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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 of a crude type of Faraday Cage.
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Incidence of strikes
Nearly 2000 people per year in the world are injured by lightning strikes. In the USA between 9-10% of those struck die,1 amounting to an average of 75 fatalities annually.2 In the United States, it is the #2 weather killer (second only to floods), killing 100 annually and injuring ten times that number.3 The odds of an average person living in the USA being struck by lightning in a given year is 1:700,000. Roy Sullivan has the record for being the human who has been struck by lightning the most times. Working as a park ranger, Roy was struck seven times over the course of his 35 year career. He lost a nail on his big toe, and suffered multiple injuries to the rest of his body.4
Path to victim
Lightning can incapacitate humans in four different ways:
- Direct strike
- 'Splash' from nearby objects struck
- Ground strike near victim causing a difference of potential in the ground itself (due to resistance to current in the Earth), amounting to several thousand volts per foot, depending upon the composition of the earth that makes up the ground at that location (sand being a fair insulator and wet, salty and spongy earth being more conductive).
- EMP or electromagnetic pulse from close strikes - especially during positive lightning discharges
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. 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.5
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.
Damage
Telephones, modems and other devices can be damaged by lightning, as harmful overcurrent can reach them through the phone jack or electricity outlet.6
Safety mechanisms
Lightning rods
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.
Predicting strikes
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"7 and 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.8 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.
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 miles). 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.
Personal lightning safety
The US National Lightning Safety Institute 9 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.
See also
References
- ^ Cherington, J. et al. 1999: Closing the Gap on the Actual Numbers of Lightning Casualties and Deaths. Preprints, 11th Conf. on Applied Climatology, 379-80.[1].
- ^ Mythbusters ("Son of a Gun"). The Discovery Channel.
- ^ http://www.lightningsafety.noaa.gov/resources/Ltg%20Safety-Facts.pdf OSAA Lightning Safety Facts
- ^ http://www.guinnessworldrecords.com/content_pages/record.asp?recordid=48497 Roy's record at Guinness.
- ^ Vastag B (2007). "fryPod: Lightning strikes iPod users". Science News 172 (3).
- ^ "Summer tips for telecom users". blog.anta.net. 2008-06-17. ISSN 1797-1993, http://blog.anta.net/2008/06/17/summer-tips-for-telecom-users/. Retrieved on 18 June 2008.
- ^ NWS Pueblo Lightning Page - Bolts From The Blue
- ^ Detection of lightning - US Patent 7254484
- ^ Personal Lightning Safety Tips National Lightning Safety Institute . Accessed July 2008
External links
Wikipedia content modification information:
- This page was last modified on 11 November 2008, at 12:02.
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