Atmospheric Physics

☁️Atmospheric Physics Unit 8 – Atmospheric Electricity and Lightning

Atmospheric electricity encompasses electrical phenomena in Earth's atmosphere, including electric fields, currents, and charges. This unit explores the formation of lightning, charge separation in clouds, and the global atmospheric electric circuit. Understanding these processes is crucial for studying weather patterns and atmospheric dynamics. The unit delves into various types of lightning, their formation processes, and associated phenomena like thunder. It also covers detection techniques, environmental impacts, safety measures, and current research directions in atmospheric electricity. This knowledge is essential for meteorologists, physicists, and environmental scientists.

Basics of Atmospheric Electricity

  • Atmospheric electricity refers to the electrical phenomena occurring in the Earth's atmosphere
  • Includes the study of electric fields, currents, and charges in the atmosphere and their interactions with the Earth's surface
  • Plays a crucial role in the formation of lightning, which is a sudden electrical discharge in the atmosphere
  • Atmospheric electric field strength typically ranges from 100 to 300 V/m near the Earth's surface under fair weather conditions
  • Conductivity of the atmosphere increases with altitude due to the presence of ions and free electrons
  • Global atmospheric electric circuit maintains a potential difference of ~300 kV between the Earth's surface and the ionosphere
  • Thunderstorms act as generators in the global electric circuit, transferring negative charge to the Earth's surface through lightning and precipitation

Charge Separation in Clouds

  • Charge separation in clouds is a prerequisite for lightning formation
  • Occurs primarily due to collisions between ice particles and graupel (soft hail) in the presence of supercooled water droplets
  • Graupel particles tend to acquire a negative charge, while smaller ice crystals gain a positive charge during collisions
  • Updrafts in the cloud carry the positively charged ice crystals to the upper regions of the cloud
  • Negatively charged graupel particles remain in the lower and middle regions of the cloud
    • Gravitational settling of graupel contributes to the formation of a negative charge center in the lower part of the cloud
  • Charge separation mechanisms are still an active area of research, with several theories proposed (convective charging, inductive charging, and ion capture)
  • Electric field strength within the cloud can reach values of 100 kV/m or more due to charge separation

Types of Lightning

  • Intracloud (IC) lightning occurs within a single thunderstorm cloud, between regions of opposite charge
    • Most common type of lightning, accounting for about 75% of all lightning events
  • Cloud-to-ground (CG) lightning occurs between a thunderstorm cloud and the Earth's surface
    • Negative CG lightning (−CG) is the most common type, where negative charge is transferred from the cloud to the ground
    • Positive CG lightning (+CG) is less frequent but often more intense and long-lasting than −CG lightning
  • Cloud-to-cloud (CC) lightning occurs between two separate thunderstorm clouds
  • Ground-to-cloud (GC) lightning, also known as upward lightning, initiates from tall structures (radio towers) and propagates towards the cloud
  • Spider lightning refers to the horizontal branching of a lightning channel within a cloud or along the base of a cloud
  • Ball lightning is a rare and poorly understood phenomenon, described as a luminous sphere that can persist for several seconds

Lightning Formation Process

  • Lightning formation involves a complex sequence of events, starting with charge separation in the thunderstorm cloud
  • As the electric field strength within the cloud increases, electrical breakdown of air occurs, initiating a lightning discharge
  • Stepped leader is a negatively charged channel that propagates downward from the cloud in a series of discrete steps
    • Each step is typically 50-100 m long and lasts for a few microseconds
  • When the stepped leader approaches the ground, it induces positive charges on the Earth's surface and objects
  • Positive streamers, also known as connecting leaders, propagate upward from the ground or objects towards the descending stepped leader
  • Attachment process occurs when one of the positive streamers connects with the stepped leader, completing the electrical pathway between the cloud and the ground
  • Return stroke is a bright, high-current discharge that propagates upward from the ground to the cloud along the established channel
    • Responsible for the visible flash of lightning and can reach temperatures of up to 30,000 K
  • Dart leader may propagate downward along the same channel, followed by subsequent return strokes, causing a flickering appearance of lightning
  • Lightning discharge typically lasts for a few hundred milliseconds and can consist of multiple return strokes

Thunder and Sound Propagation

  • Thunder is the acoustic signal generated by a lightning discharge
  • Rapid heating and expansion of the air along the lightning channel create a shock wave that propagates outward as sound waves
  • Sound of thunder can be heard up to a distance of about 25 km from the lightning strike, depending on atmospheric conditions
  • Rumbling or prolonged sound of thunder is due to the different arrival times of sound waves from different parts of the lightning channel
    • Sound waves from the closer parts of the channel reach the observer earlier than those from the farther parts
  • Acoustic refraction caused by temperature and wind gradients in the atmosphere can affect the propagation of thunder
  • Lightning-to-thunder time delay can be used to estimate the distance of the lightning strike from the observer
    • Sound travels at approximately 343 m/s at sea level, so a 3-second delay indicates a distance of about 1 km

Detection and Measurement Techniques

  • Lightning detection networks are used to monitor and locate lightning strikes on a regional or global scale
  • Most common lightning detection method is based on the measurement of electromagnetic signals emitted by lightning discharges
    • Very Low Frequency (VLF) and Low Frequency (LF) radio waves are used for long-range detection
    • Very High Frequency (VHF) and Ultra High Frequency (UHF) radio waves are used for more precise, short-range detection
  • Time-of-arrival (TOA) technique uses the time differences between the signals received at multiple stations to triangulate the location of the lightning strike
  • Magnetic direction finding (MDF) technique uses the orientation of the magnetic field generated by the lightning discharge to determine its direction
  • Satellite-based lightning detection systems, such as the Lightning Imaging Sensor (LIS) and the Geostationary Lightning Mapper (GLM), provide global coverage and continuous monitoring of lightning activity
  • Electric field mills and field change meters are used to measure the local electric field and its variations during thunderstorms
  • High-speed cameras and optical sensors are used to study the fine structure and propagation of lightning channels

Environmental Impacts and Safety

  • Lightning is a significant source of nitrogen oxides (NOx) in the atmosphere, which can affect air quality and contribute to the formation of ozone and acid rain
  • Wildfires can be ignited by lightning strikes, particularly in dry and vegetated areas
  • Lightning strikes can cause damage to buildings, infrastructure, and electrical systems
    • Surge protection devices and proper grounding are important for mitigating the effects of lightning-induced transients
  • Human safety is a primary concern during thunderstorms, as lightning strikes can cause injury or death
    • Outdoor activities should be avoided during thunderstorms, and shelter should be sought in enclosed buildings or vehicles
  • 30/30 rule: If the time between seeing a lightning flash and hearing thunder is 30 seconds or less, seek shelter and remain there for at least 30 minutes after the last thunder is heard
  • Lightning protection systems, including lightning rods and Faraday cages, are used to protect structures and minimize the risk of damage and injury
  • Education and awareness campaigns are important for promoting lightning safety and reducing the number of lightning-related incidents

Current Research and Future Directions

  • Improving the understanding of charge separation mechanisms in thunderstorms and the role of different hydrometeors (graupel, hail, ice crystals) in the process
  • Investigating the relationship between lightning activity and severe weather events, such as tornadoes and hail storms
  • Studying the effects of climate change on global lightning activity and the potential implications for atmospheric chemistry and wildfire risk
  • Developing more advanced lightning detection and localization techniques, including the use of interferometry and 3D mapping of lightning channels
  • Exploring the potential of lightning data assimilation in numerical weather prediction models to improve the forecasting of severe thunderstorms
  • Investigating the role of cosmic rays and solar activity in modulating the global atmospheric electric circuit and lightning activity
  • Studying the occurrence and characteristics of transient luminous events (TLEs), such as sprites, elves, and blue jets, which are associated with lightning in the upper atmosphere
  • Developing more effective lightning protection strategies and technologies for aircraft, wind turbines, and other vulnerable infrastructure


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Š 2024 Fiveable Inc. All rights reserved.
APÂŽ and SATÂŽ are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.