Senin, 19 November 2012

Lightning and diving

Oceans Rarely Hit By Lightning

NASA has shown with satellite imaging that the oceans rarely get hit with lightning. Apparently the surface water does not heat up enough to cause the positive charge needed for lightning to occur. Potentially, lightning is the biggest weather danger for divers. Every year, lightning kills more people in the US than tornadoes or hurricanes. Only floods are more deadly. During the last three decades, floods killed an average of 139 people a year, lightning 87, tornadoes 82 and hurricanes 27 in the US, according to national Weather Service figures.

The question should be "what cautionaries should the scuba diver take?" Should he get out of the water? Is he safer in the water than in the boat? If shore diving - stay in the water or go ashore?

Over the years around ten percent of the lightning deaths in the US have been in or near the water- the statistics don't show how manyvictims were diving or swimming, how many were on boats and how many were on beaches. Lightning is likely to strike the highest thing around, which is why we're told not to take shelter under trees during thunderstorms. If you're in a boat during a thunderstorm, the boat and everyone in it are the highest things around; they're prime targets for lightning bolts. While people on land can take shelter in buildings or vehicles, those aboard a dive boat that's caught by a thunderstorm far from shore have no place to take shelter. Diving underwater may not be an option. Lightning that hits the water could be deadly because its electricity flows through water.

Scientists know little about what happens when lightning hits water. The electrical current probably spreads in all directions, weakening as it spreads out. Since large numbers of dead fish aren't found after thunderstorms move across bodies of water, the current probably weakens in short distances.

Still, a diver who happens to be underwater when a lightning bolt hits nearby could become part of an unintended scientific experiment on just how quickly the current weakens.

I can find nothing about a possible stand off distance, probably since a prediction cannot be made regarding the location of the strike. Assume that it will hit the highest point, however, and the answer would be to get out of the boat or stay in the water if shore diving. We just don't know the answer.

Ron Holle, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. has this to say about lightning and water:

"Large numbers of lightning flashes strike oceans, lakes, rivers, and ponds. If there is nothing protruding higher than a body of water or flat ground, then a flat surface is hit.

The area of a swimming pool is small, so it's not usually directly hit. However, the area affecting a pool is quite large. This area includes the surrounding power and telephone lines, and the plumbing around the pool and inside the bathhouse and other structures. These are usually unsafe places during a thunderstorm because the current from a lightning strike will travel easily through the standing water, showers and other plumbing. Since the pump, lights and other facilities have power lines linked to the plumbing, a hit to any part of a pool complex can affect all of it.

Water does not "attract" lightning. It does, however, conduct current very well. It's not clear how far lightning travels through water. People have been killed or injured by direct or indirect strikes while in or on the water, boats, docks, piers, surf, surfboards, canoes, while fishing, and so on. In most cases, it appears that the strike was within a few tens of yards of the person. But the current can extend farther through plumbing or wiring so the distance of influence can be greater.

In general, there is less lightning per area over water than over land. This is due to the fact that water bodies are usually cooler than land during summer. For this reason thunderstorms are less likely to build or continue to develop over water than over heated land.

The current in a lightning bolt is as high as 30,000 amperes --- about 150 times more than ordinary house current of 200 amps. It's easy to see how this much electricity is deadly. Fortunately, many lightning victims aren't hit directly, if all the charge doesn't go through them. This is why about six out of every seven persons hit by lightning survive, although sometimes with serious injuries and lasting ill effects.

Lightning will often hit something, such as a tree, a wire fence or a boat mast. Most of the electricity flows through the object that it hits, but some jumps to hit a person or people. This is called a 'side flash'. Such side flashes have killed people talking on the telephone or taking a shower inside otherwise safe buildings, following through phone lines or water pipes after hitting the ground. People are also sometimes injures when lightning hits the ground and follows it to where they are standing.

Lightning can affect all parts of the body, but the usual cause of deaths is heart stoppage. The electrical charge disrupts the heart's rhythm, stopping it. Usually, however, the heart will quickly resume beating. The electricity is more likely to paralyze the brain's respiratory center. The victim will die from lack of oxygen unless someone nearby can quickly perform artificial respiration to get the victim's breathing going again. This may have to continue for hours before the victim begins to breath normally.

A Lightning Safety Group, which met at the 1998 American Meteorolgical Society convention to update safety recommendations, noted in it's report: "Generally speaking, if an individual can see lightning and/or hear thunder, he or she is already at risk. Louder or more frequent thunder indicates that lightning activity is approaching, increasing the risk for lightning injury or death."

The report also notes that "many lightning casualties occur in the beginning, as the storm approaches --- Also, many lightning casualties occur after the perceived threat has passed." In fact, the danger can persist as long as thirty minutes after the storm has passed and the last thunder is heard.

Lightning always comes from a thunderstorm cloud, but has been known to hit as far as 20 miles from the nearest cloud, far from the storm's rain. This is why the group says seeing just one lightning bolt or hearing one clap of thunder should be a warning to get into a safe place.

If you're caught in a boat, about all you can do is crouch down in the boat's center and stay as far away from any metal surfaces, and radios or other electrical gear that might be attached to an antenna.

If you see lightning, knowing which way the wind is pushing the clouds will make a big difference. If the wind is pushing the clouds your way, it's time to head for shore. If you see lightning, the flash to bang method can also help determine whether lightning is moving closer. ( sound travels about one mile every five seconds).


If caught at sea in a thunderstorm:

-Stay in the center of the cabin

--Keep arms and legs in the boat - do not dangle in the water.

--Discontinue all water activities

--Disconnect and do not touch any electronic equipment.

--Lower, remove or tie down the radio antenna and other protruding devices if not part of the lightning protection system

--Avoid making contact with any part of the boat connected to the lightning protection system. Avoid touching two components that are connected to the system at the same time, such as the gear lever and spotlight handle.

NOAA has this to say about lightning and water:

-Get out of the water, it's a great conductor of electricity.

-Stay off the beach and out of small boats or canoes.

-If caught in a boat, crouch down in the center away from metal hardware.

-Swimming, wading, snorkeling and scuba diving are NOT safe.

-Lightning can strike the water and travel some distance beneath and away from its point of contact.

-Don't stand in puddles of water.

The US Navy Manual does not address the problem, that I can find.

So-- what to do? If I were diving in a thunderstorm, I'd get out of the water. Before diving? I'd not dive during and for thirty minutes after the storm.

However, we might want to alter our recommendations considering this answer to a question to Science Update posed about lightning and fish kills. This was sent to us by Jeff Wiberg.

"Does lightning fry fish? I'm Bob Hirshon and this is Science Update.

Today's question comes from Matthew Dabney of Longmont, Colorado.


"Why is it that we're directed to get out of water during a lightning storm to avoid electrocution? Do fish get electrocuted when the lightning strikes a lake?"

We asked Don MacGorman, a physicist at the National Severe Storms Laboratory in Norman, Oklahoma. He says that as long as the fish are underwater, they're probably okay.


"Basically lightning stays more on the surface of the water rather than penetrating it. That's because water is a reasonably good conductor, and a good conductor keeps most of the current on the surface."

So, when lightning hits the water, the current zips across the surface in all directions. And if you're swimming anywhere in the vicinity, it'll probably hit you. But below the surface, most of the electricity is instantly neutralized. So the fish are generally spared.

Of course, if the fish happen to be surfacing, they're at risk just like you are. And Dr. MacGorman adds that some electricity does penetrate the water, right at the strike point.


"So fish under a lightning strike can be killed, if it's close enough to the surface. But it has to be much closer than you do on the surface of the water."

Considering this answer - and if you have diving gear and adequate air - the best place for you to be would be underwater (not on the surface swimming).


Links for Further Reading

Lightning Safety, National Weather Service, Melbourne, FL

Lightning Injuries, eMedicine, Mary Ann Cooper, MD

Lightning Injuries, eMedicine, Scott Bjerke, MD FACS

Lightning Injury Research Program
ditayangkan ulang oleh : dr.erick Supondha (hyperbaric&diving medicine Consultant) jakarta, indonesia , hiperbarik oksigen terapi RS. Bethsaida 021 99070050