Strokes of cloud-to-ground lightning during a thunderstorm
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 billion joules of energy. This discharge may produce a wide range of electromagnetic radiations, 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 is often followed by thunder, an audible sound caused by the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. It occurs commonly during thunderstorms and other types of energetic weather systems.
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", "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".
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)
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 to the right. 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.
When
the rising ice crystals collide with graupel, the ice crystals become
positively charged and the graupel becomes negatively charged.
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 to the left. 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
upper part of the thunderstorm cloud becomes positively charged while
the middle to lower part of the thunderstorm cloud becomes negatively
charged.
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.
General considerations
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. 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.
Distribution and Frequency
World
map showing frequency of lightning strikes, in flashes per km² per year
(equal-area projection), from combined 1995–2003 data from the Optical
Transient Detector and 1998–2003 data from the Lightning Imaging Sensor.
Lightning is not distributed evenly around the planet, as shown in the map.
On Earth, the lightning frequency is approximately 44 (± 5) times per second, or nearly 1.4 billion flashes per year and 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 tropics where 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 the influence of small or absent land masses in the vast
stretches of the world's oceans limits the differences between these
variants in the atmosphere, lightning is notably less frequent there
than over larger landforms. The North and South Poles are limited in their coverage of thunderstorms and therefore result in areas with the least amount of lightning.
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.
Lightning in Belfort, France
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 (0.6–1.25 miles) above the ground and tops up to 15 km (9.3 mi) in height.
Lightning hotspots: 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, where the elevation
is around 975 m (3,200 ft). On average, this region receives 158
lightning strikes per 1 square kilometer (0.39 sq mi) per year. Lake Maracaibo in Venezuela averages 297 days per year with lightning activity. Other lightning hotspots include Catatumbo in Venezuela, Singapore, and Lightning Alley in Central Florida.
Necessary conditions
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.
Electrical field generation
View of lightning from an airplane flying above a system.
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.
Flashes and strikes
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.
A lightning strike from cloud to ground in the California, Mojave Desert
An intracloud flash. A lightning flash within the cloud, illuminates the entire blanket.
Lightning leaders
A downward leader travels towards earth, branching as it goes.
Lightning strike caused by the connection of two leaders, positive shown in blue and negative in red
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 get 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. 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,
describes random connectivity phenomena, which produce an evolution of
connected structures similar to that of lightning strikes.
Upward streamers
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.
Attachment
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.
Discharge
Return stroke
High-speed photography showing different parts of a lightning flash during the discharge process as seen in Toulouse, France.
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.
Re-strike
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.
Transient currents during flash
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. 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.
Types
There are three primary types of lightning, defined by what is at the "ends" of a flash channel.
- Intracloud (IC), which occurs within a single thundercloud unit
- Cloud to cloud (CC) or intercloud, which starts and ends between two different "functional" thundercloud units
- Cloud to ground (CG), that primarily originates in the thundercloud and terminates on an Earth surface, but may also occur in the reverse direction, that is ground to cloud
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)
Cloud to ground lightning
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.
Positive and negative lightning
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.
A Bolt from the blue lightning strike which appears to initiate from the clear, but turbulent sky above the anvil cloud
and drive a bolt of plasma through the cloud directly to the ground.
They are commonly referred to as positive flashes despite the fact that
they are usually negative in polarity.
There are six different mechanisms theorized to result in the formation of downward positive lightning.
- Vertical wind shear displacing the upper positive charge region of a thundercloud, exposing it to the ground below.
- The loss of lower charge regions in the dissipating stage of a thunderstorm, leaving the primary positive charge region.
- A complex arrangement of charge regions in a thundercloud, effectively resulting in an inverted dipole or inverted tripole in which the main negative charge region is above the main positive charge region instead of beneath it.
- An unusually large lower positive charge region in the thundercloud.
- Cutoff of an extended negative leader from its origin which creates a new bidirectional leader in which the positive end strikes the ground, commonly seen in anvil-crawler spider flashes.
- The initiation of a downward positive branch from an intracloud lightning flash.
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 500 megajoules of energy. Large bolts of negative lightning can carry up to 120 kA and 350 coulombs.
The average positive ground flash has roughly double the peak current
of a typical negative flash, and can produce peak currents up to 400,000
amperes (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 tornadoes and 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.
Cloud to cloud (CC) and intra-cloud (IC)
Branching of cloud to cloud lightning, New Delhi, India
Multiple paths of cloud-to-cloud lightning, Swifts Creek, Australia.
Cloud-to-cloud lightning, Victoria, Australia.
Cloud-to-cloud lightning seen in Gresham, Oregon.
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.
Anvil
Crawler over Lake Wright Patman south of Redwater, Texas on the
backside of a large area of rain associated with a cold-front
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.
Observational variations
- Anvil crawler lightning, sometimes called Spider lightning is created when leaders propagate through horizontally-extensive charge regions in mature thunderstorms, usually the stratiform regions of mesoscale convective systems. These discharges usually begin as intracloud discharges originating within the convective region; the negative leader end then propagates well into the aforementioned charge regions in the stratiform area. If the leader becomes too long, it may separate into multiple bidirectional leaders. When this happens, the positive end of the separated leader may strike the ground as a positive CG flash or crawl on the underside of the cloud, creating a spectacular display of lightning crawling across the sky. Ground flashes produced in this manner tend to transfer high amounts of charge, and this can trigger upward lightning flashes and upper-atmospheric lightning.
- Ball lightning may be an atmospheric electrical phenomenon, the physical nature of which is still controversial. The term refers to reports of luminous, usually spherical objects which vary from pea-sized to several meters in diameter. It is sometimes associated with thunderstorms, but unlike lightning flashes, which last only a fraction of a second, ball lightning reportedly lasts many seconds. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists. Scientific data on natural ball lightning is scarce owing to its infrequency and unpredictability. The presumption of its existence is based on reported public sightings, and has therefore produced somewhat inconsistent findings. Brett Porter, a wildlife ranger, reported taking a photo at Queensland of Australia in 1987.
- Bead lightning is the decaying stage of a lightning channel in which the luminosity of the channel breaks up into segments. Nearly every lightning discharge will exhibit beading as the channel cools immediately after a return stroke, sometimes referred to as the lightning's 'bead-out' stage. 'Bead lightning' is more properly a stage of a normal lightning discharge rather than a type of lightning in itself. Beading of a lightning channel is usually a small-scale feature, and therefore is often only apparent when the observer/camera is close to the lightning.
- Cloud-to-air lightning is a lightning flash in which one end of a bidirectional leader exits the cloud, but does not result in a ground flash. Such flashes can sometimes be thought of as failed ground flashes. Blue jets and gigantic jets are a form of cloud-to-air or cloud-to-ionosphere lightning where a leader is launched from the top of a thunderstorm.
- Dry lightning is used in Australia, Canada and the United States for lightning that occurs with no precipitation at the surface. This type of lightning is the most common natural cause of wildfires. Pyrocumulus clouds produce lightning for the same reason that it is produced by cumulonimbus clouds.
- Forked lightning is cloud-to-ground lightning that exhibits branching of its path.
- Heat lightning is a lightning flash that appears to produce no discernible thunder because it occurs too far away for the thunder to be heard. The sound waves dissipate before they reach the observer.
- Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.
- Rocket lightning is a form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently.
- Sheet lightning is cloud-to-cloud lightning that exhibits a diffuse brightening of the surface of a cloud, caused by the actual discharge path being hidden or too far away. The lightning itself cannot be seen by the spectator, so it appears as only a flash, or a sheet of light. The lightning may be too far away to discern individual flashes.
- Smooth channel lightning is an informal term referring to a type of cloud-to-ground lightning strike that has no visible branching and appears like a line with smooth curves as opposed to the jagged appearance of most lightning channels. They are a form of positive lightning generally observed in or near the convective regions of severe thunderstorms in the north central United States. It is theorized that severe thunderstorms in this region obtain an "inverted tripole" charge structure in which the main positive charge region is located below the main negative charge region instead of above it, and as a result these thunderstorms generate predominantly positive cloud-to-ground lightning. The term "smooth channel lightning" is also sometimes attributed to upward ground-to-cloud lightning flashes, which are generally negative flashes initiated by upward positive leaders from tall structures.
- Staccato lightning is a cloud-to-ground lightning (CG) strike which is a short-duration stroke that (often but not always) appears as a single very bright flash and often has considerable branching. These are often found in the visual vault area near the mesocyclone of rotating thunderstorms and coincides with intensification of thunderstorm updrafts. A similar cloud-to-cloud strike consisting of a brief flash over a small area, appearing like a blip, also occurs in a similar area of rotating updrafts.
This
CG was of very short duration, exhibited highly branched channels and
was very bright indicating that it was staccato lightning near New
Boston, Texas.
- Superbolts are rather loosely defined as strikes with a peak source power of more than 100 gigawatts (most lightning strikes come in at around 1 gigawatt). Events of this magnitude occur about as frequently as one in 240 strikes. They are not categorically distinct from ordinary lightning strikes, and simply represent the uppermost edge of a continuum. Contrary to popular misconception, superbolts can be either positively or negatively charged, and the charge ratio is comparable to that of "ordinary" lightning.
- Sympathetic lightning is the tendency of lightning to be loosely coordinated across long distances. Discharges can appear in clusters when viewed from space.
- Upward lightning or ground-to-cloud lightning is a lightning flash which originates from the top of a grounded object and propagates upward from this point. This type of lightning can be triggered by a preceding lightning flash, or it may initiate entirely on its own. The former is generally found in regions where spider lightning occurs, and may involve multiple grounded objects simultaneously. The latter usually occurs during the cold season and may be the dominant lightning type in thundersnow events.
- Clear-air lightning describes lightning that occurs with no apparent cloud close enough to have produced it. In the U.S. and Canadian Rockies, a thunderstorm can be in an adjacent valley and not observable from the valley where the lightning bolt strikes, either visually or audibly. European and Asian mountainous areas experience similar events. Also in areas such as sounds, large lakes or open plains, when the storm cell is on the near horizon (within 26 km (16 mi)) there may be some distant activity, a strike can occur and as the storm is so far away, the strike is referred to as a bolt from the blue. These flashes usually begin as normal intracloud lightning flashes before the negative leader exits the cloud and strikes the ground a considerable distance away. Positive clear-air strikes can occur in highly sheared environments where the upper positive charge region becomes horizontally displaced from the precipitation area.
Effects
Lightning strike
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.
Thunder
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, and sound
travels through air at about 343 m/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
(0.213 mi) in distance; a delay of three seconds would indicate a
distance of about one kilometer (0.62 mi) (3×343 m). A flash preceding
thunder by five seconds would indicate a distance of approximately one
mile (1.6 km) (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 100 miles
whereas the thunder travels about 20 miles. 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.
High-energy radiation
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.
Air quality
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
Volcanic material thrust high into the atmosphere can trigger lightning.
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.
- Powerful and frequent flashes have been witnessed in the volcanic plume as far back as the 79 AD eruption of Vesuvius by Pliny The Younger.
- Likewise, vapors and ash originating from vents on the volcano's flanks may produce more localized and smaller flashes upwards of 2.9 km long.
- Small, short duration sparks, recently documented near newly extruded magma, attest to the material being highly charged prior to even entering the atmosphere.
Extraterrestrial
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.
- Airplane contrails have also been observed to influence lightning to a small degree. The water vapor-dense contrails of airplanes may provide a lower resistance pathway through the atmosphere having some influence upon the establishment of an ionic pathway for a lightning flash to follow.
- Rocket exhaust plumes provided a pathway for lightning when it was witnessed striking the Apollo 12 rocket shortly after takeoff.
- Thermonuclear explosions by providing extra material for electrical conduction and a very turbulent localized atmosphere, have been seen triggering lightning flashes within the mushroom cloud. In addition, intense gamma radiation from large nuclear explosions may develop intensely charged regions in the surrounding air through Compton scattering. The intensely charged space charge regions create multiple clear-air lightning discharges shortly after the device detonates.
Scientific study
Properties
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 million volts 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.
Detection and monitoring
Lightning strike counter in a museum
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.
Artificially triggered
- Rocket-triggered lightning can be "triggered" by launching specially designed rockets
trailing spools of wire into thunderstorms. The wire unwinds as the
rocket ascends, creating an elevated ground that can attract descending
leaders. If a leader attaches, the wire provides a low-resistance
pathway for a lightning flash to occur. The wire is vaporized by the
return current flow, creating a straight lightning plasma channel in its
place. This method allows for scientific research of lightning to occur
under a more controlled and predictable manner.[99]
- The International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida typically uses rocket triggered lightning in their research studies.
- Laser-triggered
- Since the 1970s, researchers have attempted to trigger lightning strikes by means of infrared or ultraviolet lasers, which create a channel of ionized gas through which the lightning would be conducted to ground. Such triggering of lightning is intended to protect rocket launching pads, electric power facilities, and other sensitive targets.
- In New Mexico, U.S., scientists tested a new terawatt laser which provoked lightning. Scientists fired ultra-fast pulses from an extremely powerful laser thus sending several terawatts into the clouds to call down electrical discharges in storm clouds over the region. The laser beams sent from the laser make channels of ionized molecules known as "filaments". Before the lightning strikes earth, the filaments lead electricity through the clouds, playing the role of lightning rods. Researchers generated filaments that lived a period too short to trigger a real lightning strike. Nevertheless, a boost in electrical activity within the clouds was registered. According to the French and German scientists who ran the experiment, the fast pulses sent from the laser will be able to provoke lightning strikes on demand. Statistical analysis showed that their laser pulses indeed enhanced the electrical activity in the thundercloud where it was aimed—in effect they generated small local discharges located at the position of the plasma channels.
Physical manifestations
Lightning-induced
remanent magnetization (LIRM) mapped during a magnetic field gradient
survey of an archaeological site located in Wyoming, United States.
Magnetism
The movement of electrical charges produces a magnetic field.
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 surface but 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 meters of a severe
lightning storm.
Solar wind and cosmic rays
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 culture and religion
Lightning by Mikalojus Konstantinas Ciurlionis (1909)
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. 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.
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.
