Lightning strikes are electrical discharges caused by lightning, typically during thunderstorms.
Humans can be hit by lightning directly when outdoors. Contrary to popular notion, there is no 'safe' location outdoors. People have been struck in sheds and makeshift shelters. However, shelter is possible within an enclosure of conductive material such as an automobile, which is an example[1] of a crude type of Faraday cage.
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An estimated 24,000 people are killed by lightning strikes around the world each year and about 240,000 are injured.[2] In the U.S., between 9 and 10% of those struck die,[3] for an average of 40 to 50 deaths per year (28 in 2008).[4] In the United States, it is the #2 weather killer (second only to floods).[5] The odds of an average person living in the U.S. being struck by lightning in a given year is 1/500,000.
U.S. National Park Ranger Roy Sullivan has the record for being struck by lightning the most times. Sullivan was struck seven times during his 35 year career. He lost the nail on one of his big toes, and suffered multiple injuries to the rest of his body.[6]
Lightning strikes injure humans in several different ways:[7]
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.[7] 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 hit the electrical charge strikes the victim first. 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. Metallic objects in contact with the skin may concentrate the lightning strike, preventing the flashover effect and resulting in more serious injuries. At least two cases have been reported where a lightning strike victim wearing an iPod suffered more serious injuries as a result.[8] However, during a flash the current flowing around the body will generate large magnetic fields, which may induce electrical currents within organs such as the heart. This effect might explain the cases where cardiac arrest followed a lightning strike that produced no external injuries.[7]
Splash hits occur when lightning prefers a victim (with lower resistance) over a nearby object that has more resistance, and strikes the victim on its way to ground. Ground strikes, in which the bolt lands near the victim and is conducted through the victim and his or her connection to the ground (such as through the feet, due to the voltage gradient in the earth, as discussed above), can cause great damage.
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.[9] A secondary effect of lightning on users of telephone equipment can be hearing damage, as the strike may cause bursts of extremely loud noise. Close strikes can also generate electromagnetic pulses (EMPs) – especially during 'positive' lightning discharges.
Trees are frequent conductors of lightning to the ground.[10] 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. In sparsely populated areas such as the Russian Far East and Siberia, lightning strikes are one of the major causes of forest fires.[11] The smoke and mist expelled by a forest fire can cause electric charges, multiplying the intensity of a forest fire.[11] It is commonly thought that a tree standing alone is more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.
The two most frequently struck tree types are the oak and the elm.[12] 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 root system that goes down into the water table.[13] Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to its being targeted are a high resin content, loftiness, and its 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. 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 lightnings charge.[14]
Lightning currents have a very fast risetime, 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 conductor skin.[15]
Lightning strikes on sandy soil can produce fulgurites. These root-shaped tubes of melted and fused sand grains are sometimes called petrified lightning.
Several different types of devices, including lightning rods and electrical charge dissipators, are used to prevent lightning damage and safely redirect lightning strikes.
A lightning rod (or lightning protector) is a metal strip or rod, usually of copper or similar conductive material, used as part of lightning safety to protect tall or isolated structures (such as the roof of a building or the mast of a vessel) from lightning damage. Its formal name is lightning finial or air terminal. Sometimes, the system is informally referred to as a lightning conductor, arrester, or discharger; however, these terms actually refer to lightning protection systems in general or specific components within them. Lightning protection systems alter lightning streamer behavior.
The field of lightning protection is almost totally void of systems or concepts designed to deal with the general problem area as a whole. Chaff and silver iodide crystals 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.
Although commonly associated with close thunderstorms, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is known as "A Bolt From the Blue"[16] and it is because 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.[17] 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 prediction 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 National Lightning Safety Institute recommends using the F-B (flash to boom) method. The flash of a lightning strike and resulting thunder occur at roughly the same time. But light travels at 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 3 to determine the distance in kilometers, or by 5 for miles. All of the precautions above should be taken from the time the F-B is 25 seconds or less, that is, the lightning is closer than 8 km (5.0 mi). Do not rely on the F-B method for determining when to relax the safety measures, because lightning typically occurs in multiple locations, and just because some strikes are far away does not mean another is not close. Precautions should not be relaxed until thunder cannot be heard for 30 minutes.
The US National Lightning Safety Institute[18] 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, since 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.
If a person is injured by lightning they do 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. If a victim appears lifeless, it is important to begin artificial resuscitation immediately to prevent death by suffocation.[19]
