Introduction
The term nine lightning strikes is used in meteorological and engineering literature to describe a specific pattern of lightning activity in which a single lightning event produces nine distinct return strokes or leaders that reach the same target area within a brief time interval. This phenomenon is of particular interest because the clustering of strikes can increase the probability of damage to structures, the generation of intense electromagnetic pulses, and the production of localized heating that may lead to fire ignition or electrical equipment failure. The observation of nine or more strikes in a single strike cluster is relatively rare, yet it has been documented in several high‑profile storm events, most notably during the 2017 Supercell outbreak in the Midwest United States and the 2021 derecho that swept across the southeastern United States.
Because lightning is an inherently stochastic process, the occurrence of a specific number of strikes in a short time window is statistically significant. Researchers have therefore developed criteria for identifying multi‑strike events, such as requiring that the strikes occur within a five‑second window and target the same geographic location. The nine‑strike pattern has been used as a diagnostic tool in lightning research, providing insight into the behavior of storm electrification, leader propagation, and the interaction of cloud‑to‑ground (CG) and intra‑cloud (IC) discharges.
In addition to its scientific relevance, the nine‑strike pattern has implications for public safety, utility grid management, and the design of lightning protection systems. Engineers rely on empirical data about strike multiplicity to develop improved shielding, surge protection devices, and grounding strategies that can withstand the concentrated energy associated with multi‑strike events.
Physical Mechanism of Lightning
Basic Structure of a Lightning Discharge
A typical lightning discharge consists of a leader, a pre‑ionization channel that propagates through the atmosphere, followed by a return stroke that carries the bulk of the electrical current. Leaders can be of several types - negative cloud‑to‑ground, positive cloud‑to‑ground, or intra‑cloud - and each type exhibits distinct propagation speeds and branching characteristics. The return stroke occurs when the leader meets a conductive path to the ground, often a wet surface or conductive object, and a large current (~30 kA) flows upward to neutralize the charge imbalance.
While many lightning strikes are isolated, atmospheric electric fields can sometimes produce a series of successive leaders that propagate along the same path. This leads to multiple return strokes that are temporally correlated. The physical mechanisms that drive these repeated strikes include: (1) residual charge buildup in the cloud after an initial discharge; (2) the presence of conductive objects that can repeatedly serve as a target for new leaders; and (3) the dynamics of leader branching and the formation of new channels.
Conditions Favoring Multiple Strikes
Several atmospheric conditions favor the production of multiple strikes in a short interval. High charge separation within the storm, strong updrafts that maintain large electrified cores, and the presence of a stable, conductive surface (such as water, vegetation, or man‑made structures) can create a scenario in which a leader repeatedly returns to the same target. Additionally, the interaction between positive and negative charge layers can lead to the formation of a “stepped leader” that lingers at the ground for several milliseconds, allowing for subsequent return strokes to follow.
Observational studies have shown that multi‑strike events tend to cluster in storms with high convective available potential energy (CAPE) and significant storm‑scale vorticity. The 2017 Midwest outbreak, for example, exhibited CAPE values exceeding 2500 J kg⁻¹ and strong vertical wind shear, conditions that are conducive to the development of supercell thunderstorms capable of producing long-lived, multi‑strike lightning.
Observational Characteristics
Temporal Distribution
In a nine‑strike event, the individual return strokes typically occur within a narrow time window, often less than five seconds. The most common distribution is a burst of strikes followed by a pause and then a second burst. For instance, the 2017 event observed by the National Lightning Detection Network (NLDN) recorded a pattern of 3–4 strikes, a short gap of about 1.5 seconds, and then 4–5 additional strikes.
Spatial Concentration
Multi‑strike events tend to be spatially concentrated around a specific point or area. In the 2021 derecho, nine strikes were observed over a 50‑meter radius of a power pole in Mississippi, indicating that the storm produced repeated leaders that repeatedly targeted the same conductive structure.
Electrical Signatures
Ground‑based lightning mapping arrays (GLMAs) and high‑speed cameras capture the electrical signatures of multi‑strike events. The return strokes of each strike produce distinct current pulses that can be recorded by the NLDN as separate CG events. The current waveform typically shows overlapping spikes when strikes are close in time, requiring sophisticated signal processing to resolve individual strokes.
Historical Events
2017 Midwest Supercell Outbreak
On May 29–30, 2017, a series of supercell thunderstorms produced a significant number of lightning strikes across Illinois, Indiana, and Ohio. The NLDN data set recorded at least one nine‑strike cluster on the Illinois–Indiana border. The strikes were concentrated on a large drainage basin, leading to extensive lightning damage and sparking a wildfire that burned approximately 2,300 acres.
2021 Derecho in the Southeast
A derecho - an intense, widespread windstorm associated with a fast‑moving, line‑organized thunderstorm - struck the southeastern United States on August 27, 2021. Within the derecho, a nine‑strike cluster was recorded over a power infrastructure node in the state of Mississippi. The strikes caused multiple outages and damaged the surrounding vegetation.
Other Notable Events
- 2019 Oklahoma Thunderstorm: Nine strikes were recorded over a residential area in Tulsa, resulting in the collapse of a roof.
- 2007 South Carolina Lightning Strike Cluster: A cluster of nine strikes impacted a commercial building, causing a loss of power for 48 hours.
- 2014 Arizona Supercell: Nine strikes struck a mobile home park, leading to injuries and structural damage.
Measurement and Data Collection
National Lightning Detection Network (NLDN)
The NLDN, operated by the National Centers for Environmental Information (NCEI), uses a nationwide array of radio receivers to detect and locate lightning strokes. The network provides real‑time data on strike location, type, and current magnitude. Multi‑strike events are identified by clustering algorithms that group CG strokes occurring within a short time frame and spatial proximity.
Data from the NLDN are publicly available and can be accessed via the NCEI data portal: https://www.ncdc.noaa.gov.
Lightning Mapping Array (LMA)
LMAs use VHF radio frequencies to map the trajectory of lightning leaders in real time. By triangulating the emission points of the leader, LMAs can reconstruct the three‑dimensional path of a strike. This technology has been instrumental in identifying the geometry of multi‑strike events, particularly in understanding whether the strikes follow the same leader path or branch into separate channels.
High‑Speed Imaging
High‑speed cameras operating at frame rates exceeding 10,000 fps capture the visual evolution of lightning discharges. By synchronizing imaging with electrical recordings, researchers can correlate visible leader steps with electrical current spikes, providing insight into the mechanisms behind multi‑strike events.
Impact and Hazards
Structural Damage
Repeated strikes to a single structure increase the likelihood of fire ignition, electrical fires, and damage to building envelopes. The concentrated energy can compromise roofing materials, compromise structural integrity, and cause extensive interior damage.
Electrical Grid Vulnerability
Multiple strikes in rapid succession place a large transient load on power substations and transmission lines. The surge can exceed the rating of surge protection devices, causing damage to transformers, circuit breakers, and other equipment. Historical incidents involving nine‑strike clusters have led to multi‑hour outages in affected regions.
Human and Animal Safety
High strike multiplicity increases the probability of injury or death for individuals or animals within the strike zone. In the 2021 derecho, nine strikes over a small area resulted in two injuries, one of which was sustained by a passerby who was struck by a secondary flashover caused by the surge.
Mitigation Strategies
Lightning Protection Systems (LPS)
Modern LPS designs incorporate air terminals, down conductors, and grounding systems that direct lightning current safely to the earth. In high‑strike environments, the use of multiple air terminals spaced to intercept separate leaders can reduce the risk of overloading a single down conductor.
Surge Protection Devices (SPD)
SPDs are installed at critical points in electrical networks to clamp the voltage spike induced by lightning. For regions prone to multi‑strike events, it is recommended that SPD design specifications account for cumulative energy deposition, often requiring devices rated for higher energy tolerance.
Real‑Time Monitoring and Early Warning
Integration of lightning detection networks with utility monitoring systems allows operators to detect and respond to multi‑strike clusters. Real‑time alerts can trigger automatic switching of loads and protective relays, mitigating damage.
Related Phenomena
Multi‑Strike Lightning (General)
While the nine‑strike pattern is notable, multi‑strike events can involve anywhere from two to dozens of strokes. Studies indicate that the probability of multi‑strike events increases with storm intensity and CAPE values. The statistical distribution of strikes per event often follows a Poisson process, with a small tail representing high‑multiplicity clusters.
Storm‑Associated Lightning Clusters
Lightning clusters that form within a single storm system may involve thousands of strikes over a 24‑hour period. While these clusters are usually spread across large geographic areas, localized high‑multiplicity clusters can still occur within them.
Lightning Outbreaks and Flares
Flares - brief, high‑current events that produce an intense burst of electromagnetic radiation - are sometimes associated with multi‑strike activity. Research into flare mechanisms has shown that they can precede or follow high‑multiplicity clusters, potentially providing a predictive indicator.
Scientific Research
Laboratory Simulations
High‑voltage discharge laboratories simulate multi‑strike conditions by generating controlled leader propagation in the presence of conductive surfaces. Experiments conducted at the National Institute of Standards and Technology (NIST) have replicated nine‑strike clusters using a 2‑megajoule power source. The results demonstrate that repetitive return strokes can be triggered by the partial neutralization of the ground path after each strike.
Statistical Analyses
Analyses of NLDN data spanning 2010–2020 reveal that nine‑strike events occur with a frequency of approximately 0.1% of total CG strikes in the United States. The same dataset indicates that the average current per stroke in nine‑strike clusters is 1.3 times the mean of isolated strikes, implying an elevated energy deposit.
Electromagnetic Pulse (EMP) Modeling
Numerical modeling of EMP signatures from multi‑strike events utilizes finite‑difference time‑domain (FDTD) methods. Models published in the journal Atmospheric Research show that nine‑strike clusters produce distinct broadband spectral peaks at 30–40 MHz, which can be used to differentiate them from single‑strike events.
Geospatial Correlation with Severe Weather Indices
Research at the University of Oklahoma’s Weather Research Center correlates nine‑strike clusters with severe weather indices. The study finds a statistically significant correlation (p < 0.01) between nine‑strike events and storms with CAPE values above 3000 J kg⁻¹, suggesting that CAPE can serve as a partial predictor.
Future Directions
Future research aims to refine real‑time detection algorithms for multi‑strike clusters, integrating machine‑learning models trained on historical data to predict high‑multiplicity events. The development of more robust SPDs capable of absorbing cumulative energy from nine or more strikes is a priority for utility companies in lightning‑prone regions.
Additionally, interdisciplinary studies combining atmospheric science, electrical engineering, and cultural anthropology seek to understand how lightning patterns influence human perception and risk management. This holistic approach promises to yield both scientific insights and practical safety measures.
References
- National Centers for Environmental Information (NCEI)
- National Lightning Detection Network (NLDN) Data Portal, 2021
- Lightning Mapping Array Technical Report, University of Illinois, 2018
- NIST Laboratory Discharge Experiments, 2019
- Atmospheric Research Journal, “Statistical Analysis of Multi‑Strike Lightning”, 2020
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