Have you seen this graphic on any radar image for a region you were occupying before?

If so, there was a very good chance it was accompanied by extreme winds, extensive lightning, and generous amounts of precipitation.

A squall line is an organized line of thunderstorms. It is classified as a multi-cell cluster, meaning a thunderstorm complex comprising many individual updrafts. They are also called multi-cell lines. Squalls are sometimes associated with hurricanes or other cyclones, but they can also occur independently. Most commonly, independent squalls occur along front lines, and may contain heavy precipitation, hail, frequent lightning, dangerous straight line winds, and possibly funnel clouds, tornadoes and waterspouts. Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical wind shear from an angle behind the frontal boundary. The strong winds at the surface are usually a reflection of dry air intruding into the line of storms, which when saturated, falls quickly to ground level due to its much higher density before it spreads out downwind.
The main driving force behind squall line creation is attributed to the process of in-filling of multiple thunderstorms and/or a single area of thunderstorms expanding outward within the leading space of an advancing cold front.
The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of an updraft, rising from ground level to the highest extend of the troposhere, condensing water and building a dark, ominous clouds to one with a noticeable overshooting top and anvil (thanks to synoptic scale winds). Because of the chaotic nature of updrafts and downdrafts, pressure perturbations are important.
Pressure perturbations within an extent of a thunderstorm are noteworthy. With buoyancy rapid within the lower and mid-levels of a mature thunderstorm, one might believe that low pressure dominates in the mesoscale environment. However, this is not the case. With downdrafts ushering colder air from mid-levels, hitting ground and propagating away in all directions, high pressure is to be found widely at surface levels, usually indicative of strong (potentially damaging winds).
Following the initial passage of a squall line, light to moderate stratiform precipitation is also common. A Bow echo is frequently seen on the northern and southern most reaches of squall line thunderstorms (via satellite imagery. This is where the northern and southern ends curl backwards towards the middle portions of the squall line, making a "bow" shape. Bow echoes are frequently featured within supercell mesoscale systems.

Wind shear is an important aspect to measuring the potential of squall line severity and duration. In low to medium shear environments, mature thunderstorms will contribute modest amounts of downdrafts, enough to turn will aid in create a leading edge lifting mechanism – the gust front. In high shear environments created by opposing low level jet winds and synoptic winds, updrafts and consequential downdrafts can be much more intense (common in supercell mesocyclones). The cold air outflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.
As thunderstorms fill into a distinct line, strong leading-edge updrafts – occasionally visible to a ground observer in the form of a shelf cloud, appear as an ominous sign of potential severe weather.
Beyond the strong winds because of updraft/downdraft behavior, heavy rain (and hail) is another sign of a squall line. In the winter, squall lines can occur albeit less frequently – bringing heavy snow and/or thunder and lightning – usually over inland lakes (i.e. Great Lakes region)
Shelf clouds and roll clouds are usually seen above the leading edge of a squall, also known as a thunderstorm's gust front. From the time these low cloud features appear in the sky, one can expect a sudden increase in the wind in less than 15 minutes.
Tropical cyclones normally have squalls coincident with spiral bands of greater curvature than many mid-latitude systems due to their smaller size. These squalls can harbor waterspouts and tornadoes due to the significant vertical wind shear which exists in the vicinity of a tropical cyclone's outer bands.
From Wikipedia, the free encyclopedia Article titled: Squall
References
1.The Weather Channel. Weather Glossary: S.
Retrieved on 2006-11-19.
2.Australian Bureau of Meteorology. Weather Words.
Retrieved on 2006-11-19.
3.squall: Definition and Much More from Answers.com
Georoots News. Georoots News V.1#5: Changes in the Wind.
Retrieved on 2006-12-30.
4.Oxford English Dictionary. Oxford University Press. 10Rev Ed edition (7 April 2005)
5.WGN-TV. Weather Words – B.
Retrieved on 2006-11-19.
6.Wind Names
a b Golden Gate Weather Services. Names of Winds.
Weatherquestions.com. What is a Squall Line?
Retrieved on 2006-11-19.
7.Wilfried Jacobs. EUMeTrain: Case Study on Squall Line.
Retrieved on 2006-11-19.
8.Thinkquest. Meteorology Online: Squall.
Retrieved on 2006-11-19.
9.Robert H. Johns and Jeffry S. Evans. Storm Prediction Center. Derecho Facts.
Retrieved on 2006-11-19.
10.National Weather Service Forecast Office, Springfield, Missouri. Storm Spotter Online Training.ThunderBolt Features an Enhanced Squall line program

A typical stroke of lightning stretches as long as eight miles, and forms when a negatively charged region in a storm cloud begins to send out a stepped leader. This leader is met by another leader rising from the earth, allowing the cloud to discharge to the ground, reducing the negative electrical charge. That's typically how it works…

…but not always.

And when it doesn't work that way, things get interesting.

Occasionally, a severe storm will create a large thunderhead which develops a very strong positive charge in its upper reaches. When this charge becomes strong enough, it can produce what's called a positive bolt of lightning. Positive lightning develops in the same way as typical lightning bolts, but the positive bolt draws electrons upward from the ground. These lightning bolts tend to be much, much stronger than regular lightning, and may carry as much as a hundred times the energy of a normal flash of lightning.

These "superbolts" of lightning, thankfully, are very rare. Only about five superbolts occur for every ten million normal lightning strokes; typically, they're found only in severe storms during the winter, and are more common in and near Japan than anywhere else in the world.

Superbolts can reach way beyond the normal eight to ten miles of a typical lightning stroke. The longest superbolt on record reached from Waco, Texas to Dallas, after having traveled about a hundred and ten miles.

A hit from one of these positive superbolts tends to be a "no survivors" event. Such bolts often strike the ground twenty-five miles or more away from the storm front, without warning at all.

And it could be worse.

Lightning superbolts here on earth are downright feeble compared to superbolts in the upper reaches of Jupiter's atmosphere.

Lightning on Jupiter is very similar to lightning on earth, and is common in the planet's belt of water clouds. Weather conditions on Jupiter are far more intense, though, and produce much more violent storms than any that occur here.

On earth, weather patterns are driven by the sun. As we rotate, the sun heats the side of the planet facing it; uneven heating and cooling form the basic engine that drives our weather. On Jupiter, weather is driven by heat escaping from deep inside the planet's gas envelope.

The New Horizons satellite probe, currently on its way to Pluto, made a very close flyby of Jupiter in March of this year. Armed with instruments more sensitive than those on the Cassini probe before it, New Horizons detected for the first time massive superbolts near Jupiter's poles, a thousand times more powerful than superbolts on earth. Previous superbolts had been observed by the Galileo probe near the planet's equator (pictures of which are shown below), but the new images show that these massive lightning discharges are not confined to the turbulent zones around the planet's center.

Lightning is not always confined to thunderclouds. For hundreds of years, people have observed lightning in the pillar of ash from an erupting volcano; these lightning displays are sometimes huge and much more fierce than the lightning from ordinary thunderstorms.

Scientists have long believed that these fearsome lightning displays are caused by friction between particles of soot and ash, which leads to the the development of static charges high inside the ash cloud. But recent information suggests that the lightning in a volcanic eruption may be caused by water and ice, just like the lightning in an ordinary thunderstorm.

The mechanism by which lightning forms in volcanic clouds is not well-understood, in part because it's never been a priority for geologists. Small flecks of dust and ash will hold static electric charges, so the working model in the past has been "Well, you get static electricity built up in the columns of ash, and that's that."

A new model of volcanic lightning proposed by researchers at MIT casts doubt on this idea, and proposes a mechanism where water, the same thing that is responsible for lightning on Jupiter and Saturn as well as during storms on earth, causes these huge lightning displays.

Magma, the molten rock that spews out of a volcano, contains a great deal of water–in fact, the water carried to the earth's surface by erupting volcanoes, called "juvenile water," makes up in some cases as much as six percent of the total mass of material ejected from the volcano.

This water is initially ejected as vapor–steam–but quickly condenses out. The explosive force of a large eruption sends material into the upper atmosphere, where the temperature is typically well below freezing; the huge quantity of water blasted out along with the explosion expands and condenses on the particles of dust and ash, coating them with liquid water and ice.

And, as we know, wherever you have strong updrafts through clouds containing water and ice crystals, you get enormous static differentials. Those static electrical charges produce lightning.

In fact, lightning activity is so strongly associated with volcanism that volcanologists are now using lightning detectors to indicate volcanic activity.

When you were growing up, you probably heard all kinds of stories about lightning. In this post, we'll talk about some of the ideas you've probably heard. Some of the popular conceptions about lightning are true; others are not. We've talked about some of these beliefs in previous posts, so we'll link to other posts when appropriate.

Belief: Lightning never strikes the same place twice.
Status: FALSE

This is probably the most common of all beliefs about lightning. We dedicated an entire post to this belief here. The short version is: Lightning can and does strike the same place twice; in fact, the Empire State Building and other tall structures get hit many times a year.

Belief: Lightning can strike out of a clear blue sky, with no rain and no clouds overhead.
Status: TRUE

Everyone's heard of the "bolt out of the blue" that strikes with no warning when there's not a cloud in the sky. This can and does happen–quite frequently, in fact. Most lightning bolts come from the front and back edges of a storm system, and can travel for many miles before they touch the ground. Lightning can easily reach eight miles away from a storm cloud, and hit the ground in a spot that's under clear sunny sky. In rare circumstances, lightning can travel for very long distances indeed; the longest lightning bolt ever recorded, in October 2001, started from a storm in Waco, Texas, and hit the ground in Dallas, after traveling a total distance of some 110 miles!

Belief: You can tell if lightning is dangerous by counting the number of seconds between a lightning flash and the sound of thunder.
Status: FALSE

Many, many people are taught to believe that counting the time between lightning and thunder is a good way to tell if the lightning is close enough to be dangerous. This particular myth is especially dangerous, because lightning can reach many miles from a thunderstorm, as we just mentioned. Thunder can't be heard very far; depending on the landscape, you may not be able to hear thunder from any farther than two miles away or so. Even in ideal circumstances, it's hard to hear thunder from more than four miles. But lightning can often reach more than eight miles from a thunderstorm.

Belief: You are protected from lightning by wearing rubber-soled shoes, or by the rubber tires inside a car.
Status: FALSE

We've talked about this myth before as well. A car is a safe place to be, but not because of the rubber tires; the car's body acts as a Faraday cage to protect the people inside. Rubber-soled shoes, though, will not help you at all during a thunderstorm. A lightning bolt that can jump through miles of air can't be stopped by a few inches of rubber!

Belief: A lightning bolt is hotter than the sun.
Status: TRUE

The temperature of the sun's outer photosphere is about eleven thousand degrees Fahrenheit. A lightning bolt can heat the column of air around it to about fifty thousand degrees–nearly five times hotter.

Belief: Lightning is attracted to metal jewelry, metal hair clips, and personal electronic devices like iPods.
Status: FALSE

As a lightning bolt descends from the cloud, streamers begin rising from the ground in the area around the descending lightning bolt. These upward-rising streamers eventually touch the downward stroke, and the lightning bolt is complete.

Small metal items such as hair clips or the earphone wires from a music player don't make any difference at all; they're far too small to influence the formation of the streamers. Wearing an iPod or metal jewelry won't put you at greater risk of a lightning bolt.

However, metal objects can alter the flow of electricity if you are struck by lightning. One person who was struck while listening to an iPod suffered burns and damage to his ears when the current flowed up the headphone wires.

Very large electrical discharges tend to be most dangerous if they penetrate the chest, where they can disrupt the normal rhythm of the heart. For this reason, personnel who work with large electrical sources (such as army radios and photoflash devices) often work with one hand in their pocket. If they touch energized equipment with one hand, they may suffer burns, but touching energized equipment with two hands may cause the electrical surge to flow through their body from one hand to the other, stopping the heart.

Belief: A person who is struck by lightning is electrified and should not be touched.
Status: FALSE

This popular myth is based on a lack of understanding of the way electricity works. A lightning bolts lasts only a few microseconds, and a person's body can not store significant amounts of electricity. If someone is struck by lightning, give medical attention immediately!

Belief: A rubber raincoat or other insulating clothing will protect you from lightning.
Status: FALSE

As with rubber-soled shoes and rubber car tires, rubber rainwear poses no obstacle to lightning. The high voltage and high current of a lightning bolt can easily pass through such clothing.

Belief: Carrying an umbrella in a storm can increase the chance of being struck by lightning.
Status: TRUE

Anything which makes you taller can increase the odds that you'll be struck by lightning. Lightning doesn't always hit the highest point, but the odds are greater that it will strike tall targets–particularly if those objects are made of metal. The umbrella can act as a crude lightning rod, directing a lightning stroke right into you.

Belief: If lightning strikes the ground nearby, the stroke is "grounded" and harmless.
Status: FALSE

When lightning strikes the ground, the electrical discharge can travel for considerable distances through the ground, as we talked about here. The indirect "splash current" from nearby lightning strokes can still be dangerous even sixty feet or more from the point of discharge. In water, it's even worse; dangerous currents can travel for six hundred feet from the place where the bolt hits the water. This creates an area of about eleven thousand square feet on land, or a whopping million square feet in water, that can carry potentially lethal amounts of current. Every year, lightning is responsible for the deaths of thousands of farm animals and millions of fish. The USDA estimates that about 80% of all accidental livestock deaths are caused by lightning.

Belief: You should not take a shower or use a conventional land-line telephone during a thunderstorm.
Status: TRUE

Lightning strokes can travel for significant distances through the ground, and these distances are greatly increased if lightning strikes a conductive metal object such as a telephone line. In fact, many computers are destroyed every year by lightning strikes on telephone lines. People will often disconnect their computers from the wall during a storm, but may not consider disconnecting them from the telephone line or cable modem. Lightning that strikes the phone line or cable can travel through a phone, cable, or DSL modem and into the computer, destroying it. The same thing can happen if you are using the phone or taking a shower; a lightning hit a good distance away can travel through metal pipes or phone lines and into your house.

Surge suppressors are not always effective at protecting electronics in a house. They are good at defending against small surges, but a direct lightning hit on a power or telephone line near the house will easily overwhelm a surge suppressor and damage the equipment connected to it.

Belief: You should stay away from metal bleachers, guard rails, railroad tracks, metal sheds, and fences during a storm.
Status: TRUE

Metal is metal is metal. All metals conduct electricity, and large metal objects can conduct the energy of a lightning bolt for long distances. A lightning hit on a train track or a fence can be conducted down its length, and discharge into a person standing on or near it. Metal sheds and bleachers offer no protection from lightning, and may conduct a far-off lightning strike directly to a person seeking shelter under them.

Belief: Lightning rods protect buildings by "discharging" a storm cloud or draining the electricity out of it.
Status: FALSE

A lightning rod offers an easy point for a streamer to form, increasing the odds that a bolt of lightning will hit the rod and be conducted safely to the ground rather than hitting the roof of a building. Lightning rods do not drain the electricity out of a cloud; instead, they just offer an appealing place for a lightning bolt to hit.

Belief: "Heat lightning" is caused by hot air and poses no threat.
Status: FALSE

What people call "heat lightning" is really just the visible flashes of lightning too far away to hear thunder. Since lightning bolts can travel farther than you can hear thunder, "heat lightning" in the distance may still pose a threat.

Let's start with an interesting fact: A typical cloud-to-ground lightning bolt is hot. Very hot. The air surrounding a lightning stroke is superheated plasma, which can be anywhere between 30,000 and 50,000 degrees Fahrenheit. By way of comparison, the outer layer of the sun is around 11,000 degrees Fahrenheit.

Worldwide, there are about six thousand lightning flashes per minute. Not all these flashes are cloud-to-ground strokes, but even so, that's a lot of lightning strikes every minute.

Some of these lightning bolts hit sand. Sand is primarily silicon dioxide, SiO₂. When sand is heated above about 900 degrees Fahrenheit or so, it melts and fuses into an amorphous solid that you're probably familiar with; in this form, it's called "glass."

When lightning meets sand, all the energy in the lightning bolt spreads out, melting it into glass as it goes. The result is a formation called a "fulgurite," a brittle glass tube that traces the pattern of the lightning in the sand.

Fulgurites are typically hollow tubes, and may be anywhere from a few inches to several yards long. The length and width are proportional to the strength of the lightning bolt, and also depend on the characteristics of the surface where the bolt hit. The heat of the lightning bolt rapidly melts the sand, which cools just as quickly to a solid tube that preserves the path the lightning took through the ground. The tube is usually very fragile, partly because the sand is rarely pure (inclusions in the glass weaken it) and partly because it cools much too rapidly to anneal, so thermal stress is locked into the glass.

Fulgurites can spread for a great distance underground, depending on the composition of the material beneath the point of impact. Large fulgurites are uncommon, mostly because people who dig them up tend not to be careful with them; you can find fulgurites by digging at almost any beach, but it takes great skill and delicacy to dig up a large fulgurite without destroying it. This Web page shows the process of discovering and unearthing an enormous fulgurite in Seven Springs, Arizona.

Occasionally, lightning will strike an outcropping of rock (or other hard material like concrete). When this happens, the lightning will melt a channel through the rock, creating a "rock fulgurite". Rock fulgurites are a part of the surrounding material, and trace the exact path the lightning took as it passed through the material. The heat of the lightning is sufficient to change the material as the bolt passes through it, leaving a kind of "fossil" behind. (Click this thumbnail for a bigger image.)

Fulgurites can be created artificially as well. Two techniques are used for doing this. One is to attach a lightning rod to a metal stake in the ground by a thick cable, and then wait for lightning to strike; when lightning hits the rod, the energy is conducted into the cable, and a fulgurite forms where the stake is driven into the ground.

The second (and more dramatic) way to create fulgurites is to use small rockets to trigger lightning, like we talk about here.
Images: University of Florida

"One one-thousand, two one-thousand…" Counting the time between a flash of lightning has long been a popular way to figure out how far away a lightning flash is. Even today, some people still recommend this method to determine if you're safe from a direct strike, though this is something of a fallacy–thunder isn't audible very far, and a lightning bolt can reach eight miles in front of a storm.

But what's even more interesting is that the effects of a single lightning bolt can cause a chain reaction of events over hundreds of thousands of square miles, hundreds of miles away. Before we talk about why, we'll have to talk about some of the things that go on high above the earth, in the Van Allen radiation belts.

The earth has a magnetic field. Everyone knows this; if you've ever used a magnetic compass, you know that the earth's magnetic field is what makes the needle point north. This NASA model of the magnetic field shows how complex it is, and how the magnetic lines of force loop from the north pole to the south pole.

The magnetic field does more than just make compass needles move. It also helps make life on earth possible. It deflects charged particles streaming from the sun, protecting us from radiation that would otherwise make conditions on earth inhospitable to life. This stream of charged particles is called the "solar wind," and where it meets the magnetic field, all kinds of fun and vigorous things happen.

This illustration (click for a larger, beautiful version) shows the way the solar wind interacts with the earth's magnetic field. Most of the stream of charged particles (protons and electrons, primarily) is deflected away from the earth; but some of it becomes trapped by the magnetic force. These trapped particles create regions of charged ionizing radiation above the earth, called the Van Allen radiation belts. The auroras over the north and south poles are the result of these charged particles interacting with the upper atmosphere.

So what does this have to do with lightning? Bear with us; we're getting to that.

The Van Allen radiation belts are actually divided into two discrete sections. The outer layer is composed primarily of highly energetic electrons, which spiral around the lines of magnetic force in the earth's magnetic field and bounce back and forth between the north and south poles. These electrons come from the solar wind; as time goes on, more and more electrons become trapped here until finally an event causes them to break free of the magnetic field and stream down into the upper atmosphere. (The inner layer is primarily made up of energetic protons.)

Often, that event is lightning.

Researchers have recently discovered that lightning strikes at certain latitudes can cause electrons to rain out into the upper atmosphere.

Every bolt of lightning creates a characteristic electromagnetic pulse. This pulse can travel great distances, and can reach high into the outermost layers of Earth's atmosphere. At this altitude, the electromagnetic pulse interacts with the Earth's magnetic field; if the electromagnetic pulse is oriented correctly, it will scatter the electrons trapped high above the earth, breaking them free from the magnetic field and causing them to precipitate down into the upper atmosphere. The EMP can affect an area of tens or even hundreds of thousands of square miles, causing electrons in a very wide area to descend into the ionosphere.

Lightning, when you get right down to it, is essentially nothing but static electricity on a grand scale. Rising water vapor condenses into droplets or tiny ice crystals, which acquire a static electric charge as they pass through the air. This mechanism is responsible for all the forms of lightning found in thunderstorms–cloud to cloud lightning, cloud to ground lightning, and in-cloud lightning that you'll see during a storm.

The same mechanisms work anywhere water vapor exists in cloud formations. Lightning flashes have been observed in the upper atmospheres of both Jupiter and Saturn, albeit on a far grander scale than the lightning flashes on earth. These gas giants have water vapor in their atmospheres, along with methane, ammonia, and elemental hydrogen and helium. The clouds found on gas giants tend to form in layers, with each layer composed predominantly of different chemical compounds; the outer layer of Jupiter's clouds, for example, is predominantly ammonia, with a second cloud layer beneath it made up mostly of ammonium hydrosulfide (NH₄SH), and below that, a layer of water clouds. Jupiter's atmosphere is extraordinarily turbulent, creating plenty of friction for massive lightning discharges in the water cloud layer.

This is interesting stuff, but not all that surprising. The mechanisms for lightning on Jupiter and lightning on earth are very similar, despite the radical differnces in the composition and size of the planets. Water vapor forms clouds, motion of water droplets in the clouds creates static electricity, you get lightning. Neat, but not earth-shaking.

Very recently, however, the European Space Agency's Venus Express probe has confirmed a suspicion many astronomers have had for a long time: You don't need water vapor for lightning. Clouds of sulphuric acid will do just as well.

The ESA's probe has discovered evidence of lightning on Venus, our (relatively) nearby neighboring planet. Venus is an inhospitable place, with high surface temperatures, a crushing atmosphere, and clouds of acid streaming by overhead in ceaseless windstorms stronger than the strongest terrestrial hurricane. And, apparently, lightning.

This artist's conception of cloud to ground lightning on Venus shows a lightning bolt descending from the haze of acidic clouds overhead. The probe only recorded direct evidence of cloud to cloud lightning, high in the sky at an altitude of 55 kilometers, but if the processes underlying lightning on earth are similar, cloud to ground lightning is possible as well.

These findings are significant for two reasons. First, lightning played a strong role in shaping the chemistry of our atmosphere early in our planet's history, and lightning almost certainly has changed the atmospheric composition of Venus as well; understanding Venus' atmosphere requires understanding the lightning that occurs there.

Second, we now know that the process by which water clouds form static electrical potentials is not unique to water. The method by which electrical charges become segregated in thunderheads is not entirely understood, but knowing that the same kind of charge segregation can happen in clouds of sulphuric acid helps give us insight into the process here at home.

Besides, lightning on Venus is just plain cool.

Normally, a lightning bolt (as we talked about before) hits the ground after a long, complex chain of events that concludes with a streamer from the ground reaching up and making contact with a descending ionization channel.

Throughout history, that's the way lightning has worked. People have not always understood the mechanism, of course; lightning's often been attributed historically to the action of irate gods, and it wasn't until Benjamin Franklin applied a scientific methodology to the question that modern understanding of lightning took hold.

And today, we know that lightning mostly strikes when a rising leader touches a descending ionization channel. Mostly.

Occasionally, though, that isn't the way it happens. Occasionally, a descending ionization channel makes contact with a small solid-fuel rocket rising from the ground trailing a copper wire behind it:

This is what happens when you fire a rocket trailing a wire into a thunderhead. A copper wire is even better at conducting electricity than an ionization channel is, and ninety-nine out of a hundred lightning bolts surveyed say they'd rather discharge along it than along a leader.

Ahem. Anyway, this is called "triggered lightning," and it's a research tool for understanding lightning. When such a rocket is fired into a descending ionization channel, scientists can control precisely when and where the lightning hits, which offers them the ability to measure everything from the energy contained in the bolt to the number of individual strokes.

This image, from the University of Florida, shows a triggered lightning bolt meeting an ascending rocket, and the discharge following the wire back down to the ground. In this case, the triggered lightning was part of an experiment to investigate the way different minerals affect the formation of fulgurites, which are fused glassy tubes that form when lightning strikes sandy material. (We'll talk about those in a later post.)

These rockets make a one-way trip; the rocket, and its trailing wire, are vaporized by the lightning bolt.

A single lightning bolt is often made up of several discrete flashes, each of which is a separate electrical discharge and each of which follows so closely on the heels of the one before it that to the unaided eye they look like a single bolt.

In the high-speed photograph from Lawrence Livermore above, the result of these multiple flashes can be seen; as each flash occurs, the wind blows the glowing, superheated air sideways, producing the appearance of a sideways wall of flame moving away from the straight vertical line (which is the rocket's exhaust).

Today we're not going to talk about lightning of the natural variety, and instead talk about lightning of the man-made variety. Specifically, we're going to talk about the Z-Machine, an enormous X-ray generator at Sandia National Laboratory.

How enormous? This enormous:

(You can click on the picture for a bigger version. Believe me, it's worth it.)

What you see here is the Z-Machine after it has just fired. Essentially, it's a gigantic electrical discharge fed into a very small space, and directed at a tiny target made of a cylinder of hair-fine tungsten wires.

But let's back up a little first.

The Z-Machine was originally designed to test theories about atomic explosions. It used extreme heat and pressure confined to a very, very small space to study the behavior of materials at very high temperatures–the kinds of temperatures you'd get in the center of a nuclear bomb blast.

The way it works is surprisingly simple, really. Power is fed into several Marx generators, which store the electricity. Marx generators are banks of capacitors arranged in such a way that they are discharged in series, multiplying the voltage that was used to charge them considerably.

The Z-machine uses a series of huge Marx generators arranged in a circle. Each of these is charged from ordinary household-level current for a while. When they're all charged, several laser triggers fire, discharging the Marx generators through several massive copper wires, each about five feet across, arranged like spokes in a wheel. The wires conduct the electricity to the target, which is promptly, and vigorously, annihilated.

The total amount of electrical power that's used to zap the target is huge–about 290 trillion watts, or roughly eighty times the total output of all the power generating plants in the world. This electrical surge is very short; it only lasts for a few billionths of a second. The power instantly vaporizes the tungsten target, by heating it to nearly two million degrees. The ultrahot tungsten is turned into plasma, which quickly implodes under the tremendous magnetic field generated by this huge surge of electricity. The plasma crushes the target within the cylinder of tungsten wires using forces and temperatures comparable to what you'd find at the surface of the sun.

Originally, this was done to help design more efficient nuclear weapons. By studying the density and flow of energy inside a nuclear blast, scientists could design nuclear bombs that operated more efficiently. Today, the Z-Machine is used to research nuclear fusion–a hypothetical power source that can produce virtually limitless amounts of energy from ordinary seawater, without radioactive waste.

The electrical discharge you see all over the place in the photograph above isn't electricity from the Marx generators themselves. It's electricity induced in any nearby metal objects, such as ladders and support structures, just by the electromagnetic pulse produced from the collapsing plasma inside the target chamber in the middle of the ring. The ring itself is filled with deionized water, to help buffer and cool the enormous cables that bring the electricity from the Marx generators to the target in the center.

COLORADO SPRINGS, Colo. — A lightning strike indirectly killed a man who had taken cover inside a tent with three other people, reported KMGH-TV in Denver.

This story illustrates one of the hidden dangers of high-voltage electricity in general and lightning in particular: splash current. A lightning bolt can kill you without even touching you.

When a lightning bolt strikes an object, the object becomes strongly electrified and may emit electrical discharges itself. These bolts, called "side splash," will arc from the object that's been hit to just about anything nearby, or to the ground, or both. By now, you probably know better than to stand beneath a tree during a lightning storm (and if you don't, you should–don't stand beneath a tree during a lightning storm!). The danger is only partly in being struck directly; even if by some stroke of fortune the riser coming from the tree makes contact with a descending stroke before the riser coming from your head does, if the tree gets struck you'll quite likely get hit by splash.

People can also be killed by lightning even if the stroke lands nowhere near. When lightning goes to ground, there will be an enormous surge of current in the ground that can extend away from the point of impact for some distance in all directions. This "splash current" can electrocute people standing nearby even if there's no arc and the lightning stroke doesn't hit them directly. This is what happened to the unfortunate person in the news article linked above.

The risk of injury or death from splash current increases as the amount of surface area in contact with the ground increases. This is particularly dangerous to campers, who may seek shelter from a storm inside their tent–and lie down in the tent. If more of your body is in contact with the ground, you are more likely to be injured or killed by splash current.

There are a couple of lessons in here. First off, high-voltage electricity tends to seek ground any way it can. A lightning bolt carries tremendous voltage at incredibly high current, and it tends not to be neat and tidy about the way it seeks ground. It'll arc all over the place, sometimes in unpredictable and counterintuitive ways. It can jump from object to object, it can jump between nearby buildings, and it can even jump all over the inside of a metal shed. (If you're caught in a lightning storm, most buildings provide excellent protection, but metal sheds and similar structures are an exception. A metal shed won't act as a Faraday cage because it's not completely enclosed; they don't have metal floors. So when lightning hits a shed, it has a tendency to produce arcs all over the inside–which is bad news if you happen to be there!)

The second is that there really isn't a safe place to be outdoors during a lightning storm. Prevention is the name of the game. In the story above, it took rescue workers three hours to reach the hiker and get him to the bottom of the mountain after he'd been hit by splash current–and by then it was too late.