8.2 Lightning formation
10 min read•august 21, 2024
Lightning formation is a complex process driven by electrical in thunderstorms. It involves interactions between ice particles, graupel, and supercooled water droplets, leading to the buildup of electric fields within clouds.
When these fields exceed critical thresholds, lightning is initiated through electron avalanches and leader formation. Different types of lightning, such as cloud-to-ground and intracloud, result from varied discharge paths and polarities, each with unique characteristics and implications for atmospheric physics.
Electrical charge separation
- Electrical charge separation forms the foundation for lightning formation in thunderstorms
- Understanding this process is crucial for predicting and analyzing lightning activity in the atmosphere
- Charge separation involves complex interactions between various cloud particles and atmospheric conditions
Cloud electrification processes
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- Non-inductive charging dominates cloud electrification
- Involves collisions between ice particles in the presence of supercooled water droplets
- Temperature gradient within the cloud influences charge transfer direction
- Rebounding collisions between particles lead to net charge accumulation
- Updrafts and downdrafts in the cloud contribute to charge separation
Ice particle collisions
- Ice crystals and graupel particles collide in mixed-phase regions of clouds
- Smaller ice crystals typically acquire positive charge
- Larger graupel particles tend to acquire negative charge
- Collision efficiency depends on particle size, shape, and relative velocities
- Temperature and liquid water content affect the charge transfer magnitude
Graupel and ice crystal interactions
- Graupel forms through riming of supercooled water droplets on ice particles
- Serves as the primary charge carrier in thunderstorms
- Descends through the cloud due to its larger size and weight
- Collides with smaller ice crystals rising in updrafts
- Charge reversal temperature (~-15°C) influences the polarity of charge transfer
- Differential fall speeds between graupel and ice crystals enhance charge separation
Lightning initiation
- Lightning initiation marks the transition from charge accumulation to discharge
- Occurs when electric fields within the cloud exceed critical thresholds
- Involves complex processes of electron avalanches and leader formation
- Understanding initiation mechanisms is crucial for accurate lightning forecasting
Electric field thresholds
- Conventional breakdown requires fields of ~3 x 10^6 V/m at sea level
- Actual observed fields in clouds are typically an order of magnitude lower
- Runaway breakdown theory explains initiation at lower field strengths
- Cosmic rays contribute to the initial process
- Local field enhancements near hydrometeors lower the required threshold
Stepped leader formation
- Negatively charged stepped leader initiates the lightning discharge
- Propagates in discrete steps of ~50 meters
- Each step lasts for ~1 microsecond
- Branches out in multiple directions as it descends
- Creates an ionized channel connecting the cloud to the ground
- Stepped leader velocity ranges from 10^5 to 10^6 m/s
Return stroke mechanism
- Occurs when the stepped leader approaches the ground
- Upward-moving connecting leader meets the descending stepped leader
- Creates a continuous ionized channel from cloud to ground
- Massive current surge (~30,000 amperes) flows upward
- Heats the channel to temperatures exceeding 30,000 K
- Produces the bright flash and associated with lightning
Types of lightning
- Lightning manifests in various forms depending on the discharge path and polarity
- Each type has unique characteristics and implications for atmospheric physics
- Understanding different lightning types is essential for accurate detection and risk assessment
Cloud-to-ground vs intracloud
- Cloud-to-ground (CG) lightning connects the cloud to the Earth's surface
- Intracloud (IC) lightning occurs entirely within the cloud
- CG lightning poses greater risks to human safety and infrastructure
- IC lightning typically precedes CG lightning in storm development
- IC:CG ratio varies with latitude, season, and storm type
- Total lightning (IC + CG) provides better insight into storm intensity
Positive vs negative discharges
- Negative CG lightning transfers negative charge to the ground
- Accounts for ~90% of CG lightning strikes
- Positive CG lightning transfers positive charge to the ground
- Less common but typically more powerful and destructive
- Positive CG often associated with severe weather and sprite formation
- Polarity influences the electromagnetic signature and detection methods
Ball lightning phenomenon
- Rare and poorly understood form of lightning
- Appears as luminous spheres lasting several seconds
- Reported to move horizontally and pass through solid objects
- Theories include vaporized silicon, oxidizing nanoparticles, and microwave cavity formation
- Difficult to study due to its unpredictable and short-lived nature
- Remains a subject of scientific debate and investigation
Lightning detection methods
- Accurate lightning detection is crucial for weather forecasting and safety
- Various techniques are employed to detect and locate lightning discharges
- Combining multiple detection methods enhances coverage and accuracy
Ground-based networks
- Utilize sensors that detect electromagnetic signals from lightning
- Time-of-arrival and magnetic direction finding techniques determine strike location
- National Lightning Detection Network (NLDN) covers the continental United States
- Detection efficiency varies with sensor density and lightning type
- Provide real-time data for weather services and research applications
- Can detect both cloud-to-ground and some intracloud lightning
Satellite-based observations
- Geostationary Lightning Mapper (GLM) on GOES-R series satellites
- Detects optical emissions from lightning in both day and night
- Provides continuous coverage over a large area (Western Hemisphere)
- Helps track storm development and intensity changes
- Complements ground-based networks for global lightning monitoring
- Useful for detecting lightning in remote or oceanic regions
Lightning mapping arrays
- Consist of multiple VHF receivers in a local network
- Map the three-dimensional structure of lightning channels
- Provide detailed information on lightning initiation and propagation
- Useful for studying lightning physics and storm electrification processes
- Help distinguish between different types of lightning discharges
- Typically cover smaller areas with high spatial and temporal resolution
Thunderstorm electrification
- electrification drives the charge separation process
- Involves complex interactions between cloud dynamics and microphysics
- Understanding these processes is key to predicting lightning activity
Convective updrafts
- Strong updrafts transport water vapor and cloud particles upward
- Create mixed-phase regions where ice and supercooled water coexist
- Enhance collision rates between ice particles and graupel
- Contribute to the vertical charge separation within the cloud
- Updraft strength correlates with the intensity of electrification
- Typically reach speeds of 10-50 m/s in mature thunderstorms
Charge distribution in clouds
- Tripole structure common in mature thunderstorms
- Main negative charge region typically located at -10 to -20°C level
- Upper positive charge region found above -20°C isotherm
- Lower positive charge region near the freezing level
- Screening layer of opposite charge often forms at cloud boundaries
- Charge structure can vary with storm type and stage of development
Non-inductive charging mechanism
- Primary mechanism for charge separation in thunderstorms
- Relies on collisions between ice crystals and riming graupel
- Does not require pre-existing electric fields
- Charge transfer direction depends on temperature and liquid water content
- Explains observed charge structures in various types of storms
- Laboratory experiments have validated this mechanism
Lightning frequency and distribution
- Lightning occurrence varies significantly across the globe
- Understanding these patterns is crucial for climate studies and risk assessment
- Influenced by various geographical and meteorological factors
Global lightning patterns
- Lightning flash rate density highest in tropical and subtropical regions
- African continent experiences the most lightning activity globally
- South America and Southeast Asia also have high lightning frequencies
- Lightning chimney over the Catatumbo River in Venezuela
- Ocean lightning less frequent but still significant in some areas
- Global average of ~44 ± 5 lightning flashes per second
Seasonal variations
- Lightning activity peaks during local summer in most regions
- Monsoon seasons greatly influence lightning patterns in Asia
- Spring and fall secondary peaks observed in some mid-latitude areas
- Winter lightning more common in certain coastal and mountainous regions
- El Niño and La Niña cycles affect global lightning distribution
- Long-term climate changes may alter seasonal lightning patterns
Land vs ocean occurrence
- Lightning occurs ~10 times more frequently over land than oceans
- Land-sea temperature contrast drives convection in coastal areas
- Maritime thunderstorms typically less intense but can produce unique phenomena
- Warm ocean currents can enhance lightning activity in certain regions
- Ship tracks may influence cloud electrification over oceans
- Island effect can locally increase lightning frequency in oceanic areas
Lightning physics
- Lightning involves complex physical processes at various scales
- Understanding these processes is crucial for accurate modeling and prediction
- Spans from microscopic electron interactions to large-scale atmospheric effects
Plasma channel formation
- Lightning channel consists of highly ionized air ()
- Initial breakdown creates a weakly ionized path
- Stepped leader propagation further ionizes the channel
- Return stroke rapidly heats the channel to ~30,000 K
- Channel diameter expands from ~1 cm to ~10 cm during return stroke
- Subsequent strokes often reuse the existing ionized channel
Electromagnetic radiation emission
- Lightning emits electromagnetic radiation across a wide spectrum
- Radio frequency emissions used for lightning detection and location
- Optical emissions in visible and infrared wavelengths
- X-ray and gamma-ray emissions observed during leader propagation
- Terrestrial gamma-ray flashes associated with upper atmospheric discharges
- Electromagnetic pulse (EMP) can affect electronic systems
Thunder generation
- Rapid heating of lightning channel causes explosive expansion
- Creates a shock wave that transitions to an acoustic wave
- Thunder can be heard up to ~25 km from the
- Low-frequency components of thunder can travel further
- Multiple return strokes and channel tortuosity affect thunder characteristics
- Thunder propagation influenced by atmospheric temperature and wind profiles
Environmental factors
- Various environmental conditions influence lightning formation and characteristics
- Understanding these factors is crucial for accurate forecasting and risk assessment
- Interactions between different environmental factors can lead to complex effects
Temperature and humidity effects
- Higher temperatures generally increase convection and lightning probability
- Humidity provides the moisture necessary for thunderstorm development
- Dry air entrainment can enhance or suppress lightning depending on altitude
- Freezing level height affects the depth of the mixed-phase region
- Inversions can inhibit convection and reduce lightning activity
- Diurnal temperature variations influence thunderstorm timing and intensity
Atmospheric instability
- Convective Available Potential Energy (CAPE) correlates with lightning frequency
- Lifted Index and K-Index used to assess lightning potential
- Wind shear influences storm organization and longevity
- Capping inversions can suppress or enhance convection depending on strength
- Mesoscale boundaries (fronts, sea breezes) can trigger thunderstorm development
- Orographic lifting enhances instability in mountainous regions
Aerosol concentration impact
- Aerosols serve as cloud condensation nuclei and ice nuclei
- Can increase or decrease lightning activity depending on concentration
- Urban heat islands and pollution may enhance lightning in some areas
- Smoke from wildfires can suppress or invigorate convection
- Desert dust affects cloud microphysics and electrification processes
- Long-range transport of aerosols influences
Lightning protection
- is crucial for safeguarding lives and infrastructure
- Involves various strategies to mitigate the risks associated with lightning strikes
- Continuous research and development improve protection technologies
Lightning rods and grounding
- Franklin rod provides a preferential strike point for lightning
- Faraday cage principle used to protect buildings and sensitive equipment
- Proper essential for effective lightning protection
- Surge protection devices safeguard electrical and electronic systems
- Rolling sphere method used to determine protected zones
- Regular maintenance and inspection crucial for system effectiveness
Aircraft lightning protection
- Aircraft often initiate lightning strikes while flying
- Composite materials present unique challenges for protection
- Faraday cage principle applied to aircraft fuselage
- Static dischargers reduce charge buildup during flight
- Fuel tanks and critical systems require special protection measures
- Certification standards ensure aircraft can withstand lightning strikes
Personal safety measures
- "When thunder roars, go indoors" - primary safety rule
- 30-30 rule: seek shelter if thunder follows lightning within 30 seconds
- Avoid tall objects and open areas during thunderstorms
- Stay away from windows, plumbing, and electrical equipment indoors
- Avoid water activities during thunderstorms
- Wait 30 minutes after the last thunder before resuming outdoor activities
Climate change impacts
- Climate change affects various aspects of lightning activity
- Understanding these impacts is crucial for long-term risk assessment and adaptation
- Complex interactions between climate variables and lightning processes
Lightning frequency projections
- Global warming expected to increase lightning frequency
- Projections suggest ~12% increase per degree Celsius of warming
- Regional variations in lightning changes likely to occur
- Some areas may experience decreased lightning activity
- Changes in storm dynamics and microphysics influence projections
- Uncertainty remains due to complex interactions in the climate system
Wildfire ignition potential
- Lightning is a significant natural cause of wildfire ignition
- Increased lightning frequency may lead to more wildfire starts
- Changes in precipitation patterns affect fuel moisture and fire susceptibility
- Positive polarity strikes more likely to ignite fires
- Dry thunderstorms pose a particular risk for wildfire ignition
- Feedback loops between wildfires, aerosols, and lightning possible
Atmospheric composition effects
- Lightning produces nitrogen oxides (NOx) in the atmosphere
- NOx influences ozone formation and overall air quality
- Changes in lightning patterns affect global NOx distribution
- Potential feedbacks between air pollution and lightning activity
- Lightning-produced NOx impacts methane lifetime in the atmosphere
- Understanding these effects crucial for climate and air quality modeling
Key Terms to Review (25)
Benjamin Franklin: Benjamin Franklin was a polymath and one of the Founding Fathers of the United States, known for his contributions to science, politics, and the understanding of electricity. His experiments with lightning and his theories on electrical charge significantly advanced the study of atmospheric electricity, providing insights into charge separation in clouds, lightning formation, and the global electric circuit.
Charge separation: Charge separation is the process through which positive and negative electric charges are distributed unevenly, often occurring in atmospheric phenomena. This imbalance of charge is critical in cloud formation, leading to various weather events, including lightning, and plays a vital role in the Earth’s global electric circuit.
Cloud-to-ground lightning: Cloud-to-ground lightning is a type of electrical discharge that occurs between a charged cloud and the Earth's surface. This phenomenon plays a significant role in weather systems, as it can indicate the presence of severe thunderstorms and is associated with various atmospheric processes, including the formation of lightning, the types of lightning that occur, the creation of thunder, and upper atmospheric discharges.
Convective updrafts: Convective updrafts are vertical air movements that occur when warm, moist air rises in the atmosphere, contributing to the formation of clouds and storms. These updrafts are critical in thunderstorm development, as they help to lift moisture-laden air to higher altitudes where it can cool, condense, and potentially lead to precipitation and lightning events.
Coulomb's Law: Coulomb's Law describes the force between two charged objects, stating that the force is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. This principle is essential for understanding the behavior of electric charges, including those involved in lightning formation, as it helps explain how charge separation occurs within clouds and between clouds and the ground.
Cumulonimbus cloud: A cumulonimbus cloud is a towering vertical cloud associated with thunderstorms and severe weather, characterized by its anvil-shaped top and significant vertical development. These clouds form when warm, moist air rises rapidly, leading to strong updrafts that create intense weather phenomena, including lightning and thunder, as well as heavy rain, hail, and sometimes tornadoes.
Development stage: The development stage refers to a crucial phase in the lifecycle of thunderstorms where the storm matures and actively produces intense weather phenomena, including lightning. During this stage, strong updrafts and downdrafts interact, creating a highly charged environment conducive to lightning formation. This stage is characterized by increasing cloud heights and the organization of storm structure, leading to significant atmospheric electrical activity.
Dissipation stage: The dissipation stage is the final phase of a thunderstorm's lifecycle, characterized by a decrease in intensity and the eventual weakening of storm features. During this stage, the storm's updraft weakens significantly, leading to a reduction in precipitation and the fading of lightning activity. As the storm dissipates, it may transition into a lighter rain or stop altogether, marking the end of its energy release.
Electric Field: An electric field is a region around a charged particle where a force would be exerted on other charged particles. This invisible field is created by the presence of electric charges, which can influence the motion of nearby charges and is crucial in understanding phenomena like lightning formation.
Gauss's Law: Gauss's Law states that the electric flux through a closed surface is proportional to the charge enclosed within that surface. This principle connects electric fields with the distribution of electric charge and is crucial in understanding how lightning forms, as it helps explain the behavior of electric fields in storm clouds and the buildup of charge that leads to discharge events like lightning.
Global lightning patterns: Global lightning patterns refer to the distribution and frequency of lightning strikes around the world, influenced by various climatic and geographical factors. These patterns reveal the intensity of thunderstorm activity, with certain regions experiencing significantly more lightning than others due to humidity, temperature, and atmospheric conditions that foster storm development.
Grounding: Grounding refers to the process by which electrical charges are safely dissipated into the Earth, reducing the risk of damage or injury during electrical discharges, such as lightning strikes. This concept is crucial in understanding how lightning interacts with the atmosphere and the Earth's surface, as it helps to mitigate the potentially dangerous effects of electrical currents generated by thunderstorms.
Intra-cloud lightning: Intra-cloud lightning refers to electrical discharges that occur within a single cloud, typically within cumulonimbus clouds. This type of lightning accounts for a significant portion of total lightning activity and involves the movement of electrical charges within the cloud, leading to the rapid discharge of energy. Understanding intra-cloud lightning is crucial in recognizing its role in the overall lightning formation process, differentiating between types of lightning, and comprehending its relationship with thunder.
Ionization: Ionization is the process in which an atom or molecule gains or loses electrons, resulting in the formation of charged particles called ions. This phenomenon is crucial in atmospheric physics as it plays a significant role in various electrical and chemical processes, including the formation of lightning and the occurrence of upper atmospheric discharges such as sprites. Understanding ionization helps explain how electrical energy is transferred through the atmosphere and the resulting effects on weather and climate.
Lightning frequency projections: Lightning frequency projections are estimates or forecasts that predict the occurrence and intensity of lightning events in a given area over a specific time frame. These projections are crucial for understanding how changing climate conditions, such as temperature and humidity, influence lightning formation and frequency. By analyzing historical data and utilizing climate models, researchers can project future lightning activity, which helps in assessing potential impacts on ecosystems, infrastructure, and human safety.
Lightning protection: Lightning protection refers to the methods and systems designed to safeguard structures, people, and equipment from the damaging effects of lightning strikes. This involves creating pathways for the electric charge to safely travel to the ground, reducing the risk of fires, electrical surges, and injuries during thunderstorms. Effective lightning protection is crucial given the dynamic nature of lightning formation and its unpredictable behavior in severe weather conditions.
Lightning strike: A lightning strike refers to a sudden discharge of electricity from the atmosphere to the ground, occurring during a thunderstorm. This phenomenon is the result of an imbalance in electrical charges between clouds and the earth, leading to the formation of lightning channels that release energy in the form of a bright flash and a loud sound known as thunder.
Non-inductive charging mechanism: A non-inductive charging mechanism refers to the process by which electrical charges accumulate in a thunderstorm without the direct influence of contact or induction. This method primarily involves the collision and interaction of ice particles and water droplets within the cloud, resulting in a transfer of charge that contributes to the overall electrical imbalance, ultimately leading to lightning formation.
Plasma: Plasma is one of the four fundamental states of matter, consisting of highly energized ions and free electrons. This state occurs when gas is heated to the point that the atoms become ionized, allowing for electrical conductivity and the generation of magnetic fields. Plasma is crucial in various atmospheric phenomena, particularly in lightning formation, where the intense energy creates ionized paths through the air, leading to the visible discharge.
Return stroke mechanism: The return stroke mechanism refers to the rapid upward movement of electrical energy that occurs during a lightning strike, specifically the flow of current that travels from the ground up to the charged region in the atmosphere. This process is part of the lightning discharge cycle, where the return stroke is characterized by a bright flash and is responsible for the majority of the visible light emitted during a lightning event.
Stepped leader formation: Stepped leader formation refers to the process by which a series of downward-moving electrical discharges, called stepped leaders, create a pathway for lightning to travel from a cloud to the ground. This formation is characterized by its incremental descent in a series of steps, where each step can be likened to a short burst of electric current that establishes a conductive channel, eventually allowing for the return stroke, which is the bright flash of lightning that we see.
Thunder: Thunder is the sound produced by the rapid expansion of air surrounding a lightning bolt as it heats up to around 30,000 degrees Fahrenheit (approximately 16,649 degrees Celsius). This explosive heating causes the air to expand quickly, creating a shockwave that travels through the atmosphere, resulting in the characteristic rumbling or cracking sound we hear. Understanding thunder helps in grasping the formation and types of lightning, as they are closely linked phenomena.
Thunderstorm: A thunderstorm is a localized weather phenomenon characterized by the presence of thunder and lightning, often accompanied by heavy rainfall, strong winds, and sometimes hail. Thunderstorms develop from the rising of warm, moist air that cools as it ascends, leading to cloud formation and the release of energy through convective processes. Understanding thunderstorms involves recognizing their formation, the dynamics of lightning generation, and the sound of thunder that follows.
Wildfire ignition potential: Wildfire ignition potential refers to the likelihood of a fire starting in a given area, influenced by environmental conditions, fuel availability, and ignition sources. This potential is particularly significant when considering the role of natural phenomena, such as lightning, which can act as a catalyst for wildfires under the right atmospheric conditions, including dry vegetation and high temperatures.
William Reid: William Reid was a significant figure in atmospheric science, particularly known for his work on the processes of lightning formation. He contributed to the understanding of electrical activity in storms, which is crucial for comprehending how lightning occurs and the conditions that lead to its formation.