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THE NATURE OF LIGHTNING A brief summary you are welcome to bring home for your young'uns Lightning is electricity created as electrons are shifted in the atmosphere by many different processes. Electricity is a movement of electrons, those nearly-weightless specks orbiting atoms and often zipping around loose. Substances that conduct electrons (metals like copper and aluminum for example) will allow them to flow with little trouble and strong imbalances of charge will not occur. If the charge has to blast through non-conducting or resistive materials, however, it will heat up the material as it forces its way through (the best advantage it can create is a plasma, the forth state of matter...glowing dissociated nuclei and electrons or vaporized matter). Air is an insulator with high resistance to the movement of electrons (if you remember the chart of elements on your classroom wall, Nitrogen, Oxygen, Argon, are non-metals and non-conductors).This allows a large buildup of electricity. When enough electrons (which repel each other) have been contained by the air and accumulate force sufficient to break down the insulating power of air (about 75000 volts/inch in DRY air) a bundle of electrons will squirt out. This develops a thin strip of plasma which follows every imperfection it can find. It is called a stepped leader and moves in quick crooked steps toward less-charged things, developing a charged pathway. Once the thin leader makes final contact, a lightning bolt ( a bundle of electrons about the size of a five-foot finger) leaps through the pathway. This has the amperes that do the damage. Charges can surge along this easy plasma channel many times, sustaining it, until the charge potential weakens and the episode is over. The brightness of lightning is due to the plasma the air channel becomes, its crookedness the random inequalities of air, and its flicker is due to the repeated surges. Because air resists the passage of charge the explosive heating-expansion of air causes the compressionwave we know as thunder. The soundwave expands at about 5 seconds per mile along the whole miles-long channel of the lightning's crooked path (this, and echoes, explains the long rumbling of a distant stroke). You can estimate the distance of a stroke by counting the interval between flash and thunder...5 seconds for each mile ("one-thousand, two-thousand, three-thousand.." slowly). One cloud churning away up there can generate repeated lightning bolts as it drifts across the landscape. This means that if the flash and crash are separated by less than a few seconds YOU MIGHT Soon BE TOAST unless you become a less attractive target than something else. Lightning selects its final target within only a few yards (Perhaps 50 or less) of the strike. Once the charge strikes it must dissipate, which it will do along the easiest pathway; if the strike is in rather uniform medium, such as a field or water, the concentration weakens evenly with distance. If there are several lower-resistance pathways they might all get some charge, with one getting most of it and the others experiencing "side-flashes". People conduct electricity a lot better than air and can become "innocent bystanders". If the path from cloud to ground has little resistance there is little heat and little need for the charge to side-flash...little damage or danger. Stainless steel is a much less conductive material than aluminum so your shrouds and stays might be less attractive a path than down the aluminum mast to the cabintop, across the air-gap (filled by terrified wife and crew) to the big steel or lead keel immersed in the water. Because the bolt must do its thing within about 100 millionths of a second it will probably side-flash all over the place. If it must force its way through the hull trapped water in voids and thru-hull fittings turns to steam and blows out holes. A tall object provides a "cone of protection" of about 45 degrees..(.remember the row of connected lightning rods on a single building?). Lightning protection systems provide thick copper cables leading to large metal plates underground in the moist earth. Boats provide for large metal surfaces below the waterline to which cables connect, often the propeller and rudders on powerboats are adequate. Saltwater is much more conductive than freshwater because of the ions in it; perhaps a square foot or so of surface will suffice. Much more is necessary, perhaps over three times as much, in freshwater. Metal boats have it made but don't be the tallest thing on it! Lightning hazards are, in order of severity: (1) direct strike: having the bolt hit you and travel through, or over the surface of, your body. (2) side flash, having charges jump to you on their way elsewhere. (3) step potential, charges flowing outward from impact point flowing through you. (4) touch potential, charges caused by touching any metal that is charged during the strike. YOU WON'T STOP LIGHTNING. Your goal is merely to direct its charge to where it can travel easily (cooly) and ground out to earth (water) away from where you and crew will be. You should check over the layout of your boat to see where lightning might most likely go and set up preferred entry, traverse, and exit routes for lightning so that you and your crew can be safe as possible. While you're at it you might consider how to protect your electronics and stuff (but thats what insurance is for). Lightning trivia (?) Electricity measurements: A volt is electrical push, amp is flow, and watt is the combination, or power. Like a waterfall: volt is the height, amp is the size of the river, and watt is the pounding you'll get under it. There are perhaps 5 intracloud discharges for every cloud-to-ground strike (in the US). Ground-to-cloud lightning is much less common, but generation is in the cloud Thunder can be heard only 11 to 15 miles away. Peak currents are generally 10,000 to 20,000 amps but can be up to hundreds of thousands of amps.( Typical household circuits carry 15 amps at 110 volts). Each cloud-to-ground flash involves between 100 million and a billion volts or about 1 to 10 billion watt-seconds.This would run a 100 watt bulb for only a few months, however, because most of the energy is converted to heat, light, and thunder. The temperature of the air-plasma (lightning) is 15,000 to 60,000degrees F and is bluish-purple because that is the color that the atmospheric gases make. (like the red of neon signs). Lightning rods do not attract lightning beyond a few feet, they are intended to provide attachment points for charges to enter the system of cables and ground plates for a bolt that was going to hit anyway. Lightning deaths and injuries statistics: (1950-1969 old data but representative enough) Activity at time of strike killed injured In, on, or near open water 200 177 golfing 95 164 camping or picknicking 36 188 Note that boaters are top of the list! Also note that most people hit survive, especially if CPR is applied when necessary. The water catagory shows many don't receive this. A quarter-acre of flat land can be expected to be struck every hundred years, put a 50 foot structure on it and figure every 4 to 6 years. A 1000 ft tower is struck by lightning about 10 times per year. (So much for "lightning never strikes twice...") 4 to 20% of moored boats in Florida get hit every year. One in Sarasota has been hit 5 times. Palatka can expect 90 days per year when thunder can be heard, we are at the edge of the greatest intensity zone in the US Resources include: Becker, William Boating-Lightning Protection National Ag Safety Database http://www.cdc.gov/niosh/docs/as04800.html Huck, Michael V. Lightning and Boats Seaworthy Productions 1995 Sitarz, Walter Boating Safety--Thunderstorms (MAP-5) Florida Sea Grant College Program, U. of FL. Gainsville FL 32605 Standards and Recommended Practices for Small Craft. Standard E-4 Lightning Protection. American Boat and Yacht Council, PO Box806 Amityville, NY 11701 Thomson, Ewen M. Lightning and Sailboats Florida Sea Grant College Program, U. of Florida, Bldg.803, U. of FL. Gainsville FL 32611http://www.thomsen.ece.ufl.edu./lightning/SGEB17.html Uman, Martin All about Lightning Dover publications, New York 1986 (an excellent source for detailed values in laymans terms). Websites: http://www.lightningsafety.noaa.gov/ http://www.boltlightningprotection.com https://www.patrick.af.mil/45og/45ws/lightningsafety the "s" is important http://www.trailersailor.com and search archives on lightning...lots of stories
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