In our last post, we talked about the way lightning forms in stages, where a high electrical field will cause the formation of an ionization channel that grows in stages from a cloud toward the ground.

This same process occurs in solid materials under the influence of a large, powerful electrical field. Any time you have an object, which can be a piece of wood or acrylic or a plot of grass or even a person, and there's a sudden collapse of a powerful electrical field on or across that object, the collapse will cause the formation of ionization channels that grow in stages, just like the channels that create lightning bolts.

When this happens to a person, it's bad news. When it happens to wood or acrylic, the result is quite beautiful. The ionization channels can leave permanent marks behind; these marks are called Lichtenberg patterns.

This photograph shows a natural Lichtenberg pattern formed in grass by a lightning bolt. When the lightning touches down, the point of contact between the lightning bolt and the ground is a very, very strong electrical field, and this electrical field wants very much to drain away to ground. It does so in a series of branching, fern-like discharge paths that can leave permanent impressions in many materials.

Lichtenberg patterns are named for physicist Georg Christoph Lichtenberg (1742–1799), who discovered in 1777 that if he discharged a Leyden jar, which is essentially a big, primitive capacitor, across an insulating material like glass and then dusted the glass with lead tetroxide, the powder would stick to the areas of the glass that were electrically charged. He'd then press a sheet of paper over the powdered glass, and the powder would form an image on the paper. Effectively, he discovered the Xerox process 161 years before the process was first turned into a photocopier, though he never thought to use it that way.

Anyway, the branching patterns formed on the glass are today called Lichtenberg patterns, and they occur in other forms as well. In fact, people who are struck by lightning will sometimes end up with Lichtenberg patterns burned into their skin (scroll down in the linked page for a picture of a pattern burned into a strike survivor's arm).

These patterns will form three-dimensionally if a high-intensity discharge occurs through a suitable insulating material such as acrylic plastic, with striking results:

These are often sold as artwork. There are a number of processes by which they can be made; two of the most common are to charge a block of acrylic material with something like a Van der Graff generator and then discharge the stored electrical charge by touching the acrylic with a grounded metal spike, or by hitting the acrylic with a DC particle accelerator. You can also put the acrylic in a place where it'll be struck by lightning, though that's a somewhat less reliable method.

In any event, the end result is a discharge of the electrical field through the acrylic. This discharge permanently etches a pattern into the acrylic that shows the way the ion channels formed.

The pattern of the discharge is fractal, all the way down to a molecular level. If you could zoom in on the delicate branches of the pattern, you'd see that even the finest hair-thin line has the same sort of fingers extending from it.

This video shows a Lichtenberg pattern being formed by discharging a highly charged sheet of acrylic with a grounded metal spike:

So, if you're over the age of six or so, you probably know that lightning tends to strike the highest point around. This is why staying under a tree during a thunderstorm is a bad idea; lightning will tend to prefer tall objects, and if you're under the tree when it gets hit, you're likely to be hit by a "splash discharge."

What you might not know, though, is that lightning doesn't always hit the highest thing around, and sometimes, it may hit you even if taller objects are all around you.

In the last post, we mentioned briefly that lightning discharge is a function of the field potential of the area where the lightning bolt happens. In this entry, we'll talk a little bit more about what all that means.

In a nutshell, lightning doesn't work the way you think it does. A single lightning bolt doesn't jump from the clouds to the ground all in one stroke. The formation of a lightning bolt takes place in stages, and lightning follows a jagged path from the cloud to the ground, but the whole process takes place in only a few microseconds, so to the human eye it appears to happen all at once.

The birth of a lightning bolt starts very early in the formation of a thunderhead. Droplets of water, carried by warm rising air, ascend through the cloud; as they do, friction tends to impart an electrical charge on them. Think about happens when you rub a balloon on your clothes; same deal.

As this process continues, the cloud itself generates an enormous electrical charge, on the order of tens of millions of electron volts in potential. An electron volt is a measure of the energy of an electrical field; it's a measure of the amount of energy an electron gains when it travels from a point of low potential energy to a point of higher potential energy. A million electron volts is a lot.

Anyway, the process by which a thunderhead forms results in a cloud with a very strong electrical field. At this point, a number of different things can happen to bleed off that field. It can dissipate into the air, in the form of cloud-to-air lightning strokes; lightning can arc between two clouds, equalizing the potential between them and producing cloud-to-cloud lightning; it can produce discharges between the top and bottom of the cloud, producing "anvil crawler" lightning that climbs up the cloud itself; or it can discharge to the earth in a bolt of lightning between the cloud and the ground.

A bolt of lightning that jumps from cloud to ground begins as a place on the cloud where the electrical field is particularly strong. The field surrounds the cloud, but it isn't uniform everywhere; anything from air currents to the distribution of water droplets within parts of the cloud itself will cause areas where the field is stronger or weaker. If the electrical field exceeds a certain strength, then some really interesting stuff starts happening.

Normally, air is an insulator; electricity can't pass through it. But if an electrical field is powerful enough, an insulator can become a conductor. A powerful electrical field will actually rip the electrons off of their atoms, creating an electrically charged "plasma," a soup of free electrons and positively charged atoms.

If the electrical field in one spot of a cloud becomes strong enough for this to happen, it will create what's called an "ionization channel"–a column of air that's been turned into plasma. This column reaches away from the cloud, looking for an area of lower electrical potential. The ground is an area of lower electrical potential, so this column will often tend to reach toward the ground.

As the column grows, the electrical field drops in strength. Normally, by the time it's reached about fifty meters long or so, the electrical field has dropped to the point where it can no longer ionize the air, so it stops.

However, this ionization channel tends to draw electrical charge away from other parts of the cloud, so it soon picks up enough strength to start growing again. It will begin extending outward once more, still reaching for an area of lower electrical potential, until it's grown another fifty meters or so, where it stops again.

This is why lightning bolts aren't straight; they're jagged. Each part of the lightning bolt is one of these ionization channels, that grows outward and stops. Lightning reaches for the ground in a series of steps, growing outward and then stopping, gathering more electrical potential from the cloud, growing outward and then stopping, rinse and repeat. As the channel reaches toward the ground, it may fork, splitting into multiple different ionization channels that head downward in different directions. Sometimes, some of these channels don't quite make it all the way to the ground.

All this happens fast, and I do mean fast. An ionization channel grows in about a microsecond, then stops for about ten microseconds as the field gathers strength, then grows outward in another microsecond or so. A microsecond is 1/1,000,000 of a second; the lightning bolt happens in far less than the blink of an eye.

When the ionization channel starts to get close top the ground, it begins to attract "streamers," or areas of strong positive electrical charge, from the ground. These streamers grow upward toward the ionization channel from any place on the ground where there's any variation in the electrical field; this means towers, metal objects, the top of your head, small barnyard animals, masts of ships, or any place else where an electrical field can form.

The picture above, from the ROADNet database, shows the process happening; on the right-hand bolt, you can see that the streamer has not quite made contact with the descending channel yet, whereas in the left-hand bolt, the streamer has only just made contact.

These streamers race upward from the ground like blind fingers, drawn toward the descending ionization channel. As soon as a streamer makes contact with the channel, the electrical circuit between the cloud and the ground is complete, and there is an enormous discharge of electrical current from the cloud to the ground. Boom!

Because the streamers rise from multiple points from the ground, it's pretty much a roll of the dice where the lightning bolt will land. Streamers that start from the tops of tall objects are more likely to make contact with the descending ionization channel first–but that isn't always the case. Streamers begin rising from all over the place near where the ionization channel is coming down, and it's merely a question of which one touches the downward-moving ionization channel first.

There are a number of factors which influence how the streamers form. They tend to form from metal or electrically conductive objects more easily than from other objects, and they tend to form from high objects more easily than from low objects, but none of these things is a guarantee of where the lightning will hit. The size and shape of the objects in the vicinity will influence how easily the streamers form and how long they grow, and random variation in the electrical field near the ground will also influence how the streamers form.

In the end, we're talking about probabilities more than anything else. The streamers usually form most easily from tall objects–but not always. The streamers usually meet the downward ionization channel most readily from high objects–but not always. Whichever streamer touches the ionization channel first, wins–and that's not always a streamer forming from the tallest spot around you!

Once the circuit is complete, the discharge doesn't happen all at once. A single lightning bolt can be made of several "flashes" of current; parts of the ionization channel may break up and re-form very quickly, causing the bolt to "walk" across the ground for a few milliseconds. So even if lightning does happen to hit the tallest point in the neighborhood, it won't necessarily stay there. Several large current discharges may happen on one another's heels, so fast that they look like a single lightning bolt–but the point at which the bolt touches the ground may travel many yards, or more, across these separate "flashes." So if you're standing next to the point that got hit, you may be in trouble a few thousandths of a second later. If you're caught near a lightning bolt without warning, better be able to run pretty fast…

File this one under "things you know that just aren't so," right next to "humans only use 10% of their brains" and "science says bumblebees can't fly:"

Lightning never strikes the same place twice.

The fact is, lightning doesn't have a memory. Thunderheads don't keep a record of where they've sent lightning bolts before, and in many cases storm systems may preferentially hit the same spot again and again. The Empire State Building is hit about twenty-five times a year, on average.

Now, granted, the Empire State Building is a special case. It's a tall, grounded structure with a lightning rod on the top, so as far as lightning's concerned, it's the Promised Land.

And speaking of lightning rods, most people know that the lightning rod was invented by Benjamin Franklin, but what's less well-known is that it quickly became a matter of political dispute between King George III and the Republic during the Revolutionary War.

Franklin's original lightning rod design, an example of which is shown here, called for a sharpened iron rod, mounted to the top of a building and connected by a copper cable to the ground. Franklin reasoned that a sharpened lightning rod would more effectively attract a lightning bolt, because it would more efficiently concentrate electrostatic charge at its tip. (This particular lightning rod is one of his originals; it's bent because of a lightning strike, which softened the metal and melted the tip.)

King George III favored designs with rounded balls on the top, in part because Franklin was one of the signers of the Declaration of Independence. In 1776, a powder magazine in London was struck by lightning and burned; the Franklin lightning rod protecting it was quickly blamed, and lightning rods with rounded ends were soon mandated by law in England.

As it turns out, ol' King George was actually right, and modern lightning rods in the US have a rounded top. Lightning doesn't work the way most people think it does. When a lightning stroke travels from a cloud toward the ground, it's met by a riser, or "streamer," that rises from the ground to meet the incoming stroke.

A lightning rod does not prevent lightning from occurring. Instead, a lightning rod provides a convenient focal point for the streamer; the idea is that the lightning stroke will preferentially hit the lightning rod, rather than hitting, say, the top of the house, or the car parked out front, or the neighbor's cat. (Lightning does not always strike the highest point in the area; instead, it strikes the point of highest charge density, where the streamer forms most readily.)

Charge density increases dramatically around a pointed object, so a pointed lightning rod is most effective at initiating a streamer. However, charge density remains uniformly high around a smooth surface, so a lightning rod with a rounded top is more effective at sustaining the streamer for long enough to attract the downward stroke. For this reason, most modern lightning rods, at least in the United States, have rounded ends, not pointed ends. (Some lightning rods try to get the best of both worlds by using a metallic tuft on the end, with lots of sharp bits to create high field potential and a large surface area to help support the riser.)

In any event, lightning can and does hit the same place twice. The only thing a lightning stroke cares about is the field potential of the area where it hits, and an area that's appealing enough for lightning to strike once will often be struck again. In fact, you can even predict the number of times lightning will strike in a given area, based on the part of the country you're in.

If you're a dedicated storm chaser, or even a person who's interested in lightning, you probably know that there are three kinds of lightning: cloud-to-cloud, cloud-to-ground, and in-cloud lightning. Three states of matter, three kinds of lightning, three dimensions, three stooges, three strikes and you're out, everything comes in three, right?

Well, no.

Leaving aside for the moment that there were six stooges (Moe, Larry, Curly Howard, Curly Joe, Shemp, and Joe Besser) and seven states of matter (Einstein-Bose condensate, superfluid, solid, glassy solid, liquid, gas, and plasma), some very unusual types of lightning have recently been discovered in the atmosphere above storm clouds.

Two forms of lightning discharge that occur above thunderstorms are red sprites and blue jets. These types of discharges are still poorly understood; red sprites were only discovered in 1989, and blue jets were discovered even more recently than that. These types of lightning discharges are very large (red sprites can extend 80 km or more above a thunderstorm), and are very dim, difficult to see without specialized equipment.

This is an image of a red sprite captured on video by NASA. Red sprites are complex structures that occur high above a thundercloud, usually within milliseconds of a cloud-to-ground lightning stroke. They are predominantly red, and can reach 95 km into the upper atmosphere. Sprites usually appear in clusters; each cluster is a large, reddish structure with many long blue or red tendrils reaching below. A single red sprite can pack a whopping 50 gigawatts(!) of power.

Blue jets are brief upward flashes of lightning that form a jet or fountain from the top of a thunder cloud into the upper atmosphere. Blue jets occur independently of cloud-to-ground lightning, and shoot up as far as 80 km. These were first recorded in 1994, and are extraordinarily difficult to capture images of. They're also quite short-lived; the discharges begin at cloud level and propagate upward at about 300 times the speed of sound, fanning outward as they go.

Below is a rendered animation of blue jets and red sprites, which will give you a sense of the size and scale of these lightning phenomena:

If you know anything about lightning, you probably know that you're safe from lightning inside a car–at least as long as you aren't touching anything metal.

A lot of people believe that the car's rubber tires act as an insulator, and that this is the reason a car is safe during a lightning storm. This is a common myth; the truth is actually a little more complex.

Air is an insulator, which is why you can walk by a power outlet in the wall without getting shocked. (If air conducted electricity, we'd all be in a lot of trouble!) A lightning bolt can reach eight miles from the leading edge of a storm and hit someone standing under clear, sunny sky without a cloud in sight; if eight miles of an insulator like air won't stop a lightning bolt, a few inches of an insulator like rubber certainly won't. For this reason, you are not safe from lightning by wearing rubber-soled shoes, another common (and dangerous) myth about lightning.

The reason a car protects you from lightning is that the metal skin of the car acts like a Faraday cage. We briefly talked about Faraday cages in an earlier post, so now we're going to talk a little bit more about them.

Put most simply, a Faraday cage is any enclosure that's made of a conducting material such as metal. Electrical fields are conducted around the outside of a Faraday cage without penetrating the enclosure. Faraday cages can be made of metal mesh, or any other conducting material, and are very effective at preventing electrical and electromagnetic fields from penetrating them.

If you own a microwave oven, the inside of the oven is a Faraday cage designed to prevent microwave-frequency electromagnetic signals from escaping; that's why the window in the door has a metal mesh over it. In Japan, some movie theaters are paneled with wood panelling that has a fine copper mesh inlaid in the back; the copper mesh acts as a Faraday cage to block radio-frequency signals used by cellular telephones.

Faraday cages are named after physicist and chemist Michael Faraday (1791-1867), who was one of the principle discoverers of electromagnetism and who invented the first working electric motor. Faraday proposed that a conductive shell might block electromagnetic signals and protect the objects inside it from strong electrical fields; he was the first scientist to propose that electromagnetic fields extend from electrical conductors, and developed the forerunners of modern electrical transformers and generators. He was also the first scientist to propose that light and magnetism are related.

A car's outer surface is made of sheet metal; for that reason, a car acts as a crude Faraday cage, protecting its occupants from lightning by conducting the electrical discharge around its outer surface, away from the interior. Cars are not terribly efficient Faraday cages; the large glass windows offer a way for radio-frequency signals to enter the car, which is why your cell phone still works when you're inside. But for a huge electrical surge on the order of a lightning bolt, which can carry 30 kA or more of electrical current, a car works just fine.

Today, Faraday cages are so ubiquitous that there is even specialized clothing used in industrial applications that's designed to protect its wearer from high-strength electromagnetic fields. Workers on high-tension lines use this clothing to protect themselves from electrocution, as this interesting video shows:

There are a number of different approaches used in lightning warning systems. Most lightning safety systems fall into one of two broad categories: lightning prediction and lightning detection.

Lightning Prediction

The older technique, lightning prediction, attempts to anticipate lightning strokes by using an antenna designed to sense sudden collapse in electrical field density in an area. This approach is used in devices that are designed to predict the occurrence of lightning strokes within the protected area, most typically within a given radius around the sensing antenna.

One such prediction system, called ThorGuard, uses an antenna like the one pictured above to monitor electromagnetic field density and attempt to predict lightning by monitoring changes in the field density. While the company that manufacturers this system refuses to supply statistics about the accuracy of these predictions, the system is claimed to be able to predict lightning within a five-mile radius.

The advantage of a lightning prediction system is that it can be effective before the first stroke of lightning occurs. The disadvantage of such systems is their cost, the fact that the systems tend to be cumbersome, and the fact that they do not tract active thunderstorms. Maintenance of such systems tends to be nontrivial as well.

Lightning Detection

The second approach to lightning warning is lightning detection. This approach relies on systems that actually detect the unique electromagnetic signal generated by a stroke of lightning.

Lightning detection is relatively straightforward. A lightning stroke produces a characteristic burst of radio-frequency noise, centered at around 10 kH or so. Such systems are extremely good at responding to a lightning stroke; in fact, the NOAA uses a network of detectors like this to plot every lightning hit in the United States. Aircraft sometimes have built-in storm detection systems to alert the pilot of thunderstorm activity in his path.

Lightning safety systems that work by lightning detection have not been around as long as the lightning prediction systems. Early models of lightning detection safety devices could detect lightning strokes and determine the approximate range to the lightning strokes, but were not able to track storms or provide information about storm movement.

Modern lightning detection systems like the one shown here, a ThunderBolt, can detect lightning strokes and calculate the distance to the stroke, and also contain computer programs that can track motion of the storm relative to the owner's location and provide information about the storm's motion and estimated time of arrival.

These systems use a small coil antenna (visible on the bottom of the upper shell on the left side of this image) which feed a signal to a sensitive analog amplifier. The output of the amplifier is digitized, then processed via fast Fourier transform to look for the characteristic signature of lightning. The distance to the lightning strokes can be deduced from the strength of the signature and other data, and the computer can monitor the average distance to the leading strokes created by a thunderstorm to determine the direction and speed of the storm's motion.

The advantages of lightning detection systems are that they are easily portable, they do not rely on external antennas so they can be used anywhere, they can locate storm activity and provide accurate information about the storm's distance and rate of travel, they can provide information about the storm severity, and they can provide specific information about the time for the storm to become a danger to the user.

The disadvantage of storm detection systems is that they do not provide warnings before the very first stroke of lightning. Storm activity can form directly overhead, and a storm detection system will not provide warning until the storm has produced at least one bolt of lightning.

If you fly commercially often, the odds are that sooner or later you'll be in a plane that gets hit by lightning in the air. Boeing claims that passenger aircraft are struck by lightning, on average, about twice a year.

Sounds scary, but normally it's not a big deal. Most aircraft are made of aluminum, and the metal body acts like a Faraday cage, drawing the current harmlessly around the skin of the aircraft and keeping it away from the interior (the mechanical and electrical systems and, more importantly, the passengers).

Lightning does occasionally damage or destroy aircraft. Typically, small or general aviation planes aren't as well-protected as passenger jets, and most cases of lightning-related crashes, rare as they are, involve small planes.

There are exceptions; the National Lightning Safety Institute reports one case of a lightning-related crash of a Boeing 747 operated by the Iranian Air Force. According to the report:

The Boeing was operated on a military logistic flight from Tehran to McGuire AFB via Madrid. [...] At 14.30 the crew advised Madrid that they were diverting to the left because of thunderstorm activity, and at 14.32 Madrid cleared ULF48 to 5000ft and directed him to contact Madrid approach control. At 14.33 the crew contacted approach control and advised them that there was too much weather activity ahead and requested to be vectored around it. Last radio contact was when ULF48 acknowledged the 260deg heading instructions and informed Madrid that they were descending to 5000ft. The aircraft was later found to have crashed in farmland at 3000ft msl following left wing separation. It appeared that the aircraft had been struck by lightning, entering a forward part of the aircraft and exiting from a static discharger on the left wingtip. The lightning current's conductive path to the static discharger at the tip was through a bond strap along the trailing edge. Concentration of current at the riveted joint between this bond strap and a wing rib were sufficient conductive to cause the flash to reattach to this rivet and to leave the discharger. Fuel vapors in the no.1 fuel tank then ignited. The explosion caused the upper wing skin panel to separate, causing a drastic altering of the aeroelastic properties of the wing, and especially the outboard section of wing. The outer wing began to oscillate, developing loads which caused the high-frequency antenna and outer tip to separate. The whole wing failed a little later.

Accidents of this type are very, very rare, and commercial passenger planes are safe places to be during a lightning storm.

However…

Modern passenger aircraft, most notably the new Boeing 787 Dreamliner, have skins made of composite material rather than aluminum. This composite material does not conduct electricity, which means that there is no discharge path around the outside surface of the aircraft.

Boeing has reportedly been struggling with the lightning safety systems on the Dreamliner, which have presented a more difficult engineering challenge than the engineers anticipated. The lightning safety system on the Dreamliner has added cost and weight to the aircraft, and also raised accusations by a former Boeing engineer that the lightning suppression systems are inadequate and insufficiently tested.

Detecting lightning remotely is easy. A lightning stroke produces a characteristic electromagnetic signal that's simple to pick up; there are plans on the Internet for simple homebrew lightning detectors.

But determining distance to a lightning stroke is a little more tricky.

Lower Strength (10kA or less) lighting events are more frequently statistically. However, since this includes the common strength levels of cloud-to cloud- strokes, it is not a good frequency for establishing a consistent body of information for determining the distance to storm activity. Because of their orientation, cloud-to-cloud strokes require analysis of polarization effects in order to calculate a distance. In most cases this is very difficult to do.

The statistical avg. stroke amperage is tied to latitude, since stroke amperage is a function of the height of the freeze-line in the atmosphere. The lower the freeze-line altitude, the higher the avg. amplitude of the lightning stroke. In the U.S. for instance, the avg strength of a cloud-to-ground stroke rises in amperage as you move north, but the total number of strokes declines.

When you factor in this effect, and remove the effects of the lower strength cloud-to-cloud strokes from the statistics, you arrive at the value of 30kA as the best “average” signal strength. It probably wouldn’t be the exact average number for a particular storm if you are just counting events, but it is the best amperage to average to determine the most accurate storm distance information.