From Wikipedia, the free encyclopedia - View original article
A lightning strike is an electric discharge on a massive scale between the atmosphere and an earth-bound object. They mostly originate on the ground and terminate in the thundercloud, called ground to cloud (GC) lightning. Upward propagating lightning is initiated from a tall grounded object and reaches into the clouds. About 25% of all lightning events worldwide are ground to cloud. The bulk of lightning events are intracloud (IC) or cloud to cloud (CC), where discharges only occur high in the atmosphere. Some lightning strikes go from cloud to ground (CG).
A single lightning event is a "flash", which is a complex, multi-stage process, some parts of which are not fully understood. Most ground to cloud flashes only "strike" one physical location, referred to as a "termination". The primary conducting channel, the bright coursing light that may be seen and is called a "strike", is only about one inch in diameter, but because of its extreme brilliance, it often looks much larger to the human eye and in photographs. Lightning discharges are typically miles long, but certain types of horizontal discharges can be upwards of tens of miles in length. The entire flash lasts only a fraction of a second. Most of the early formative and propagation stages are much dimmer and not visible to the human eye.
Thunder is always produced by lightning, but very distant lightning may be seen but not heard. Lightning cannot happen in a vacuum devoid of ions, nor can thunder occur without molecules to vibrate.
Lightning strikes can injure humans in several different ways:
Lightning strikes can produce severe injuries, and have a mortality rate of between 10% and 30%, with up to 80% of survivors sustaining long-term injuries. These severe injuries are not usually caused by thermal burns, since the current is too brief to greatly heat up tissues; instead, nerves and muscles may be directly damaged by the high voltage producing holes in their cell membranes, a process called electroporation.
In a direct strike, the electrical currents in the flash channel pass directly through the victim. The relatively high voltage drop around poorer electrical conductors (such as a human being), causes the surrounding air to ionize and break down, and the external flashover diverts most of the main discharge current so that it passes "around" the body, reducing injury.
Metallic objects in contact with the skin may "concentrate" the lightning's energy, given it is a better natural conductor and the preferred pathway, resulting in more serious injuries, such as burns from molten or evaporating metal. At least two cases have been reported where a strike victim wearing an iPod suffered more serious injuries as a result.
However, during a flash, the transient stroke current flowing through the channel and around the body will generate large electromagnetic fields and EMPs, which may induce electrical transients (surges) within the nervous system or pacemaker of the heart, upsetting normal operations. This effect might explain the cases where cardiac arrest or seizures followed a lightning strike that produced no external injuries. It may also point to the victim not being directly struck at all, but being very close to the strike termination.
Another effect of lightning on bystanders is to their hearing. The resulting shock wave of thunder can damage the ears. Also, electrical interference to telephones or headphones may result in damaging acoustic noise.
According to the NOAA, over the last 20 years, the United States averaged 51 annual lightning strike fatalities, placing it in the second position, just behind floods for deadly weather. In the US, between 9% and 10% of those struck die, for an average of 40 to 50 deaths per year (28 in 2008). The chance of an average person living in the US being struck by lightning in a given year is estimated at 1 in 500,000, while the chance of being struck by lightning in a lifetime is 1 in 6250 (estimated lifespan of 80 years).
These statistics do not reflect the difference between direct strikes, where the victim was part of the lightning pathway; indirect effects of being close to the termination point, like ground currents; and resultant, where the casualty arose from subsequent events, such as fires or explosions. Even the most knowledgeable first responders may not recognize a lightning related injury, let alone particulars, which a medical examiner, police investigator or on the rare occasion a trained lightning expert may have difficulty identifying to record accurately. This ignores the reality that lightning, as the first event, may assume responsibility for the overall and resulting accident.
Direct strike casualties could be much lower than reported numbers.
Trees are frequent conductors of lightning to the ground. Since sap is a relatively 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.
In sparsely populated areas such as the Russian Far East and Siberia, lightning strikes are one of the major causes of forest fires. The smoke and mist expelled by a forest fire can cause electric charges, multiplying the intensity of a forest fire.
The two most frequently struck tree types are the oak and the elm. Pine trees are also quite often hit by lightning. Unlike the oak, which has a relatively shallow root structure, pine trees have a deep central tap root system that goes down into the water table. Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to pines being targeted are a high resin content, loftiness, and their sharp needles which lend themselves to a high electrical discharge during a thunderstorm.
Trees are natural lightning conductors and are known to provide protection against lightning damage to nearby buildings by diverting lightning strikes away from structures. Tall trees with high biomass for the root system provide good lightning protection. An example is the teak tree (Tectona grandis). When planted near a building, its height helps to capture the oncoming lightning leader, and the high biomass of the root system helps in dissipation of the lightning's charge.
Telephones, modems, computers and other electronic devices can be damaged by lightning, as harmful overcurrent can reach them through the phone jack, Ethernet cable, or electricity outlet. Close strikes can also generate electromagnetic pulses (EMPs) – especially during "positive" lightning discharges.
Lightning currents have a very fast rise time, on the order of 40 kA per microsecond. Hence, conductors of such currents exhibit marked skin effect, causing most of the currents to flow through the outer surface of the conductor.
In addition to electrical wiring damage, the other types of possible damage to consider include structural, fire, and property damage.
The field of lightning protection systems is an enormous industry world-wide due to the impacts lightning can have on the constructs and activities of man. Lightning, as varied in properties measured across orders of magnitude as it is, can cause direct effects or have secondary impacts; lead to the complete destruction of a facility or process or simply cause the failure of a remote electronic sensor; it can result in outdoor activities being halted for safety concerns to employees as a thunderstorm nears an area and until it has sufficiently passed; it can ignite volatile commodities stored in large quantities or interfere with the normal operation of a piece of equipment at critical periods of time. The impacts of a lightning event are as varied and far reaching as the nearly infinite products and systems devised to mitigate the effects of lightning on our lives.
Most lightning protection devices and systems protect physical structures on the earth, aircraft in flight being the notable exception, however some attention has been paid to attempting to control lightning in the atmosphere, however all the attempts proved extremely limited in success. Chaff and silver iodide crystal concepts were devised to deal directly with the cloud cells and were dispensed directly into the clouds from an overflying aircraft. The chaff was devised to deal with the electrical manifestations of the storm from within, while the silver iodide salting technique was devised to deal with the mechanical forces of the storm.
Hundreds of devices, including lightning rods and charge transfer systems, are used to mitigate lightning damage and influence the path of a lightning flash.
A lightning rod (or lightning protector) is a metal strip or rod connected to earth through conductors and a grounding system, used to provide a preferred pathway to ground if lightning terminates on a structure. The class of these products are often called a "finial" or "air terminal". A lightning rod or "Franklin rod" in honor of its famous inventor, Benjamin Franklin, is simply a metal rod, and without being connected to the lightning protection system, as was sometimes the case in the old days, will provide no added protection to a structure. Other names include "lightning conductor", "arrester", "surgitator agitator", and "discharger"; however, over the years these names have been incorporated into other products or industries with a stake in lightning protection. Lightning arrester, for example, often refers to fused links that explode when a strike occurs to a high voltage overhead power line to protect the more expensive transformers down the line by opening the circuit. In reality, it was an early form of a heavy duty surge protection device (SPD). Modern arresters, constructed with metal oxides, are capable of safely shunting abnormally high voltage surges to ground while preventing normal system voltages from being shorted to ground.
The exact location of a lightning strike or when it will occur is still impossible to predict. However, products and systems have been designed of varying complexities to alert people as the probability of a strike increases above a set level determined by a risk assessment for the location's conditions and circumstances. One significant improvement has been in the area of detection of flashes through both ground and satellite-based observation devices. The strikes and atmospheric flashes are not predicted, however the level of detail recorded by these technologies has vastly improved in the past 20 years.
Although commonly associated with thunderstorms at close range, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is known as "A Bolt From the Blue"; lightning can strike up to 10 miles from a cloud.
Lightning interferes with AM (amplitude modulation) radio signals much more than FM (frequency modulation) signals, providing an easy way to gauge local lightning strike intensity. To do so, one should tune a standard AM medium wave receiver to a frequency with no transmitting stations, and listen for crackles amongst the static. Stronger or nearby lightning strikes will also cause cracking if the receiver is tuned to a station. As lower frequencies propagate further along the ground than higher ones, the lower medium wave (MW) band frequencies (in the 500–600 kHz range) can detect lightning strikes at longer distances; if the longwave band (153–279 kHz) is available, using it can increase this range even further.
Lightning detection systems have been developed and may be deployed in locations where lightning strikes present special risks, such as public parks. Such systems are designed to detect the conditions which are believed to favor lightning strikes and provide a warning to those in the vicinity to allow them to take appropriate cover.
The U.S. National Lightning Safety Institute advises everyone to have a plan for their safety when a thunderstorm occurs and to commence it as soon as the first lightning or thunder is observed. This is important as lightning can strike without rain actually falling. If thunder can be heard at all, then there is a risk of lightning. The safest place is inside a building or a vehicle. Risk remains for up to 30 minutes after the last observed lightning or thunder.
The National Lightning Safety Institute recommends using the F-B (flash to boom) method to gauge distance to a lightning strike. The flash of a lightning strike and resulting thunder occur at roughly the same time. But light travels 300,000 kilometers in a second, almost a million times the speed of sound. Sound travels at the slower speed of 344 m/s, so the flash of lightning is seen before thunder is heard. To use the method, count the seconds between the lightning flash and thunder. Divide by three to determine the distance in kilometers, or by five for miles. Immediate precautions against lightning should be taken if the F-B time is 25 seconds or less, that is, if the lightning is closer than 8 km (5.0 mi).
Reports differ regarding what to do if caught outside during a storm. One study shows that prostration is safer than lying down flat when there are no other alternatives. A contrasting report suggested that it did not matter whether a person was standing up, squatting, or lying down when outside during a thunderstorm, because lightning can travel along the ground; this report suggested it was safest to be inside a solid structure or vehicle. In the United States, the average annual death toll from lightning is 51 deaths per year, although there were only 23 deaths in 2013, which was a record low; the riskiest activities include fishing, boating, camping, and golf. A person injured by lightning does not carry an electrical charge, and can be safely handled to apply first aid before emergency services arrive. Lightning can affect the brainstem, which controls breathing.
All events associated or suspected of causing damage are called "lightning incidents" due to four important factors.
As such it is often inconclusive, albeit highly probably a lightning flash was involved, hence categorizing it as a "lightning incident" covers all bases.