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Crooked fire

Jan 1, 2003

As one of perhaps four dozen scientists in the world who study lightning, Joseph Borovsky, a researcher at Los Alamos National Laboratory, is happy to admit that lightning isn't well understood. The same admission surfaces in virtually every conversation one has with people who study lightning. This lack of understanding is certainly not from lack of observation, given the long history of human fascination with lightning. Mariners, with masts and antennas poking upward, are justifiably interested in lightning.

Lightning-wielding thunder gods are a common feature of any number of religions (Greek, Indian, Chinese, etc.). The Bible is replete with references of lightning as a divine tool. And the Romans even used the relative direction of lightning as a celestial Ouija board. One poses a question and waits for the next lightning bolt. Lightning to the east bodes favorably, to the west poorly (perhaps because lightning to the west is generally heading one's way).

Ben Franklin established that thunderstorms contain strong electrical charges, but the intervening 250 years haven't produced much more understanding of how lightning moves through the air, how an electric charge begins to flow as lightning, or even how storms gain their potent charge in the first place. Whatever direction it bears, and however it works, what lightning does is remarkable, occasionally frightening, and always fascinating.

The typical lightning flash one sees during a summer thunderstorm begins some 20,000 feet above the ground. As a storm grows and matures, negative electric charges build up on slushy hail stones, and those charges overwhelm the air's electrical resistance. The negative charges group together and ionize a small packet of air within the cloud for a brief period. The charge surges downward some 150 feet in just a millionth of a second (a microsecond), pauses for 50 microseconds, steps downward and pauses again, and so on. Each step has a slightly different course, and a step occasionally breaks into two branches, producing the characteristic jagged, branched shape of lightning. Don't look for the light: this "stepped leader" phase of the flash is almost dark, with only the most recent step producing any light. The negative charge from the cloud is deposited along the walls of the channel created as the stepped leader flows toward the ground. The trip from the clouds to 150 feet above the ground has taken perhaps 200 steps and one hundredth of a second.

As the stepped leader nears the ground, it has a potential of approximately -100,000,000 volts relative to the ground. This potential increases the electric field near the surface so much that streams of positive charges begin to spontaneously emerge from objects on the ground. Tall, conductive, pointed objects (masts and lightning rods, for example) focus the electric field well and are primary sites for streamer formation. These charge streamers reach up toward the descending step leader, and connect with it 150 feet above ground. This connection completes the circuit between cloud and ground.

Emptying the channel

Current immediately begins to flow through the now-completed circuit. This return stroke is what we see (and hear) as lightning. The negative charge deposited on the channel walls by the stepped leader begins to flow horizontally into the channel and then vertically into the ground; although the current flows down, the return stroke moves from the ground up into the cloud. The entire channel is emptied in less than 100 microseconds, with peak currents of 30,000 amps reached in the first five microseconds. The entire length of the channel is heated to tens of thousands of degrees. The hot, ionized air radiates light and expands to form a shock wave. Racing outward in all directions, we hear it as thunder.

With the original charge now dissipated into the Earth, charge congregates anew within the cloud. Thirty thousandths of a second (milliseconds) after the previous return stroke, a dart leader from the cloud will usually flow down the original channel, but without the branches or pauses typical of the initial stepped leader. The dart leader moves rapidly down the channel, again depositing negative charge on walls, again it meets a charge streamer, which again releases a return stroke to move up the channel, and again light and sound flash outward. A typical flash of lightning will have three or four of these leader-return stroke pairs, each separated from the other by 50 to 100 milliseconds. This is just about the time it takes for a human to resolve successive images, and these multiple strokes produce the characteristic flicker of lightning. In some flashes, a continuing current flows down the channel for perhaps 0.1 seconds after a return stroke. This continuing current is weaker, and hence makes a dimmer flash, than the first part of the return stroke.

This account of lightning represents a reasonable consensus of what a cloud-to-ground stroke is like. But cloud-to-ground strokes are only one facet of the full range of lightning styles. One of the hallmarks of a field of science still in its infancy is an emphasis on nomenclature over explanation, and certainly lightning has its share of careful pigeonholing. Fortunately only two aspects of this are essential: where lightning starts and stops, and its polarity. Cloud-to-cloud lightning begins and ends above Earth's surface, while cloud-to-ground strokes are those that end at the Earth. Both types of lightning can have either negative or positive polarity. While these distinctions have been known for decades, only in the last few years have large data sets been available to study them.

The best of this data comes from the National Lightning Detection Network (NLDN), managed by Global Atmospherics, Inc. Because lightning emits an electromagnetic pulse (EMP, the crashing on a radio you hear during a storm), a lightning discharge can be detected from hundreds of miles away. The NLDN is a network of 100 detectors across the U.S. that detect and locate (by triangulation) about 70% of the ground-strikes that occur annually in the U.S. Fans of the Weather Channel may be familiar with the lightning location map produced from this data. The NLDN measures the location, time, polarity, and number of strokes in each of these flashes. The NLDN doesn't measure, and in fact is specifically designed to avoid measuring, lightning within clouds.

That's because cloud-to-cloud lightning is by far the most common type. Depending on the latitude, the time of year, and the individual storm, some half to three-quarters of all lightning occurs within or between clouds. These do not move much charge aroundperhaps 10% that of a cloud-ground flashand despite their higher numbers are a danger only to aircraft and atmospheric chemistry. The less common cloud-ground flashes are better studied, and are more dangerous, because they transfer electric chargelots of itto structures on the ground. Roughly 21 million flashes hit the lower 48 states in 1991; that's an average of one ground hit every half second.

Within the contiguous U.S., the density of cloud-to-ground flashes varies enormously, both by location and through time. The highest density of ground hits (around 25 per square mile) is in the lightning capital of the U.S., Florida. Ground hits decrease with distance from Florida, with portions of New England and the Pacific Northwest registering fewer than 1.5 hits per square mile. Around the globe, the tropics are the primary locus of activity, with equatorial Africa having the highest frequencies. Lightning frequency over water is less well known, due to lack of measurements (The NLDN does cover most coastal waters, but not offshore areas with any accuracy.) Marine thunderstorms are less common and have less lightning then non-marine thunderstorms, and thus one expects a lower frequency of strikes in the open seas.

Researchers have known for decades that cloud-ground lightning can have either negative or positive polarity. In the U.S., and most likely for the world at large, 96% of cloud-ground flashes are negatively charged. They transfer negative charge from a cloud to the ground, in the cycle described in the opening of the article. A typical negative cloud-ground stroke might move one or two hundredths of an amp-hour of charge in each stroke (more on this surprisingly low number below). Negative cloud-ground lightning usually has two, and typically three or four, strokes. About half of the time, subsequent strokes do not hit the surface in the same place, and in fact can hit up to a mile away from the original impact point.

Positive cloud-ground lightning is not simply negative lightning with a sign change. Positive strokes are more rare, more powerful, more damaging, and more spectacular than their negatively charged brethren. The majority of lightning-induced forest fires and damage to power transmission lines are due to these mammoth strokes. Indeed, the changes wrought by a positive flash can be enough to affect the ionosphere some 40 to 50 miles above the storm. Curiously, positive flashes are not branched, and the leader flows, rather than steps, to the ground. Missing too are multiple flashes; only a sixth of positive flashes contain multiple strokes. Most important, positive flashes have much higher currents (and hence charge) than negative strokes. Positive strokes from large thunderstorms can transfer 10 times the charge carried by a negative stroke. Still, though, this amounts to only a tenth of an amp-hour of charge. A natural question: how can so little charge be so damaging?

The answer lies in the rate at which lightning moves these charges around. The total energy (and charge) in a flash is actually pretty small. The typical negative cloud-to-ground flash contains enough energy to run a 100-watt light bulb for 8 months. The damage comes from the rate at which this energy is transferred: all of that 8 months of light-bulb use flows in 0.00003 seconds. For that short time, the power in the channel is 30 times the total power generated in the U.S.all of it focused for one brief time in one tiny place.

Limited accuracy

Controlled laboratory experiments still do not accurately capture all aspects of the discharge, and they only approximate the real-world situation. Theoretical studies, such as those by Los Alamos' Borovsky, still make use of fairly limiting assumptions, which may limit their accuracy. Results of these studies, for example, are checked by comparing them to the highly uncertain studies already done!

Despite this uncertainty, the mechanism by which light and sound are produced in the lightning discharge is well known. All of the light is emitted by gasses within the channel, which act like a combination incandescent and florescent light bulb. Incandescent lights use a hot, metallic filament to produce light. All hot objects radiate this continuum light, and the hotter the temperature the brighter and more blue the light. (A rheostat-equipped light is ideal to demonstrate this: as one increases the current to the light, the filament glows both more brightly and more blue.) The hot gasses within a lightning channel are essentially the filament in an incandescent light bulb, albeit a filament half an inch wide and four miles high.

This continuum radiation provides only a portion of the total, with the remainder provided by emission from atoms ionized by the discharge. Gasses within the lightning channel become strongly ionized by the both the current and the high temperatures, and emit characteristic radiation in response.

Thunder, like lightning, is a side-effect of channel heating. In place of the light bulb, substitute a diesel engine to explain thunder's formation. A diesel engine makes elegant use of a property of all gasses, the atmosphere included. The temperature, pressure, and volume of a parcel of air are related, with the product of pressure and volume proportional to temperature. In Mr. Diesel's invention, decreasing the volume and increasing the pressure of the fuel-air mixture raises its temperature to the flash point. In a lightning channel, the sudden increase in temperature has to be balanced by an increase in pressure or volume. But the volume is more or less fixed. Magnetic fields wind around the channel, constraining it from expansion, and the air itself can't move any faster than the speed of sound. The result is an increase in the air pressure in the channelpressures of 50 atmospheres (700 lbs./sq. in) aren't uncommon. This pressure wave expands outward at the speed of sound. The boom of thunder is the passing of the pressure wave by the listener.

Typical thunder can be heard on the order of 10 miles away, although the actual distance will vary due to local conditions. At distances of more than 1,000 feet, an observer (or listener) can easily judge the distance to the lightning by use of the "five second rule." Sound waves travel at about 1,100 feet per second, or roughly one fifth of a (statute) mile every second. One observes a flash and then counts the seconds until the arrival of the thunder. The lightning is one mile distant for every five seconds of delay.

At short ranges, the nature and quality of the first moments of the thunder roll is the best way for a listener to judge his or her distance from the nearby lightning channel. Different frequencies of sound travel at different speeds, with different efficiencies, through air. Low tones travel more slowly, but much more efficiently, than high tones.

Close to the lightning channel, the pressure wave is a mixture of the complete range of audible tones, all stacked tightly together. The resulting soundunforgettable to anyone who has heard itis a loud, sharp clap that begins with the high notes immediately followed by the lower tones. The sound is loud, intense, and brief, and seems to arrive while the light of the flash is just reaching one's eyes. At greater distances, the air has absorbed the higher notes, and the lower notes, each traveling at a slightly different speed, have spread out a bit, giving the thunder a rounder, lower tone.

The preceding descriptions are just thatdescriptions of what happens, not explanations of why they happen. This omission is necessary, in that surprisingly little progress on why lightning works has been made. Even something as basic as how thunderstorms build up electrical charges, a process called electrification, is still unknown.

"There are 20 or 30 different electrification mechanisms proposed," says Dr. Terry Schurr, a meteorologist at the University of Oklahoma and the National Weather Service's Severe Storm Laboratory. The two leading explanations, each championed by a different research group, "represent the two lab's attempts to explain electrificationwith conflicting results," Schurr adds. Both groups use laboratory experiments to flesh out a long-standing idea: that collisions of ice and wet, slushy hail within the mixed phase region are the ultimate source of charge in thunderstorms. These experiments indicate that neither ice nor water alone will produce an electrically charged environment. Mix the two together, and both can be charged. The important step is the transfer of charge during collision of ice and watery hail.

Moderate amounts of water

The charging process is most efficient in air with moderate amounts of water (liquid and solid) in it. Too much or too little water, and little charging goes on. This is why lightning is rare in both winter storms (too cold and too dry) and in tropical, marine thunderstorms (too wet). Nonetheless, boatsand sailboats in particularare prime candidates for lightning hits. It is difficult to imagine a more efficient mechanism for collecting lightning then a tall, metallic mast towering over the ocean. Masts are perfect structures to nucleate streamers, the positively charged currents that connect to stepped leaders. Lightning is a constant threat to ships, a threat recognized for years. Over the first two decades of the 19th century, 150 ships in the Royal Navy were damaged by lightning. An incredible one in eight ships hit were set ablaze, and some 200 sailors were killed or wounded. One ship was totally destroyed by the ensuing fire.

That threat lives on. Data derived from Boat/U.S.'s extensive insurance archives suggest that the risk of lightning strike is roughly proportional to the height of the highest mast on a vessel. Auxiliary sail vessels have the highest risk, at around 0.6% chance per year of being struck by lightning, while power cruisers have a significantly lower risk, around 0.1% per year. While these risks may seem quite small (they are about half that of dying in an automobile accident), lighting protection systems are a high-probability way of protecting a vessel and its occupants from a potentially fatal strike. Lightning protection for vessels is explained in detail by National Fire Protection Association (NFPA) publication 780, Chapter 7. Available at libraries, and through the NFPA, Chapter 7 is good reading for every mariner. Briefly, every boat owner should consider two aspects of lightning protection: what parts of the boat need to be protected, and how to accomplish that protection. Excellent reviews of lightning protection are found in Ocean Navigator (Issue No. 49) and the January, 1997 issue of Seaworthy.

At the core of the NFPA regulations is one idea: every boat must have the conductive equivalent of a #4 AWG copper conductor leading from a sharply pointed lightning air terminal at the top of the mast to a ground plate on the hull. This simplified statement does not include all of the details, but does indicate the extremely large conductance necessary to protect a boat and its occupants. NFPA 780 lists the requirements, while the articles mentioned above review the methods needed.

Lightning protection systems are designed to protect the crew and the hull from damage in a strike. The NFPA uses a "rolling ball" model of predicting protection zones on a vessel. Imagine rolling a ball with a radius of 100 feet around your vessel. The ball rolls around any part of your vessel it touches, which is part of a properly constructed lightning protection system. Any area under the ball is protected from lightning. This ball model applies to any vesselsloop, schooner, multihulland replaces the "cone of protection" model.

A protective shadow

A stepped leader within one step of a vessel will generally hit the tallest nearby conductor, the mast-top air terminal if it forms the top of the lightning protection system. The rolling ball represents that last stepabout 100 feet in marine settingin a stepped leader. Any area below the ball is protected by the figurative "shadow" cast by the air terminal. This protection zone protects passengers, and perhaps the hull, only. Most lightning protection systems are not designed to protect electronic equipment, which may be only partially protected by even a good system. Internal protection of all electronics is necessary, in addition to a lightning protection system.

Personal protection in a lightning stormmarine or terrestrialessentially involves making oneself as uninviting a conductor as possible. The first step is to avoid being in a storm in the first place. Failing this, the easiest way to do this is to get protected and stay protected from the lightning, by finding cover in a substantial building. The majority of people struck by lightning are hit either before or after the storm, not during it. Find protection early and stay there until long after the danger has apparently passed. If caught out in the open or on a vessel one can still reduce the chances of being hit. On boats, current can arc across open spaces on deck or in the cabin in a side flash, or travel along the always-present film of water on surfaces. Minimize the chance of being hit by moving below deck, to a dry, isolated, and flat surface. Place an insulator (a dry cushion, or a dry mat) on the surface and crouch down on it, with the balls of the feet touching the floor and the ankles pressed together. Avoid contact with the boat or other people, and do not place one's hands on the deck or hull. This posture attempts to limit the conductive path in the body to the feet. Use the five-second rule to judge the distance to the channel, and recall that people have been hit by lightning originating 12 miles away. Getting good protection during a thunderstorm is clearly a good idea. n

Larry McKenna is a navigation instructor and freelance writer.