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the phenomena of Lightning
PBS/NOVA October/2005
I. How Lightning Works by Joe Dwyer
[Joe Dwyer is an Associate Professor of Physics and Space Sciences at the Florida Institute of Technology. To see a fuller bio or and ask him a question about lightning, go to Ask the Expert.]
No one is sure how lightning gets started, but one theory is that incoming cosmic rays from outer space serve as the trigger. Here, stars shine above a thunderstorm in the Alps.
Most of the 4 million lightning flashes that occur on our planet each day originate from inside thunderclouds -- often just a few miles above our heads. And yet despite decades of research, we still don't understand what is happening inside clouds to start the common lightning bolt.
In order to get a conventional spark (the kind that a spark plug makes), the electric field needs to surpass the conventional breakdown field -- the point at which air loses its insulating properties and becomes capable of conducting electricity. For air, this is about 70,000 volts-per-inch at sea level. A car accomplishes this by supplying a high voltage (say 20,000 volts) across the narrow gap of the spark plug. Thunderstorms can also generate big voltages. They do this when ice and water particles carried by updrafts collide with soft hail carried downward by gravity.
The voltages produced by the resulting charge separation are impressive, sometimes exceeding 100,000,000 volts. But unlike a spark plug, in a thunderstorm the voltage gets applied across a mile-or-more, resulting in an electric field that appears to be too small to start a spark. Indeed, decades of balloon, aircraft, and rocket measurements have rarely found electric fields even 1/10th the minimum value needed by a spark plug to make a spark in air.
Of course, lightning does occur inside thunderclouds, so clearly we must be missing something about how lightning gets started. Several ideas have been suggested including colliding raindrops, localized regions of concentrated charge, and avalanches of high-energy electrons initiated by cosmic rays from outer space. At least for now, however, no one knows what the right answer is.
Following the Leader
Lightning within clouds is far more common than lightning that strikes the ground. Left, a storm photographed near Emberger Alm, Austria in August 2003.
We do know that once lightning is initiated, somehow a hot channel called a leader quickly forms. As I noted earlier, air is normally an insulator. So in order for the lightning discharge to propagate from its point of origin to other locations such as the ground, the air along the path must be broken down, allowing electrical charges to move freely. The 'leader' accomplishes this by concentrating charge at its tip and thereby producing a large electric field, just like a spark plug. This causes a discharge in front of the leader, allowing it to move forward. To keep the process going, however, new charge must be supplied continuously to the 'leader' tip. It is the electrical current that moves this charge along the hot leader channel. The 'leader's high temperature keeps the air in the channel conductive long enough for the current to flow across many miles of air.
Interestingly, the initial leader does not travel in a smooth, continuous manner but rather in a series of short jumps called steps. This is why the initial 'leader' is called the 'stepped leader'. During a 'step', the leader shoots forward about 150 feet, often in a new direction. It takes on average about 0.007 seconds for the stepped leader to travel each mile in this stop-and-go fashion. Moreover, sometimes during the stepping process, the leaders branch into 2 channels. It is thus the stepping of the initial leader that gives lightning its characteristic jagged, forked appearance. Despite lightning's familiarity, however, no one knows why lightning 'leaders' behave this way.
Ground Strokes
Sometimes the currents that flow through the 'leader' as it propagates are large enough to make the 'leader' appear very bright. Most of the lightning that we see does not actually connect with the ground. It is instead caused by currents flowing along channels linking various parts of the same cloud, the surrounding air, or more rarely different clouds.
If the 'stepped leader' does connect with the ground, however, a short circuit between the cloud and the Earth occurs and large currents will flow -- starting at the ground and propagating upward. These currents will partially neutralize the voltage difference the thunderstorm generated. This process is called the return stroke. Electric currents in the 'return stroke' can be very large, measured in tens of thousands of amperes. The huge currents rapidly heat the air to roughly 50,000°F -- about 4 times hotter than the surface of the Sun.
It is this hot channel produced by the 'return stroke' that we see during cloud-to-ground lightning. And the rapid expansion and subsequent contraction of the hot channel begets the thunder we hear. Frequently after the 'return stroke', the thunderstorm will send down another leader -- this time called a dart leader -- that quickly recharges the channel and leads to another return stroke. The fast succession of 'leaders' and bright 'return strokes' is what causes a lightning flash to flicker.
II. Lightning Varieties
A. Cloud-to-Ground
This is the archetypal lightning bolt -- one that arcs out of the sky and smites the ground with a great, often flickering flash of light. Lightning is the sudden release of built-up charge stored in an electric field, though exactly what triggers it remains a mystery.
B. Cloud Discharge
This is lightning that occurs within a thundercloud, between 2 thunderclouds, or from a thundercloud to the air. Experts think most cloud discharges take place within an individual cloud, though few data exist to confirm that belief. Cloud discharges are certainly more common than the cloud-to-ground variety: 10-or-more cloud flashes may occur before the first one that strikes the Earth.
C. Ball Lightning
No scientific documentation such as photos, videos, or other recordings of 'ball lightning' exist. So experts have had to rely on eyewitness accounts (which have been numerous). Judging from such accounts, balls of lightning are typically between a golfball and a basketball in size; about as bright as a 60-watt light bulb; and often red, orange, or yellow in color. Observed shooting through the air, across the ground, and even into houses, they are fleeting, generally lasting a few seconds before vanishing gradually or abruptly.
D. Blue Jet
'Blue Jets' shoot upward from the tops of thunderclouds. This remoteness -- and the fact that they last but a few hundred milliseconds at most -- perhaps accounts for why they were not discovered until 1994. They are the color of sapphires, are cone-shaped in structure, and extend for many miles. Like sprites and elves, blue jets provide a mechanism for energy transfer from lightning and thunderstorms to regions of the atmosphere between thunderclouds and the lower ionosphere.
D. Red Sprites
'Red Sprites' occur above large thunderstorm systems and are generally associated with larger positive cloud-to-ground flashes far below. They are most luminous very high up in the atmosphere between altitudes of about 25 and 55 miles. Yet even at their most luminous, they are very hard to see, in part because they last for only a few thousandths of a second. Red in color and often bearing faint bluish tendrils extending downward, 'sprites' come in several shapes designated by colorful names like "carrot", "angel", and "columniform".
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E. Elves
Like celestial halos, elves are circles of light that appear some 50 miles-or-more above thunderstorms. Triggered by lightning flashes far below, these ephemeral discs spread out radially across the bottom of the ionosphere in the briefest instant, expanding up to hundreds-of-miles in diameter in less than a millisecond. Experts believe 'elves' are caused by lightning processes that accelerate electrons in the lower ionosphere.
F. Volcanic Lightning
Lightning-like discharges are sometimes observed during volcanic eruptions with no thunderstorm anywhere nearby. Hundreds or even thousands of feet in length, these bolts can flash to the ground or remain entirely within the ash cloud above the volcano. Here, lightning flashes during an eruption of Japan's Sakurajima Volcano in 1991.
G. Nuclear Lightning
Hydrogen-bomb explosions can generate their own lightning. Ground-level detonations -- like this 1952 test of an experimental thermonuclear device on Eniwetok in the South Pacific -- cause a negative charge to be deposited in the atmosphere, resulting in long discharges not produced by clouds. This is a frame from movie footage shot at a distance of about 18 miles from ground zero. The lightning channels have been enhanced for visibility.
H. Triggered Lightning
"Triggering lightning at will -- at a predetermined place and time -- is the old Promethean dream which seems more related to legend than to Science," lightning expert Pierre Hubert has written. Yet the technique has taught scientists much about lightning processes and effects. Here, a rocket trailing a thin, grounded copper wire was launched into a thundercloud, triggering a series of lightning bolts that followed the path of the wire, which was vaporized in the process (see smoke-like vapor). Wind has separated the individual bolts.
======{Program Transcript} ======
Robert Krulwich: By the way, maybe you've noticed in the old movies when it's time to zap the monster on the table and bring it to life, they always use lightning. I mean always. And why? Well, because lightning is electrical. It turns things 'on'. Though exactly what lightning does is left to your imagination because in the end, what makes Life -- that's still a mystery.
But interestingly -- and I didn't know this -- it turns out that what makes lightning is also still a mystery. In fact, it's kind of a big mystery. Here's correspondent Chad Cohen.
Chad Cohen: It's an elemental force of Nature. And still one of the most mysterious.
Joseph Dwyer (Florida Institute of Technology): Lightning is very difficult to study. I think we probably understand better how a star explodes halfway across the galaxy than how lightning propagates from 6 miles up.
Chad Cohen: Lightning strikes the Earth 4,000,000 times a day. And after hundreds-of-years of scientific scrutiny, we still do not understand the essential secret of how it begins inside a storm. That's why Professors Ken Eack and Richard Sonnenfeld and their team from New Mexico Tech are on a 10,000-foot mountain, waiting for lightning to strike.
Kenneth Eack (New Mexico Institute of Mining and Technology): We're trying to find out something new about thunderstorms and lightning. That discovery -- I think -- is worth the risk.
Chad Cohen: Whatever causes lightning to start has always been hidden inside the clouds. So unlocking that process requires waiting for the weather to reach maximum force, then launching sensitive instruments into the heart of the storm. It's hazardous and frustrating.
Richard Sonnenfeld: Come on out. Let's go. Go, go, keep going. Hey, don't drop it! Don't drop it. … Okay, let's go, let's go. …Go ahead, you get in position, you get in position. Oh! Let's go in, in, in. It hit the ground.
Dr. Martin Uman (University of Florida): A thunderstorm has got the energy of an atomic bomb.
Chad Cohen: Dr. Martin Uman is the director of the International Center for Lightning Testing and Research in Camp Blanding, Florida.
Martin Uman: It's the brightest light that we see. It's the loudest noise that we hear. The only thing hotter than lightning on Earth would be a nuclear weapon explosion.
Chad Cohen: But what triggers the release of all that power? We know that lightning is a huge electric spark. And sparks happen when positive and negative charges build up so much energy that they leap through the air to get at each other. It can only happen when the negative charge in that ball on the right and the positive charge in that metal rod on the left get so overwhelmingly strong that they cut a path through the air in the middle.
Martin Uman: It's like a hose so full of pressure that it can't hold on anymore.
Chad Cohen: That's what most scientists thought was happening inside thunderstorms as ice and water particles collide with each other, moving electric charges to opposite ends of a cloud. When the charge above and the charge below get strong enough, they leap through the air as a bolt of lightning. Except for one thing: when you actually examine the storm cloud, the strength of the positive and negative charges and the electric field around them isn't nearly enough to create that big spark.
Joseph Dwyer: Well, the problem is after decades and decades of measurements up in thunderstorms, nobody has ever managed to find an electric field anywhere near that big.
Chad Cohen: Dr. Joe Dwyer is a professor at Florida Tech.
Joseph Dwyer: Well, maybe we're looking for something that doesn't exist. Maybe there's something wrong with our understanding about how electrical discharges get started in places like thunderstorms.
Chad Cohen: So if thunderclouds -- even great big thunderclouds -- don't have electric fields big enough to generate the giant spark that lightning actually is, where's all that energy coming from? Well, here in Florida they have a pretty dramatic way of trying to figure that out. They launch rockets with really long wires attached to try to create that express lane to ground that lightning likes so much.
To get lightning from these clouds to strike where scientists can measure it requires a simple trigger.
Martin Uman: First of all, you need to have a propellant that can get this thing up there in a hurry. It has to be able to go 700 yards or so in about 2 seconds.
Chad Cohen: What is this, actually? This is ... is it just copper wire?