earth's surface) and a corresponding air-earth electrical current directed downward from the ionosphere to the ground, where the direction of the current is defined as the direction that a hypothetical positive charge would flow.
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From page 8... ...
Eventually, when the charges build up to a high enough level to cause an electrical breakdown in the air separating the charge centers, the built-up charges can discharge in a lightning stroke. This can either happen between the cloud and the ground, or between the positive and negative charge centers within the cloud.
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From page 9... ...
The actual charge transfer is, however, done by free electrons so the return stroke is really just a progressive draining of negative charge downward, with the upper limit of the drained path moving upward as electrons flow to the ground. Multiple strokes of dart leaders and return strokes can follow, producing flickering strobe-like flashes of light (see Figure 4)
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From page 10... ...
10 Figure 3. Anatomy of a lightning strike (5)
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From page 11... ...
Stepped leaders are frozen, while the multiple return strokes show up as separate strokes that follow exactly the same path (4)
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From page 12... ...
TYPICAL CLOUD-TO-GROUND LIGHTNING BETWEEN GROUND AND NEGATIVE CHARGE CENTERS DISCHARGE WITHIN CLOUD BETWEEN NEGATIVE BASE AND POSITIVE TOP (INTRA-CLOUD) DISCHARGE BETWEEN NEGATIVE AND POSITIVE CHARGE CENTERS Figure 5.
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From page 13... ...
Nearby lightning discharges will produce sudden changes in the strength of the local electrical field, and these distinctive changes can be used to detect lightning -- although without any direct way of measuring the distance or range to the lightning flash. Nearby charge centers, such as a cloud developing directly overhead, can dominate the local electric field and may limit the detection of distant lightning strikes.
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From page 14... ...
Ground-based lightning detection networks are primarily designed to detect CG lightning and can provide information about each individual stroke within a lightning flash. With recent improvements to these same detectors they can now detect a significant percentage of the nearby IC lightning strokes, but at a variable and as yet not well characterized detection efficiency that depends on the properties of the stroke and the distance from the network sensors.
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From page 15... ...
While there are no universally recognized standards for issuing alerts or alarms for airport ramp operations, the American Meteorological Society and the National Oceanic and Atmospheric Administration (NOAA) have endorsed the "30-30 rule." This rule states that outdoor activities should be limited or curtailed whenever there has been a lightning strike detected within 6 mi (based on 30 sec between an observed flash and the sound of the thunder)
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From page 16... ...
The illustration on the right, provided by Bill Rison from the New Mexico Institute for Mining and Technology, is a vertical cross-section of a storm, as seen by a research radar, overlaid with a full depiction of a lightning stroke based on a specialized lightning mapping system capable of detecting each step in the lightning stroke. In this case, the lightning strikes the ground about 5 km (3 mi)
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From page 17... ...
Electric Field Mills (or Other Electric Field Monitoring Systems) By monitoring the buildup of the local electric field strength, electric field mills (or other electric field monitoring systems)
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From page 18... ...
The Vaisala thunderstorm warning system is based on realtime lightning observations provided by Vaisala's NLDN. The system can optionally be augmented by the addition of up to seven electric field mills.
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From page 19... ...
provides a simple, direct view of nearby lightning strikes (see Figure 11)
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From page 20... ...
The first, based on monitoring the buildup of the atmospheric electric field in response to nearby charged clouds, represents a true prediction. Electric field measurements will not, however, necessarily predict all nearby lightning strikes, and they can be expected to produce occasional false alarms (19, 20)
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From page 21... ...
Monitoring the Local Atmospheric Electric Field As already discussed, electric field measurements can detect the presence of high levels of charge separation in nearby clouds that suggests a strong likelihood of current or future lightning activity. These systems have the unique potential to provide advance warning of the first lightning strike from a developing storm.
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From page 22... ...
In this storm, the initial mid-level strengthening of the radar echo preceded an intensive growth period, with the highest lightning flash rates well correlated with the period of the maximum updrafts. This storm's ratio of IC to CG lightning strikes was unusually high, but follows the normal pattern of IC lightning developing several minutes before the first CG stroke.
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From page 23... ...
As a predictor, total lightning is also attractive since it is based on an observed event, rather than dependent on extrapolated storm behaviors. While current lightning detection networks can detect some IC lightning flashes, high-efficiency detection of IC lightning events will require the network sensors to be enhanced, combined with a significant increase in the number of network sensors.
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From page 24... ...
24 Figure 16. An example of the WDT lightning prediction algorithm running within the WDT LDSS.
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