(John R. Southern;Wikipedia)
Air Must Ionize For Lightning To Happen
When the electric field (or electric potential energy, electric potential, or voltage - all terms defined in the previous article) exceeds the dielectric strength of the air, the air can no longer resist current. The electric field begins to ionize the air. The electrons of the air atoms and molecules are pushed so hard in the direction of the electric field that they are pulled off and become free and mobile.
Most of the air is composed of nitrogen and oxygen molecules, (78% and 21% respectively). Because they strongly resist ionization, a very strong electric field must be applied before they will ionize. Once air does become ionized, with freely mobile electrons, it turns into an excellent electrical conductor.
Ions are not all built the same. For example, water molecules behave very differently from nitrogen or oxygen molecules. Water, a highly polar covalent molecule, contains positively and negatively charged regions. These regions are attracted to other charges around them, making water's molecular bonds less stable. In fact, water self-ionizes, acting a bit like an ionic molecule or salt (but with itself). It fairly easily dissociates into positive H30+ ions and negative OH- ions, shown below.
The double arrow means the reagent (water) and products (H30+ and OH-) ions reach an equilibrium. A small percentage of molecules of a glass of pure water, for example, will exist in ion form. Random electric field fluctuations due to molecular motion occasionally produce a (very localized) field strong enough to break the fairly weak O-H bond.
Oxygen and nitrogen molecules are nonpolar covalent molecules. They cannot dissociate into positive and negative ions, separating charge this way, because their bonding electrons are snugly and equally shared between them. Instead, the electric field must be intense enough to rip tightly bound electrons off the atoms in these molecules, ionizing them that way. At this point there is enough energy to break some of the molecules themselves apart, and this too is difficult because they have very strong bonds, especially the nitrogen-nitrogen triple bond. As air becomes ionized, some oxygen and nitrogen molecules remain intact but contain excited atoms. The outermost electrons in these molecules have extra energy. Other molecules are split apart and these lone atoms are excited. Some of the excited atoms gain even more energy and become partially ionized. This means that the ionized air under a thunderstorm contains a mixture of atoms (shown below right) - partially and perhaps fully ionized atoms, excited atoms, excited molecules, a few neutral (unexcited) atoms, and a few neutral molecules - each with different energies but all contributing to a high enough average energy to consider it a plasma, a physical state that contains particles at a higher overall energy than those in a solid, liquid or gas.
In the diagram right I've drawn different nitrogen atoms and molecules to give you an idea of what is happening to them. Only the five valence (outer shell) electrons in each nitrogen atom are shown. In reality, nitrogen atoms contain 7 electrons, with two of them confined to an inner energy shell. Lower right Is a partially ionized atom. There are degrees of ionization. A fully ionized atom (bottom left) contains only the nucleus. In this case, every electron moved into an excited energy shell and then left the atom altogether (it takes even more energy to remove the two inner electrons not shown in the other atoms).
We'll explore this ionization process further in a moment.
It takes almost twice the energy (945 kJ/mol) to break the powerful triple bond of a nitrogen molecule than it does to break the double bond of an oxygen molecule (497 kJ/mol). A kilojoule (kJ) is a measure of energy. One mole (mol) of atoms contains 6.02 x 1023 atoms. Once split, oxygen atoms are a bit more easily (partially) ionized than nitrogen atoms - 1314 kJ/mol compared to 1402 kJ/mol, respectively, to remove an outer electron (to remove all the electrons from a nitrogen atom, including those in the inner higher energy shells, would require far more energy, about 4578 kJ/mol - nitrogen in this state is also a plasma, but it has much higher energy).
The electric field building beneath a thundercloud eventually has enough energy to break apart, excite and ionize nitrogen and oxygen molecules. Nitrogen atoms by themselves are highly reactive. They will quickly recombine into nitrogen (N2) gas or into nitrous oxide (NO). Meanwhile, excited nitrogen molecules emit blue light. Oxygen molecules likewise are excited. They may also release photons of light, but more often they react with unexcited oxygen molecules to create ozone, before they have a chance to. This ozone, which only lasts about an hour before it decays back into molecular oxygen, is often linked with the fresh clean smell after a thunderstorm (and yes it is a contributor to damaging ground level ozone worldwide). The air under a thunderstorm is very humid. Ionized hydrogen atoms split apart from from water vapour contribute red to the glow, so that ionized humid air glows violet.
Ionized Air is A Type of Plasma
Most plasmas glow, just like the neon artwork shown below. They contain a mixture of ionized and excited atoms. It is the excited atoms that glow. Excited electrons in nitrogen and oxygen atoms in the air emit light as they return to their unexcited state, a process that repeats over and over. This kind of glow from atoms is explored in detail in my article, Atoms Part 2: Atoms Can Emit Light.
The beautiful neon light artwork above, created by Stephan Huber, is located in Münster, Germany. We call all these tubes "neon" lights but they can be filled with any kind of excited atom. The blue glow above comes from mercury atoms, excited by a high potential across the tube.
The air ionization process requires tremendous energy, either from heat or a powerful electric potential. As an electron absorbs energy, it will jump to higher and higher energy shells before it finally leaves the atom altogether. When atoms lose one or more electrons, the electrons become mobile and the air become electrically conductive, much like a metal, which has a delocalized electron "sea." Very energetic plasmas contain completely ionized atoms and maximum electron density depending on the atoms involved, as shown below. (The free electrons in metals means they act like a plasma; conductivity depends on the density of free electrons)
As the energy of plasma drops, free electrons recombine with nuclei, re-creating neutral atoms. These electrons are still excited so they shed energy by emitting photons of radiation - ultraviolet, infrared or visible light. Each electron energy shell in each atom has a unique wavelength specific to it.
The electrical conductivity of ionized air increases as it is heated. This in turn allows greater electrical current to flow through an ionized air channel, heating the plasma further. Inside a lightning strike, plasma temperature can reach almost 30,000 K (= 30,000°C), with an electron density of 1017 electrons/cm3.
The current in a lightning strike can be extremely large as a result. But what triggers lightning?
What Triggers a Lightning Bolt? The Runaway Breakdown Theory
Once the electric field exceeds the dielectric strength of the air, an electrical discharge (lightning) can occur, but a lightning strike is very specific in time and place. The entire region beneath a cloud does not break down into plasma. That would require an enormous amount of energy; the system instead finds the most energy-efficient way to discharge its excess potential energy (stored up as charge separation). What exactly triggers a lightning strike, just like the mechanisms of cloud charging we explored in the previous article, remains a mystery. However, one theory that seems to be gaining traction among researchers is the runaway breakdown theory. Some researchers believe that, although the electrical field is very powerful around a thundercloud, it is not strong enough on its own to initiate a lightning strike.
Instead, very high-energy (fast moving) electrons from outer space (which bombard Earth all the time) might be the trigger. These electrons could provide the burst of energy needed to initiate a lighting strike. It would take just a few of them to start the process because, as they strike the (slow) electrons in the ionized air, they transfer their energy to them, leading to a cascade or burst of high-energy electrons, as newly energized fast electrons bombard additional slow electrons, accelerating them in turn.
An ordinary (slow) electron in ionized air travels an average of about one centimeter before it strikes another particle. The electrons will drift in the direction of the electric field but friction between the electrons and other partially and fully ionized atoms will tend to keep them moving at a constant speed rather than allowing them to accelerate. Fast electrons (in cosmic rays), on the other hand, travel close to light speed, with energies exceeding 100 electron volts (0.1 MeV). An electron volt is a unit of energy equal to 1.6 x 10 -19 joules (J). These fast electrons have an average free path of up to a meter because they experience less friction, as shown in the graph below.
A fast electron has enough energy to ionize the air particles along the path in front of it. A fast electron, then, instead of bumping into "big" intact atoms, interacts only with smaller free (charged) particles, reducing its friction as it travels through air.
The electric field itself may further accelerate these already-fast electrons so that the electrons they strike become new fast electrons, many of which will be aligned with and accelerated along the electric field to repeat the process. Avalanches of avalanches of high-energy electrons could be produced this way, but rather than creating a giant and deadly cosmic storm under a thundercloud, the process would be limited by the resulting decay of the electric field itself. As electrons gain energy and bump into other particles, knocking off more electrons, they should eventually gain enough energy to trigger an X-ray or gamma burst, releasing energy from the electric field itself.
An alternative to this theory is possible, in which free (slow) electrons could, of their own accord, accelerate along the electric field and amass enough energy to trigger a few gamma or X-ray bursts on their own, and these bursts themselves could serve not only to trigger a lightning strike but also limit the electron cascade. The recent discovery of a surprising abundance of X-rays and gamma rays produced in some thunderstorms supports this idea.
Leaders and Streamers
As hinted at with the earlier Plexiglass ® example, the entire column of air under a thundercloud doesn't uniformly ionize. Instead, discrete channels of air ionize, leaving the rest of the air column an intact electrical insulator. One or more channels of ionized air form under the cloud, often growing like a highly branched or stepped ladder from the negatively charged base of the cloud toward the positively charged ground. This is called a leader or stepped leader. A leader travels about 320 km/h as it branches toward the ground. That's fast but not nearly as fast as the lightning bolt itself to come (as you'll see). Leaders are usually very difficult to observe, glowing faint violet against a dark sky.
As the leader approaches the ground, free electrons in the plasma drift downward in the direction of the electric field. Attracted by the approaching negative charge, one or more channels of positively charged ionized air, called streamers, might grow upward from the ground toward it. Streamers tend to form on pointy surfaces where the field is accentuated. Positive ions drift up the channel, attracted by the approaching negative charge. These streamers may glow brighter violet than leaders do, leading to an unnerving sight called St. Elmo's fire. You are most likely to see St. Elmo's fire streaming upward from lighting rods, ship masts (how it's name originated) or airplane wings, where the voltage (electric field strength) is concentrated, and where lightning is more likely to strike. This is why you want to stay well clear of trees if you're caught in a lightning storm, and it is a good idea to crouch or lie down, so that you are not a pointy surface.
Streamers and leaders glow violet, much like the colour below.
Finely branched filaments of plasma (ionized air) lead from a Tesla coil, above, a machine that can attain much higher voltage than the Van de Graaff generator described in the preceding article. The plasma in this case can be described as a corona discharge. It is not a spark, just like leaders and streamers are not sparks. The electric field around the metal tip (right) weakens with distance, allowing the electrons and positive ions in the plasma to recombine into neutral atoms at the periphery of the corona. However, a spark will happen if the metal tip comes close enough to another conductor at a lower electric potential. The conductor could be your body if you stand within about ten feet of a typical unit - a dangerous and potentially fatal situation. When a conductor is close enough, one or more filaments of ionized gas will connect with the other object. An electrical circuit is created and a spark, more accurately called an electric arc in this case, will follow, as an ongoing electric discharge. Unlike a Van de Graaf generator, a tesla coil is plugged into a typical AC (alternating current) outlet so it maintains current at a very high voltage.
Stepped leaders and streamers operate like a coronal discharge. The lightning bolt itself is a momentary electric arc, better described as a spark. When a leader and streamer eventually meet up, a complete path, or circuit, of conductive ionized air provides a path for the cloud bottom to discharge its intensely built-up negative charge.
What Is Lightning?
When a discharge path becomes available, the thundercloud can release the tremendous charge it has built up, like a powerful battery attached to a thin wire. Tremendous current overloads the "wire" which is actually a small-diameter tube of ionized air, heating it up to the point of exploding while exhausting the charge in the cloud, like a discharging battery. Countless accumulated electrons fly down the plasma "wire" as fast as they can. There is enormous current, voltage and heat in a typical lightning strike.
Volts, Current and Heat
A lightning bolt bridges a potential difference of several hundred million volts but the voltage can vary widely. It can transfer about 1020 electrons in about a millisecond, representing a current of about 10,000 amps (A), but the current in each bolt varies and currents up to 200,000 A have been recorded. The plasma "wire" is very thin (around the width of your thumb). Electrons experience incredible acceleration as they "slide down" the intense electric potential "slope." This electron movement creates intense friction, which generates an enormous amount of heat, around 30,000°C, within the lighting bolt.
Lightning glows bright bluish-white mostly because of its temperature. Like the glowing filament in an old-fashioned light bulb, the light from lightning is an example of incandescence. Atoms and partly ionized atoms within the lighting channel absorb energy and vibrate intensely. The electrons within the atoms have both electric and magnetic properties. When the electrons vibrate, they set up electromagnetic oscillations, emitting electromagnetic radiation (light), shown below.
A vibrating electron can be represented by oscillating charge, q, shown on the left. It sets up two oscillating fields - an electric field oscillation, E, and a magnetic field oscillation, B. K is the direction of the light (There are a lot of photons of light emitted, each streaming off in a different direction).
In this way the super-hot air particles act like a black body. If you'd like to know more about black body radiation, try Atoms Part 3: Atoms and Heat. The bluish-white colour of the light emitted indicates that the temperature of the air in the column, is between 10,000°C and 30,000°C - much hotter than the surface of the Sun. In addition to vibrating, atoms within the column are also highly excited, lending a purplish tinge to the bolt. This other type of light is technically called luminescence.
Neutral air atoms and molecules surrounding the plasma path are superheated by friction from the lighting bolt. A superheated gas is a gas at a temperature higher than its boiling point. In this case, the air almost instantly reaches temperatures of 10,000°C or more. It doesn't have time to expand, as it normally would, so it is compressed, up to 100 times normal atmospheric pressure. Any gas that is confined will experience increasing pressure with increasing temperature. The compressed sleeve of air explodes outward, sending shock waves through the air in every direction. The boom of thunder is the sound of this explosion, as these shock (compression) waves reach your ears.
Thunder Is a Shock Wave
The shock wave from a superheated gas explosion traveling along a lightning channel is the thunder you hear soon after a strike. Sound travels much more slowly (around 1300 km/h) than light does (300,000,000 m/s or about a billion km/h), so thunder is delayed. The drawn out roll of thunder you often hear is caused by the delay of sound coming from various sections along a long jagged and perhaps forked lightning bolt. The lightning bolt itself travels at more than 220,000 km/h. That's very fast but it's not instantaneous. It still takes time for the explosion to travel up a typical cloud-to-ground lightning bolt that is several kilometres long.
The Flash Is The Return Stroke
Although lightning is the downward rush of electrons from the cloud to the ground, the shock wave (explosion) begins at the bottom, near the ground and travels upward. The leader travels downward relatively slowly, followed by the electric discharge traveling much faster downward, as the bright light associated with the discharge travels back up. The explosion associated with the bright light traveling upward is called the return stroke. This might sound strange at first, considering that the current in the bolt flows downward. The return stroke travels upward because the electrons are accelerating in the direction of the electric field, so at the bottom, they are moving "explosively" fast. The NOAA (National Oceanic and Atmospheric Administration) website uses the following traffic analogy to explain this phenomenon:
"This is similar to cars that have been stopped by an open drawbridge. Once the drawbridge is opened for traffic, cars initially start moving forward toward the bridge but movement across the bridge works its way backward through the line of stopped cars."
Click on the NOAA link above to see two lightning animations that show the difference between charge movement and visible flash migration.
While the light traveling up the bolt seems to come all at once, the sound can be drawn out over several seconds because sound from higher and higher up the bolt takes seconds longer to reach your ears through the air. The sound also echoes off hills and buildings, etc., and this contributes to the rumbling sound as well.
Multiple Lighting Strikes
A lightning bolt usually discharges an entire region of cloud, but multiple identical strokes are common, with up to forty strikes occurring successively within the same channel, as long as it remains ionized. If you look carefully at a lighting bolt, you might notice not only the main leader glowing but also several secondary leaders glowing as well, those that aren't reaching the ground. These secondary leaders also become charged, contributing current to the main leader during the first strike, but they do not contribute to subsequent strikes. Subsequent strikes through the same channel are confined to the main leader. Multiple strikes can occur so close together than they often appear as one long lightning strike. In fact, most lighting is composed of three to four rapid-fire strikes, making the flash appear to flicker. Each strike discharges a roughly spherical region of the cloud. Each repeating strike discharges new more distant horizontal regions within the cloud, resulting in overall horizontal charge motion within the cloud, at least for cloud-to-ground negative strikes.
Current and Voltage Vary
How much current a lighting bolt carries depends on the strength of the storm, how much charge the cloud builds up through the churning of air inside it, in other words. It can vary between 5000 A and 200,000 A.
The voltage depends on the length of the lighting bolt as well as the diameter of the bolt, which likely varies between 2 and 5 cm.
The length of a cloud-to-ground lightning bolt roughly depends on the how high up the negatively charged bottom of the cloud is. The cloud bottom and ground surface are like the two plates of a capacitor. A capacitor stores charge (electrical energy), a bit like a battery does. But while a battery can induce charge movement, a capacitor can only store charge.
A capacitor is composed of two conductive plates (the bottom surface of the cloud and the ground surface in this case) separated by an insulator (air). This diagram, shown below right, is similar to a diagram I used in the previous article.
Capacitance, the ability to store charge, increases with the surface area of the plates and decreases with the distance between the plates. In other words, the electric field energy decreases as the plates are moved further apart, so the voltage needed to produce lightning increases as the distance between the ground and cloud bottom increases.
Decreasing the diameter of the lighting bolt also increases the voltage because you are increasing the resistance of the channel. The diameter of a lighting bolt may vary by a factor of almost three times. This translates into a difference in resistance of almost twelve times, so a 5 cm diameter lightning channel should experience roughly 1/12 the resistance of a 2 cm diameter channel, or 12 times more voltage, assuming the same current.
Lightning Maintains Its Awe-Factor
I hope you enjoyed this lightning dissection. As long as humans have been curious, lightning has probably been a great source of fear and wonder. The First Nations' Thunderbird, the Norse god, Thor, the Roman god, Jupiter and the Greek god, Zeus all tell stories about lightning. What they also tell us is how long we have been trying to understand it. A few centuries ago, these myths were joined by scientific explorations, and as you can see, the journey of wonder continues. There is an abundance of lighting research currently underway, leading researchers down many scientific rabbit holes - electricity, meteorology, chemistry and atomic theory. The reward is a glimpse into one of Nature's most awesome mysteries.