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Lightning

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For alternate meanings, see Lightning (disambiguation).
Multiple cloud-to-ground and cloud-to-cloud lightning strokes are observed during a night-time thunderstorm.
Cloud to cloud lightning

Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. Lightning's abrupt electric discharge is accompanied by the emission of light. The electricity passing through the discharge channels rapidly heats and expands the air into a plasma, producing lightning's characteristic thunder sound.


How lightning is formed

Lightning over Pentagon City in Arlington County, Virginia

The first process in the generation of lightning is the forcible separation of positive and negative charges within a cloud or air. The mechanism by which this happens is still the subject of research, but one widely accepted theory is the polarization mechanism. This mechanism has two components: the first is that falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field, and the second is that colliding ice particles become charged by electrostatic induction. Once charged, by whatever mechanism, work is performed as the opposite charges are driven apart and energy is stored in the e-fields between them. The positively charged crystals tend to rise to the top, causing the cloud top to build up a positive charge, and the negatively charged crystals and hailstones drop to the middle and bottom layers of the cloud, building up a negative charge. Cloud-to-cloud lightning can appear at this point. Cloud-to-ground lightning is less common. Cumulonimbus clouds that do not produce enough ice crystals usually fail to produce enough charge separation to cause lightning.

When sufficient negatives and positives gather in this way, and when the e-field becomes sufficiently strong, an electrical discharge occurs within the clouds or between the clouds and the ground, producing the bolt. It has been suggested that these discharges are triggered by cosmic ray strikes which ionise atoms, releasing electrons that are accelerated by the electric fields, ionising other air molecules and making the air conductive, then starting a lightning strike. During the strike, successive portions of air become conductive as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions (stepped channels called step leaders). The conductive filament grows in length. At the same time, electrical energy stored in the e-field flows radially inward into the conductive filament.

When a charged step leader is near the ground opposite charges appear on the ground and enhance the electric field. The e-field is higher on trees and tall buildings. If the e-field is high enough a discharge can initiate from the ground. This discharge starts as positive streamer, and if it develops as a positive leader can eventually connect to the descending discharge from the cloud.

Lightning can also occur within the ash clouds from volcanic eruptions[1],[2], or can be caused by violent forest fires which generate sufficient dust to create a static charge.

Negative lightning

A bolt of lightning usually begins when an invisible negatively charged stepped leader stroke is sent out from the cloud. As it does so, a positively charged streamer is usually sent out from the positively charged ground or cloud. When the two leader meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud. This "return stroke" is the most luminous part of the strike, and is the part that is really visible. Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect. Thunder is caused when the discharge rapidly superheats the leader channel, causing a shock wave to be sent out.

It is possible for streamers to be sent out from several different objects simultaneously, with only one connecting with the leader and forming the discharge path. Photographs have been taken on which non-connected streamers are visible [3].

This type of lightning is known as negative lightning due to the discharge of negative charge from the cloud, and accounts for over 95% of all lightning.

Statistics: an average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts, dissipates 500 megajoules (enough to light a 100 watt lightbulb for 2 months), and lasts a few milliseconds.

Positive lightning

Positive lightning makes up less than 5 percent of all lightning. It occurs when the stepped leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike several kilometers or miles distant from the clouds. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.

As a result of their power, positive lightning strikes are considerably more dangerous. At the present time aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999 [4]. It has since been suggested that it may have been positive lightning that caused the crash of Pan Am Flight 214 in 1963. Positive lightning is now also thought to be responsible for many forest fires.

Positive lightning has also been shown to trigger the occurrence of upper atmospheric lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.

Statistics (based on a small number of measurements): an average bolt of positive lightning carries a current of 300 kiloamperes, transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a thousand million volts), dissipates enough energy to light a 100 watt lightbulb for up to 95 years, and lasts for tens or hundreds of milliseconds.

Bipolar lightning

Heinz Kasemir first hypothesized that a lightning leader system actually develops in a bipolar fashion, with both a positive and a negative branching leader system connected at the system origin and containing a net zero charge. This process provides a means for the positive leader to conduct away the net negative charge collected during development, allowing the leader system to act as an extending polarized conductor. Such a polarized conductor would be able to maintain intense electric fields at its ends, supporting continued leader development in weak background electric fields.

During the Eighties, flight tests showed that aircraft can trigger a bipolar stepped leader when crossing charged cloud areas. Many scientists think that positive and negative lightning in a cloud are actually bipolar lightning.


Runaway breakdown theory

To spontaneously ionise air and conduct electricity across it, an electric field of approximately 2500 kilovolts per metre is required. However, measurements inside storm clouds to date have failed to locate fields this strong, with typical fields being between 100 and 400 kilovolts per metre. While there remains a possibility that researchers are failing to encounter the small high-strength regions of the large clouds, the odds of this are diminishing as further measurements continue to fall short.

A theory by Alex Gurevich of the Lebedev Physical Institute in 1992 proposes that cosmic rays may provide the beginnings of what he called a runaway breakdown. After striking an air molecule and rendering it extremely energetic, it strikes and ionises other air molecules in the storm's electric field, forming a chain reaction until electricity can flow through the air. This was initially considered a fringe theory, but is now becoming mainstream due to the lack of other theories.

It has been recently revealed that most lightning emits an intense burst of X-rays and/or gamma-rays which seem to be produced during the stepped-leader and dart-leader phases just before the stroke becomes visible. The X-ray bursts typically have a total duration of less than 100 microseconds and have energies extending up to nearly a few hundred KeV. The presence of these high-energy events match and support the "runaway breakdown" theory, and were discovered through the examination of rocket triggered lightning, and from satellite monitoring of natural lightning.

NASA's RHESSI satellite typically reports 50 gamma-ray events per day, and many of these are strong enough to fit the theory. Additionally, low-frequency radio emissions detected at ground level can detect lightning bolts from upwards of 4000km away; combining these with gamma-ray burst events detected from above show overlapping positions and timing.

There are problems with the "runaway breakdown" theory, however. While there seems to be a strong correlation between gamma-ray events and lightning, there are insufficient events detected to account for the amount of lightning occurring across the planet. Another issue is the amount of energy the theory states is required to initiate the breakdown. Cosmic rays of sufficient energy strike the atmosphere on average only once per 50 seconds per square kilometre. Measured X-ray burst intensity also falls short, with results indicating particle energy 1/20th of the theory's value.

Lightning facts

Old lightning scar (Georgetown, South Carolina)

A bolt of lightning can reach temperatures approaching 28,000 kelvins (50,000 degrees Fahrenheit) in a split second. This is about five times hotter than the surface of the sun. The heat of lightning which strikes loose soil or sandy regions of the ground may fuse the soil or sand into glass channels called fulgurites. These are sometimes found under the sandy surfaces of beaches and golf courses, or in desert regions. Fulgurites are evidence that lightning spreads out into branching channels when it strikes the ground.

Trees are frequent conductors of lightning to the ground (photo of a tree being struck by lightning). Since sap is a poor conductor, its electrical resistance causes it to be heated explosively into steam, which blows off the bark outside the lightning's path. In following seasons trees overgrow the damaged area and may cover it completely, leaving only a vertical scar. If the damage is severe, the tree may not be able to recover, and decay sets in, eventually killing the tree. Occasionally, a tree may explode completely, as in this Giant Sequoia struck in Geneva. It is commonly thought that a tree standing alone is more frequently struck, though in some forest areas, lightning scars can be seen on almost every tree.

Lightning injuries

Nearly 2000 persons per year in the world are injured by lightning strikes, and between 25 to 33 per cent of those struck die. Lightning injuries result from three factors: electrical damage, intense heat, and the mechanical energy which these generate. While sudden death is common due to the huge voltage of a lightning strike, survivors often fare better than victims of other electrical injuries which result in a more prolonged application of lesser voltage.

People may be hit in several different ways. In a direct hit the electrical charge strikes the victim first. Counterintuitively, if the victim's skin resistance is high enough, much of the current will flash around the skin or clothing to the ground, resulting in a surprisingly benign outcome. Splash hits occur when lightning effectively bounces off a nearby object and strikes the victim en route to ground. Ground strikes, in which the bolt lands near the victim and is conducted through the victim via his or her connection to the ground (such as through the feet), can cause great damage.

The most critical injuries are to the circulatory system, the lungs, and the central nervous system. Many victims suffer immediate cardiac arrest and will not survive without prompt emergency care, which, it is worth noting, is safe to administer, due to the fact that the victim will not retain any electrical charge after the lightning has struck (of course, the helper could be struck by a separate bolt of lightning in the vicinity). Others incur myocardial infarction and various cardiac arrhythmias, either of which can be rapidly fatal as well. The intense heat generated by a lightning strike can cause lung damage, and the chest can be damaged by the mechanical force of rapidly expanding heated air. Either the electrical or the mechanical force can result in loss of consciousness, which is very common immediately after a strike. Amnesia and confusion of varying duration often result as well. A complete physical examination by paramedics or physicians may reveal ruptured eardrums, and ocular cataracts may develop, sometimes more than a year after an otherwise uneventful recovery.

Lightning safety

Lightning is responsible for approximately 100 deaths a year in the United States alone. Lightning ranks second only to floods for storm related casualties in the U.S. every year. Many of these deaths could be prevented if basic precautions were taken when thunderstorms are expected in an area. Listening to a radio to keep up to date on storms in the area is the first step in lightning safety.

One way to prepare is to install a lightning conductor (or, lightning rod) for preventing lightning damage to a building. A lightning conductor is a metal spike that is connected to earth by a low-resistance path. Should lightning strike a building, the current will travel through the conductor rather than through the fabric of the building, causing less damage.

Electrical equipment can be protected from lightning by a lightning arrester, a device that contains one or more gas-filled spark gaps between the equipment's cables and earth. Should lightning strike one of the cables, the high voltage will cause the gas in the spark gap to break down and become a conductor, providing a path for the lightning to reach the ground without passing through the equipment.

Safer locations

USS Abraham Lincoln rides out a storm in the Arabian Sea while on station in support of Operation Southern Watch and Operation Enduring Freedom.

No place is completely safe in a thunderstorm, but some are more safe than others. Larger, better constructed structures are better than smaller or more open structures. An automobile of most sorts is generally the safest place to be during a lightning strike. This is explained by the idea of a Faraday Cage. The metal frame of the car acts as a cage and, consequently, the charge of the car polarizes exactly opposite of that of the lightning bolt. This results in the lightning bolt being cancelled out. This explains why a car is safe in a thunder storm. It is not because the rubber tires are not conductive - since a lightning bolt can and does easily bypass the tires by jumping the short distance from the car body to the ground.

When outside, avoid the following
  • High places and open fields
  • isolated trees
  • unprotected gazebos
  • rain or picnic shelters
  • baseball dugouts
  • communications towers
  • flagpoles
  • light poles
  • bleachers (stadium seating) (metal or wood)
  • metal fences
  • open top vehicles such as convertibles, tractors
  • golf carts
  • water (ocean, lakes, swimming pools, rivers, etc.)
  • metal-shafted or conductive umbrellas, golf clubs, lacrosse sticks, baseball bats, shovels, or fishing rods

If you find yourself trapped in an open area during a storm, position yourself close to the ground by squatting with your feet close together and on the balls of your feet. Crouch in a ditch if possible (but be aware of sudden flooding in heavy rain). Avoid proximity to other people (minimum 5 meters or 15 feet). Since lightning spreads when it hits the ground, you want to minimize as much contact area between you and the ground; consequently you should not lie on the ground in an attempt to minimise your height. Remember, humans are good conductors of electricity, better so than air, and lightning tends to strike the highest thing in an area, because electricity will always take the path of least resistance.

Lone tall trees are particularly dangerous; the tree being moist, the electricity generally passes down underneath the bark, splitting it in all directions, and the lightning will pass to the best conductor near it. Cattle often seek shelter under trees during a thunderstorm and are frequently killed by strikes.

When inside avoid the following
  • Use of the telephone (cellular and cordless telephone use is safe)
  • taking a shower or bath
  • washing your hands
  • doing dishes

(basically anything to do with water)

  • any contact with conductive surfaces with exposure to the outside such as metal door or window frames, electrical wiring, telephone wiring, cable TV wiring, plumbing, etc.
  • using electrical appliances that plug into the wall
  • being near external windows and doors in general
  • using a radio or TV receiver with an antenna outside the house
Quick first aid vital

Many apparently lifeless victims, especially those who received a side flash, or only a portion of the full discharge through their bodies, may have suffered cardiac arrest but surprisingly little other damage. Quick administration of cardiopulmonary resuscitation (CPR) may revive them and save their lives. First responders to places where there are multiple victims are taught to practice reverse triage. Instead of helping first those who appear the most salvageable, they try to resuscitate the unconscious victims first, since those who survived the initial hit usually survive on their own. It is important to note that lightning strike victims carry no electrical charge as a result, and it is therefore safe to handle them immediately, although it should be considered if there is a significant risk of first aiders becoming victims to a subsequent lightning bolt.

Spectacular lightning damage

There is sometimes spectacular and unconventional lightning damage. One such example is the destruction of the basement insulator of the 250 metre high central mast of longwave transmitter Orlunda, which led to its collapse.

Lightning in contemporary culture

In movies and comics of the contemporary U.S. and many other countries, lightning is often employed as an ominous, dramatic sign. It may herald a waking of a great evil or emergence of a crisis. Various novels and role playing games with fantasy tint involves wizardry of lightning bolt, weapon embodying the power of lightning, etc. The comic book character Billy Batson changed into the superhero Captain Marvel by saying the word, "Shazam!" which called down a bolt of magic lightning to make the change.

Lightning in heraldry

The bolt of lightning in heraldry is distinguished from the thunderbolt and is shown as a zigzag with nonpointed ends. It is also distinguished from the "fork of lightning".

Trivia

  • The United States is home to "Lightning Alley", a group of states in the American Southeast that collectively see a large number of lightning strikes per year. The most notable state in Lightning Alley is Florida.
  • The saying "lightning never strikes twice in the same place" is frequently disproven. The Empire State Building is struck by lightning on average 25 times each year, and was once struck 15 times in 15 minutes.
  • Some repeat lightning strike victims claim that lightning can choose its target, although this theory is entirely disregarded by the scientific community.
  • Although commonly associated with thunderstorms, lightning strikes can occur on any day, even in the absence of clouds.

See also

Reference

External links

Wikimedia Commons has media related to Lightning.

Jets, sprites & elves

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