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Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of energy.This discharge may produce a wide range of electromagnetic radiation, from very hot plasma created by the rapid movement of electrons to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms and other types of energetic weather systems, but volcanic lightning can also occur during volcanic eruptions.
The three main kinds of lightning are distinguished by where they occur: either inside a single thundercloud, between two different clouds, or between a cloud and the ground. Many other observational variants are recognized, including "heat lightning", which can be seen from a great distance but not heard; dry lightning, which can cause forest fires; and ball lightning, which is rarely observed scientifically.
Humans have deified lightning for millennia, and lightning-inspired expressions like "bolt from the blue", "to be struck by lightning" (as to having an epiphany or enlightenment), "lightning never strikes twice (in the same place)" and "blitzkrieg" are in common usage. In some languages, the notion of "love at first sight" literally translates as "lightning strike".
The details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification. The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from −15 to −25 °C (5 to −13 °F); see Figure 1. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets (small water droplets below freezing), small ice crystals, and graupel (soft hail). The updraft carries the super-cooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air.
The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged; see Figure 2. The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm.
The result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals (and positive charge) in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm (updrafts and downdrafts). In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures.
A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km (3.1 mi) tall, from within the cloud to the ground's surface. The actual discharge is the final stage of a very complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning primarily occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere.[ citation needed ] However, it can also occur during dust storms, forest fires, tornadoes, volcanic eruptions, and even in the cold of winter, where the lightning is known as thundersnow. Hurricanes typically generate some lightning, mainly in the rainbands as much as 160 km (99 mi) from the center.
The science of lightning is called fulminology, and the fear of lightning is called astraphobia .
Lightning is not distributed evenly around Earth, as shown in the map.
On Earth, the lightning frequency is approximately 44 (± 5) times per second, or nearly 1.4 billion flashes per yearand the average duration is 0.2 seconds made up from a number of much shorter flashes (strokes) of around 60 to 70 microseconds.
Many factors affect the frequency, distribution, strength and physical properties of a typical lightning flash in a particular region of the world. These factors include ground elevation, latitude, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC (in-cloud or intracloud), CC (cloud-to-cloud) and CG (cloud-to-ground) lightning may also vary by season in middle latitudes.
Because human beings are terrestrial and most of their possessions are on the Earth where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types, even though IC and CC are more common types of lightning. Lightning's relative unpredictability limits a complete explanation of how or why it occurs, even after hundreds of years of scientific investigation. About 70% of lightning occurs over land in the tropicswhere atmospheric convection is the greatest.
This occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, and it generally happens at the boundaries between them. The flow of warm ocean currents past drier land masses, such as the Gulf Stream, partially explains the elevated frequency of lightning in the Southeast United States. Because large bodies of water lack the topographic variation that would result in atmospheric mixing, lightning is notably less frequent over the world's oceans than over land. The North and South Poles are limited in their coverage of thunderstorms and therefore result in areas with the least amount of lightning.[ clarification needed ]
In general, cloud-to-ground (CG) lightning flashes account for only 25% of all total lightning flashes worldwide. Since the base of a thunderstorm is usually negatively charged, this is where most CG lightning originates. This region is typically at the elevation where freezing occurs within the cloud. Freezing, combined with collisions between ice and water, appears to be a critical part of the initial charge development and separation process. During wind-driven collisions, ice crystals tend to develop a positive charge, while a heavier, slushy mixture of ice and water (called graupel) develops a negative charge. Updrafts within a storm cloud separate the lighter ice crystals from the heavier graupel, causing the top region of the cloud to accumulate a positive space charge while the lower level accumulates a negative space charge.
Because the concentrated charge within the cloud must exceed the insulating properties of air, and this increases proportionally to the distance between the cloud and the ground, the proportion of CG strikes (versus cloud-to-cloud (CC) or in-cloud (IC) discharges) becomes greater when the cloud is closer to the ground. In the tropics, where the freezing level is generally higher in the atmosphere, only 10% of lightning flashes are CG. At the latitude of Norway (around 60° North latitude), where the freezing elevation is lower, 50% of lightning is CG.
Lightning is usually produced by cumulonimbus clouds, which have bases that are typically 1–2 km (0.62–1.24 mi) above the ground and tops up to 15 km (9.3 mi) in height.
The place on Earth where lightning occurs most often is near the small village of Kifuka in the mountains of the eastern Democratic Republic of the Congo, 975 m (3,200 ft). On average, this region receives 158 lightning strikes per square kilometre per year (410/sq mi/yr). Lake Maracaibo in Venezuela averages 297 days per year with lightning activity, an effect recognized as Catatumbo lightning. Other lightning hotspots include Catatumbo in Venezuela, Singapore, and Lightning Alley in Central Florida.where the elevation is around
In order for an electrostatic discharge to occur, two preconditions are necessary: firstly, a sufficiently high potential difference between two regions of space must exist, and secondly, a high-resistance medium must obstruct the free, unimpeded equalization of the opposite charges. The atmosphere provides the electrical insulation, or barrier, that prevents free equalization between charged regions of opposite polarity.
It is well understood that during a thunderstorm there is charge separation and aggregation in certain regions of the cloud; however the exact processes by which this occurs are not fully understood.
As a thundercloud moves over the surface of the Earth, an equal electric charge, but of opposite polarity, is induced on the Earth's surface underneath the cloud. The induced positive surface charge, when measured against a fixed point, will be small as the thundercloud approaches, increasing as the center of the storm arrives and dropping as the thundercloud passes. The referential value of the induced surface charge could be roughly represented as a bell curve.
The oppositely charged regions create an electric field within the air between them. This electric field varies in relation to the strength of the surface charge on the base of the thundercloud – the greater the accumulated charge, the higher the electrical field.
The best studied and understood form of lightning is cloud to ground (CG). Although more common, intracloud (IC) and cloud to cloud (CC) flashes are very difficult to study given there are no "physical" points to monitor inside the clouds. Also, given the very low probability lightning will strike the same point repeatedly and consistently, scientific inquiry is difficult at best even in the areas of high CG frequency. As such, knowing flash propagation is similar amongst all forms of lightning, the best means to describe the process is through an examination of the most studied form, cloud to ground.
In a process not well understood, a bidirectional channel of ionized air, called a "leader", is initiated between oppositely-charged regions in a thundercloud. Leaders are electrically conductive channels of ionized gas that propagate through, or are otherwise attracted to, regions with a charge opposite of that of the leader tip. The negative end of the bidirectional leader fills a positive charge region, also called a well, inside the cloud while the positive end fills a negative charge well. Leaders often split, forming branches in a tree-like pattern.In addition, negative and some positive leaders travel in a discontinuous fashion, in a process called "stepping". The resulting jerky movement of the leaders can be readily observed in slow-motion videos of lightning flashes.
It is possible for one end of the leader to fill the oppositely-charged well entirely while the other end is still active. When this happens, the leader end which filled the well may propagate outside of the thundercloud and result in either a cloud-to-air flash or a cloud-to-ground flash. In a typical cloud-to-ground flash, a bidirectional leader initiates between the main negative and lower positive charge regions in a thundercloud. The weaker positive charge region is filled quickly by the negative leader which then propagates toward the inductively-charged ground.
The positively and negatively charged leaders proceed in opposite directions, positive upwards within the cloud and negative towards the earth. Both ionic channels proceed, in their respective directions, in a number of successive spurts. Each leader "pools" ions at the leading tips, shooting out one or more new leaders, momentarily pooling again to concentrate charged ions, then shooting out another leader. The negative leader continues to propagate and split as it heads downward, often speeding up as it gets closer to the Earth's surface.
About 90% of ionic channel lengths between "pools" are approximately 45 m (148 ft) in length. The establishment of the ionic channel takes a comparatively long amount of time (hundreds of milliseconds) in comparison to the resulting discharge, which occurs within a few dozen microseconds. The electric current needed to establish the channel, measured in the tens or hundreds of amperes, is dwarfed by subsequent currents during the actual discharge.
Initiation of the lightning leaders is not well understood. The electric field strength within the thundercloud is not typically large enough to initiate this process by itself. [ clarification needed ] describes random connectivity phenomena, which produce an evolution of connected structures similar to that of lightning strikes.Many hypotheses have been proposed. One theory postulates that showers of relativistic electrons are created by cosmic rays and are then accelerated to higher velocities via a process called runaway breakdown. As these relativistic electrons collide and ionize neutral air molecules, they initiate leader formation. Another theory invokes locally enhanced electric fields being formed near elongated water droplets or ice crystals. Percolation theory, especially for the case of biased percolation,
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. The electric field is strongest on grounded objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a positively charged ionic channel, called a positive or upward streamer, can develop from these points. This was first theorized by Heinz Kasemir.
As negatively charged leaders approach, increasing the localized electric field strength, grounded objects already experiencing corona discharge exceed a threshold and form upward streamers.
Once a downward leader connects to an available upward leader, a process referred to as attachment, a low-resistance path is formed and discharge may occur. Photographs have been taken in which unattached streamers are clearly visible. The unattached downward leaders are also visible in branched lightning, none of which are connected to the earth, although it may appear they are. High-speed videos can show the attachment process in progress.
Once a conductive channel bridges the air gap between the negative charge excess in the cloud and the positive surface charge excess below, there is a large drop in resistance across the lightning channel. Electrons accelerate rapidly as a result in a zone beginning at the point of attachment, which expands across the entire leader network at a fraction of the speed of light. This is the 'return stroke' and it is the most luminous and noticeable part of the lightning discharge.
A large electric current flows along the plasma channel from the cloud to the ground, neutralising the positive ground charge as electrons flow away from the strike point to the surrounding area. This huge surge of current creates large radial voltage differences along the surface of the ground. Called step potentials, they are responsible for more injuries and deaths than the strike itself.Electricity takes every path available to it. A portion of the return stroke current will often preferentially flow through one leg and out another, electrocuting an unlucky human or animal standing near the point where the lightning strikes.
The electric current of the return stroke averages 30 kiloamperes for a typical negative CG flash, often referred to as "negative CG" lightning. In some cases, a ground to cloud (GC) lightning flash may originate from a positively charged region on the ground below a storm. These discharges normally originate from the tops of very tall structures, such as communications antennas. The rate at which the return stroke current travels has been found to be around 100,000 km/s.
The massive flow of electric current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly superheats the completed leader channel, forming a highly electrically conductive plasma channel. The core temperature of the plasma during the return stroke may exceed 50,000 K, causing it to brilliantly radiate with a blue-white color. Once the electric current stops flowing, the channel cools and dissipates over tens or hundreds of milliseconds, often disappearing as fragmented patches of glowing gas. The nearly instantaneous heating during the return stroke causes the air to expand explosively, producing a powerful shock wave which is heard as thunder.
High-speed videos (examined frame-by-frame) show that most negative CG lightning flashes are made up of 3 or 4 individual strokes, though there may be as many as 30.
Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds, as other charged regions in the cloud are discharged in subsequent strokes. Re-strikes often cause a noticeable "strobe light" effect.
To understand why multiple return strokes utilize the same lightning channel, one needs to understand the behavior of positive leaders, which a typical ground flash effectively becomes following the negative leader's connection with the ground. Positive leaders decay more rapidly than negative leaders do. For reasons not well understood, bidirectional leaders tend to initiate on the tips of the decayed positive leaders in which the negative end attempts to re-ionize the leader network. These leaders, also called recoil leaders, usually decay shortly after their formation. When they do manage to make contact with a conductive portion of the main leader network, a return stroke-like process occurs and a dart leader travels across all or a portion of the length of the original leader. The dart leaders making connections with the ground are what cause a majority of subsequent return strokes.
Each successive stroke is preceded by intermediate dart leader strokes that have a faster rise time but lower amplitude than the initial return stroke. Each subsequent stroke usually re-uses the discharge channel taken by the previous one, but the channel may be offset from its previous position as wind displaces the hot channel.
Since recoil and dart leader processes do not occur on negative leaders, subsequent return strokes very seldom utilize the same channel on positive ground flashes which are explained later in the article.
The electric current within a typical negative CG lightning discharge rises very quickly to its peak value in 1–10 microseconds, then decays more slowly over 50–200 microseconds. The transient nature of the current within a lightning flash results in several phenomena that need to be addressed in the effective protection of ground-based structures. Rapidly changing currents tend to travel on the surface of a conductor, in what is called the skin effect, unlike direct currents, which "flow-through" the entire conductor like water through a hose. Hence, conductors used in the protection of facilities tend to be multi-stranded, with small wires woven together. This increases the total bundle surface area in inverse proportion to the individual strand radius, for a fixed total cross-sectional area.
The rapidly changing currents also create electromagnetic pulses (EMPs) that radiate outward from the ionic channel. This is a characteristic of all electrical discharges. The radiated pulses rapidly weaken as their distance from the origin increases. However, if they pass over conductive elements such as power lines, communication lines, or metallic pipes, they may induce a current which travels outward to its termination. The surge current is inversely related to the Surge impedance ... so the higher in impedance, then the lower the current.This is the "surge" that, more often than not, results in the destruction of delicate electronics, electrical appliances, or electric motors. Devices known as surge protectors (SPD) or transient voltage surge suppressors (TVSS) attached in parallel with these lines can detect the lightning flash's transient irregular current, and, through alteration of its physical properties, route the spike to an attached earthing ground, thereby protecting the equipment from damage.
There are three primary types of lightning, defined by what is at the "ends" of a flash channel.
There are variations of each type, such as "positive" versus "negative" CG flashes, that have different physical characteristics common to each which can be measured. Different common names used to describe a particular lightning event may be attributed to the same or different events.
Cloud-to-ground (CG) lightning is a lightning discharge between a thundercloud and the ground. It is initiated by a stepped leader moving down from the cloud, which is met by a streamer moving up from the ground.
CG is the least common, but best understood of all types of lightning. It is easier to study scientifically, because it terminates on a physical object, namely the Earth, and lends itself to being measured by instruments on the ground. Of the three primary types of lightning, it poses the greatest threat to life and property since it terminates or "strikes" the Earth. The overall discharge, termed a flash, is composed of a number of processes such as preliminary breakdown, stepped leaders, connecting leaders, return strokes, dart leaders and subsequent return strokes.
Cloud-to-ground (CG) lightning is either positive or negative, as defined by the direction of the conventional electric current from cloud to ground. Most CG lightning is negative, meaning that a negative charge is transferred to ground and electrons travel downward along the lightning channel. The reverse happens in a positive CG flash, where electrons travel upward along the lightning channel and a positive charge is transferred to the ground. Positive lightning is less common than negative lightning, and on average makes up less than 5% of all lightning strikes.
There are six different mechanisms theorized to result in the formation of downward positive lightning.
Contrary to popular belief, positive lightning flashes do not necessarily originate from the anvil or the upper positive charge region and strike a rain-free area outside of the thunderstorm. This belief is based on the outdated idea that lightning leaders are unipolar in nature and originating from their respective charge region.
Positive lightning strikes tend to be much more intense than their negative counterparts. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 1 gigajoule of energy. Large bolts of negative lightning can carry up to 120 kA and 350 C. The average positive ground flash has roughly double the peak current of a typical negative flash, and can produce peak currents up to 400 kA and charges of several hundred coulombs. Furthermore, positive ground flashes with high peak currents are commonly followed by long continuing currents, a correlation not seen in negative ground flashes.
As a result of their greater power, as well as lack of warning, positive lightning strikes are considerably more dangerous. Due to the aforementioned tendency for positive ground flashes to produce both high peak currents and long continuing current, they are capable of heating surfaces to much higher levels which increases the likelihood of a fire being ignited.
Positive lightning has also been shown to trigger the occurrence of upward lightning flashes from the tops of tall structures and is largely responsible for the initiation of sprites several tens of kilometers above ground level. Positive lightning tends to occur more frequently in winter storms, as with thundersnow, during intense tornadoesand in the dissipation stage of a thunderstorm. Huge quantities of extremely low frequency (ELF) and very low frequency (VLF) radio waves are also generated.
A unique form of cloud-to-ground lightning exists where lightning appears to exit from the cumulonimbus cloud and propagate a considerable distance through clear air before veering towards, and striking, the ground. For this reason, they are known as "bolts from the blue". Despite the popular misconception that these are positive lightning strikes due to them seemingly originating from the positive charge region, observations have shown that these are in fact negative flashes. They begin as intracloud flashes within the cloud, the negative leader then exits the cloud from the positive charge region before propagating through clear air and striking the ground some distance away.
Lightning discharges may occur between areas of cloud without contacting the ground. When it occurs between two separate clouds it is known as inter-cloud lightning, and when it occurs between areas of differing electric potential within a single cloud it is known as intra-cloud lightning. Intra-cloud lightning is the most frequently occurring type.
Intra-cloud lightning most commonly occurs between the upper anvil portion and lower reaches of a given thunderstorm. This lightning can sometimes be observed at great distances at night as so-called "sheet lightning". In such instances, the observer may see only a flash of light without hearing any thunder.
Another term used for cloud–cloud or cloud–cloud–ground lightning is "Anvil Crawler", due to the habit of charge, typically originating beneath or within the anvil and scrambling through the upper cloud layers of a thunderstorm, often generating dramatic multiple branch strokes. These are usually seen as a thunderstorm passes over the observer or begins to decay. The most vivid crawler behavior occurs in well developed thunderstorms that feature extensive rear anvil shearing.
Objects struck by lightning experience heat and magnetic forces of great magnitude. The heat created by lightning currents traveling through a tree may vaporize its sap, causing a steam explosion that bursts the trunk. As lightning travels through sandy soil, the soil surrounding the plasma channel may melt, forming tubular structures called fulgurites. Although 90 percent of people struck by lightning survive,humans or animals struck by lightning may suffer severe injury due to internal organ and nervous system damage. Buildings or tall structures hit by lightning may be damaged as the lightning seeks unintended paths to ground. By safely conducting a lightning strike to ground, a lightning protection system can greatly reduce the probability of severe property damage. Lightning also serves an important role in the nitrogen cycle by oxidizing diatomic nitrogen in the air into nitrates which are deposited by rain and can fertilize the growth of plants and other organisms. Due to their metallic fuselages, aircraft are highly susceptible to lightning strikes, though it does not cause much harm to the aircraft or its passengers, aside from a small hole in the wings. Due to the conductive properties of Aluminium alloy, the fuselage acts as a Faraday cage.
Because the electrostatic discharge of terrestrial lightning superheats the air to plasma temperatures along the length of the discharge channel in a short duration, kinetic theory dictates gaseous molecules undergo a rapid increase in pressure and thus expand outward from the lightning creating a shock wave audible as thunder. Since the sound waves propagate not from a single point source but along the length of the lightning's path, the sound origin's varying distances from the observer can generate a rolling or rumbling effect. Perception of the sonic characteristics is further complicated by factors such as the irregular and possibly branching geometry of the lightning channel, by acoustic echoing from terrain, and by the usually multiple-stroke characteristic of the lightning strike.
Light travels at about 300,000,000 m/s (980,000,000 ft/s), and sound travels through air at about 343 m/s (1,130 ft/s). An observer can approximate the distance to the strike by timing the interval between the visible lightning and the audible thunder it generates. A lightning flash preceding its thunder by one second would be approximately 343 m (1,125 ft) in distance; a delay of three seconds would indicate a distance of about 1 km or 0.62 mi (3 × 343 m). A flash preceding thunder by five seconds would indicate a distance of approximately 1.5 km or 0.93 mi (5 × 343 m). Consequently, a lightning strike observed at a very close distance will be accompanied by a sudden clap of thunder, with almost no perceptible time lapse, possibly accompanied by the smell of ozone (O3).
Lightning at a sufficient distance may be seen and not heard; there is data that a lightning storm can be seen at over 160 km (100 mi) whereas the thunder travels about 32 km (20 mi). Anecdotally, there are many examples of people saying 'the storm was directly overhead or all-around and yet there was no thunder'. There is no coherent data available.[ citation needed ]
The production of X-rays by a bolt of lightning was theoretically predicted as early as 1925,but no evidence was found until 2001/2002, when researchers at the New Mexico Institute of Mining and Technology detected X-ray emissions from an induced lightning strike along a grounded wire trailed behind a rocket shot into a storm cloud. In the same year University of Florida and Florida Tech researchers used an array of electric field and X-ray detectors at a lightning research facility in North Florida to confirm that natural lightning makes X-rays in large quantities during the propagation of stepped leaders. The cause of the X-ray emissions is still a matter for research, as the temperature of lightning is too low to account for the X-rays observed.
A number of observations by space-based telescopes have revealed even higher energy gamma ray emissions, the so-called terrestrial gamma-ray flashes (TGFs). These observations pose a challenge to current theories of lightning, especially with the recent discovery of the clear signatures of antimatter produced in lightning.Recent research has shown that secondary species, produced by these TGFs, such as electrons, positrons, neutrons or protons, can gain energies of up to several tens of MeV.
The very high temperatures generated by lightning lead to significant local increases in ozone and oxides of nitrogen. Each lightning flash in temperate and sub-tropical areas produces 7 kg of NOx on average. In the troposphere the effect of lightning can increase NOx by 90% and ozone by 30%.
Volcanic activity produces lightning-friendly conditions in multiple ways. The enormous quantity of pulverized material and gases explosively ejected into the atmosphere creates a dense plume of particles. The ash density and constant motion within the volcanic plume produces charge by frictional interactions (triboelectrification), resulting in very powerful and very frequent flashes as the cloud attempts to neutralize itself. Due to the extensive solid material (ash) content, unlike the water rich charge generating zones of a normal thundercloud, it is often called a dirty thunderstorm.
Lightning has been observed within the atmospheres of other planets, such as Jupiter and Saturn. Although in the minority on Earth, superbolts appear to be common on Jupiter.
Lightning on Venus has been a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the 1970s and 1980s, signals suggesting lightning may be present in the upper atmosphere were detected.Although the Cassini–Huygens mission fly-by of Venus in 1999 detected no signs of lightning, the observation window lasted mere hours. Radio pulses recorded by the spacecraft Venus Express (which began orbiting Venus in April 2006) may originate from lightning on Venus.
Thunder is heard as a rolling, gradually dissipating rumble because the sound from different portions of a long stroke arrives at slightly different times.
When the local electric field exceeds the dielectric strength of damp air (about 3 megavolts per meter), electrical discharge results in a strike, often followed by commensurate discharges branching from the same path. (See image, right.) Mechanisms that cause the charges to build up to lightning are still a matter of scientific investigation.New study confirming dielectric breakdown is involved. Rison 2016. Lightning may be caused by the circulation of warm moisture-filled air through electric fields. Ice or water particles then accumulate charge as in a Van de Graaff generator.
Researchers at the University of Florida found that the final one-dimensional speeds of 10 flashes observed were between 1.0×105 and 1.4×106 m/s, with an average of 4.4×105 m/s.
The earliest detector invented to warn of the approach of a thunder storm was the lightning bell. Benjamin Franklin installed one such device in his house.The detector was based on an electrostatic device called the 'electric chimes' invented by Andrew Gordon in 1742.
Lightning discharges generate a wide range of electromagnetic radiations, including radio-frequency pulses. The times at which a pulse from a given lightning discharge arrives at several receivers can be used to locate the source of the discharge. The United States federal government has constructed a nationwide grid of such lightning detectors, allowing lightning discharges to be tracked in real time throughout the continental U.S.
The Earth-ionosphere waveguide traps electromagnetic VLF- and ELF waves. Electromagnetic pulses transmitted by lightning strikes propagate within that waveguide. The waveguide is dispersive, which means that their group velocity depends on frequency. The difference of the group time delay of a lightning pulse at adjacent frequencies is proportional to the distance between transmitter and receiver. Together with direction finding methods, this allows locating lightning strikes up to distances of 10,000 km from their origin. Moreover, the eigenfrequencies of the Earth-ionospheric waveguide, the Schumann resonances at about 7.5 Hz, are used to determine the global thunderstorm activity.
In addition to ground-based lightning detection, several instruments aboard satellites have been constructed to observe lightning distribution. These include the Optical Transient Detector (OTD), aboard the OrbView-1 satellite launched on April 3, 1995, and the subsequent Lightning Imaging Sensor (LIS) aboard TRMM launched on November 28, 1997.
The movement of electrical charges produces a magnetic field (see electromagnetism). The intense currents of a lightning discharge create a fleeting but very strong magnetic field. Where the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized. This effect is known as lightning-induced remanent magnetism, or LIRM. These currents follow the least resistive path, often horizontally near the surfacebut sometimes vertically, where faults, ore bodies, or ground water offers a less resistive path. One theory suggests that lodestones, natural magnets encountered in ancient times, were created in this manner.
Lightning-induced magnetic anomalies can be mapped in the ground,and analysis of magnetized materials can confirm lightning was the source of the magnetization and provide an estimate of the peak current of the lightning discharge.
Research at the University of Innsbruck has found that magnetic fields generated by plasma may induce hallucinations in subjects located within 200 m (660 ft) of a severe lightning storm.
Some high energy cosmic rays produced by supernovas as well as solar particles from the solar wind, enter the atmosphere and electrify the air, which may create pathways for lightning bolts.
In many cultures, lightning has been viewed as part of a deity or a deity in and of itself. These include the Greek god Zeus, the Aztec god Tlaloc, the Mayan God K, Slavic mythology's Perun, the Baltic Pērkons/Perkūnas, Thor in Norse mythology, Ukko in Finnish mythology, the Hindu god Indra, and the Shinto god Raijin. [Matthew 24:27] [Luke 17:24]In the traditional religion of the African Bantu tribes, lightning is a sign of the ire of the gods. Verses in the Jewish religion and in Islam also ascribe supernatural importance to lightning. In Christianity, the Second Coming of Jesus is compared to lightning.
The expression "Lightning never strikes twice (in the same place)" is similar to "Opportunity never knocks twice" in the vein of a "once in a lifetime" opportunity, i.e., something that is generally considered improbable. Lightning occurs frequently and more so in specific areas. Since various factors alter the probability of strikes at any given location, repeat lightning strikes have a very low probability (but are not impossible).Similarly, "A bolt from the blue" refers to something totally unexpected, and "A person being struck by lightning" is an imaginative or comedic metaphor for someone to experience a once in a lifetime, striking, sudden lightning-speed revelation, similar to an epiphany or an enlightenment.
Some political parties use lightning flashes as a symbol of power, such as the People's Action Party in Singapore, the British Union of Fascists during the 1930s, and the National States' Rights Party in the United States during the 1950s.The Schutzstaffel, the paramilitary wing of the Nazi Party, used the Sig rune in their logo which symbolizes lightning. The German word Blitzkrieg, which means "lightning war", was a major offensive strategy of the German army during World War II.
In French and Italian, the expression for "Love at first sight" is coup de foudre and colpo di fulmine, respectively, which literally translated means "lightning strike". Some European languages have a separate word for lightning which strikes the ground (as opposed to lightning in general); often it is a cognate of the English word "rays". The name of Australia's most celebrated thoroughbred horse, Phar Lap, derives from the shared Zhuang and Thai word for lightning.
The bolt of lightning in heraldry is called a thunderbolt and is shown as a zigzag with non-pointed ends. This symbol usually represents power and speed.
The lightning bolt is used to represent the instantaneous communication capabilities of electrically powered telegraphs and radios. It was a commonly used motif in Art Deco design, especially the zig-zag Art Deco design of the late 1920s.The lightning bolt is a common insignia for military communications units throughout the world. A lightning bolt is also the NATO symbol for a signal asset.
The Unicode symbol for lightning is ☇ U+2607.
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A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers."NWS JetStream". National Weather Service. Retrieved 26 January 2019.</ref> Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.
Thunder is the sound caused by lightning. Depending on the distance from and nature of the lightning, it can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air within and surrounding the path of a lightning strike. In turn, this expansion of air creates a sonic shock wave, often referred to as a "thunderclap" or "peal of thunder".
Ball lightning is an unexplained and potentially dangerous atmospheric electrical phenomenon. The term refers to reports of luminescent, spherical objects that vary from pea-sized to several meters in diameter. Though usually associated with thunderstorms, the phenomenon lasts considerably longer than the split-second flash of a lightning bolt. Two reports from the nineteenth century say that the ball eventually explodes, leaving behind an odor of sulfur. The actual existence of the ball lightning phenomena is not proven, but they appear in a variety of accounts over the centuries. Until the 1960s, most scientists treated reports of ball lightning skeptically, despite numerous accounts from around the world. Laboratory experiments can produce effects that are visually similar to reports of ball lightning, but how these relate to the natural phenomenon remains unclear.
The Schumann resonances (SR) are a set of spectrum peaks in the extremely low frequency (ELF) portion of the Earth's electromagnetic field spectrum. Schumann resonances are global electromagnetic resonances, generated and excited by lightning discharges in the cavity formed by the Earth's surface and the ionosphere.
A terrestrial gamma-ray flash (TGF) is a burst of gamma rays produced in Earth's atmosphere. TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 million electronvolts. It is speculated that TGFs are caused by intense electric fields produced above or inside thunderstorms. Scientists have also detected energetic positrons and electrons produced by terrestrial gamma-ray flashes.
An electrolaser is a type of electroshock weapon that is also a directed-energy weapon. It uses lasers to form an electrically conductive laser-induced plasma channel (LIPC). A fraction of a second later, a powerful electric current is sent down this plasma channel and delivered to the target, thus functioning overall as a large-scale, high energy, long-distance version of the Taser electroshock gun.
Atmospheric electricity is the study of electrical charges in the Earth's atmosphere. The movement of charge between the Earth's surface, the atmosphere, and the ionosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is an interdisciplinary topic with a long history, involving concepts from electrostatics, atmospheric physics, meteorology and Earth science.
Narrow bipolar pulses are high-energy, high-altitude, intra-cloud electrical discharges associated with thunderstorms. NBP are similar to other forms of lightning events such as return strokes and dart leaders, but produce an optical emission of at least an order of magnitude smaller. They typically occur in the 10–20 km altitude range and can emit a power on the order of a few hundred gigawatts. They produce far-field asymmetric bipolar electric field change signatures.
A lightning strike or lightning bolt is an electric discharge between the atmosphere and an object. They mostly originate in a cumulonimbus cloud and terminate on the ground, called cloud to ground (CG) lightning. A less common type of strike, called ground to cloud (GC), is upward propagating lightning initiated from a tall grounded object and reaches into the clouds. About 25% of all lightning events worldwide are strikes between the atmosphere and earth-bound objects. The bulk of lightning events are intra-cloud (IC) or cloud to cloud (CC), where discharges only occur high in the atmosphere. Lightning strikes the average commercial aircraft at least once a year, but modern engineering and design means this is rarely a problem. The movement of aircraft through clouds can even cause lightning strikes.
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael Faraday described this phenomenon as "the beautiful flash of light attending the discharge of common electricity".
A lightning detector is a device that detects lightning produced by thunderstorms. There are three primary types of detectors: ground-based systems using multiple antennas, mobile systems using a direction and a sense antenna in the same location, and space-based systems.
Upper-atmospheric lightning or ionospheric lightning are terms sometimes used by researchers to refer to a family of short-lived electrical-breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmospheric lightning is believed to be electrically induced forms of luminous plasma. The preferred usage is transient luminous event (TLE), because the various types of electrical-discharge phenomena in the upper atmosphere lack several characteristics of the more familiar tropospheric lightning.
A radio atmospheric signal or sferic is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation in the Earth–ionosphere waveguide, and can be received thousands of kilometres from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions.
Sprites or red sprites are large-scale electrical discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a quite varied range of visual shapes flickering in the night sky. They are usually triggered by the discharges of positive lightning between an underlying thundercloud and the ground.
Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.
A lightning rod or lightning conductor (UK) is a metal rod mounted on a structure and intended to protect the structure from a lightning strike. If lightning hits the structure, it will preferentially strike the rod and be conducted to ground through a wire, instead of passing through the structure, where it could start a fire or cause electrocution. Lightning rods are also called finials, air terminals, or strike termination devices.
Joseph R. Dwyer is an American physicist known for his lightning research. He is a Professor of Physics at the University of New Hampshire. Dwyer received his Ph.D. in Physics from the University of Chicago in 1994 and worked on cosmic-ray physics and gamma-ray astronomy as a research scientist at Columbia University and the University of Maryland before joining the faculty at the Florida Institute of Technology in 2000. After moving to Melbourne, Florida, Dwyer became interested in lightning physics and his research now focuses on high-energy radiation production from thunderstorms and lightning. In 2002, Dwyer and collaborators discovered that rocket-triggered lightning produced large quantities of x-rays, allowing for first the time detailed studies of an atmospheric phenomenon known as runaway breakdown. In 2014, Dwyer left the Florida Institute of Technology and joined the University of New Hampshire.
Paleolightning is the study of lightning activity throughout Earth's history. Some studies have speculated that lightning activity played a crucial role in the development of not only Earth's early atmosphere, but also early life. Lightning, a non-biological process, has been found to produce biologically useful material through the oxidation and reduction of inorganic matter. Research on the impact of lightning on Earth's atmosphere continues today, especially with regard to feedback mechanisms of lightning-produced nitrate compounds on atmospheric composition and global average temperatures.
Since the late 1980s, there have been several attempts to investigate the possibility of harvesting lightning energy. A single bolt of lightning carries a relatively large amount of energy. However, this energy is concentrated in a small location and is passed during an extremely short period of time (microseconds); therefore, extremely high electrical power is involved. 5 gigajoules over 10 microseconds is equal to 500 terawatts. Because lightning bolts vary in voltage and current, a more average calculation would be 10 gigawatts. It has been proposed that the energy contained in lightning be used to generate hydrogen from water, to harness the energy from rapid heating of water due to lightning, or to use a group of lightning arresters to harness a strike, either directly or by converting it to heat or mechanical energy, or to use inductors spaced far enough away so that a safe fraction of the energy might be captured.
The distribution of lightning, or the incidence of individual strikes, in any particular place is highly variable, but lightning does have an underlying spatial distribution. High quality lightning data has only recently become available, but the data indicates that lightning occurs on average 44 times every second over the entire Earth, making a total of about 1.4 billion flashes per year.
Behind us were frightening dark clouds, rent by lightning twisted and hurled, opening to reveal huge figures of flame.
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