Friday, January 25, 2013

Lightning Part 3: The Lightning Bolt

Lightning bolts like these three simultaneous cloud-to-ground strikes during a storm in Toronto, Canada, are mesmerizing. Have you wondered what exactly is going on inside one of these?

(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 M√ľnster, 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.

Exploding Air

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.

Except now the field is reversed  - an electric field grows in strength between the ground and cloud as negative charge builds up in the cloud and positive charge builds up in the ground. Energy is stored in this field. 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.

Wednesday, January 23, 2013

Lightning Part 2: Lightning is Electricity

Lightning is all about electricity. We need to understand (sometimes confusing) concepts like charge, current, voltage or potential difference, resistance and electric fields in order to pry open the mystery of how a bolt of electricity can appear all of the sudden out of storm cloud stuff.

Everything Is Electric

We tend to think of electricity as something out there somewhere, but it isn't. Every single building block of matter, every atom, comes with electricity built into it.

Every atom contains two kinds of charged particle - the positively charged proton and the negatively charged electron. Their electric charges are equal and opposite, so a neutrally charged atom contains an equal number of protons and electrons. Protons are relatively heavy and confined to the atomic nucleus, whereas electrons move about the nucleus, attracted by the positive charge of the protons, as shown in the Rutherford model of a lithium atom, below.

Tremendous force is required to remove a proton from an atom. Some electrons, on the other hand, can be sloughed off some atoms fairly easily (but other atoms hang on to their electrons much more tightly; check out Atoms Part 4A: Atoms and Chemistry - Atomic Orbitals and Bonding for the reason why). These free electrons are mobile. They will move in the direction of an electric field, a region of force that acts on charged particles and is created by charged particles. Each electron has a charge of 1.6 x 10-19 coulombs (C). A bunch of electrons moving together, a flow of charge in other words, is an electric current. Its flow rate is measured in amperes. One ampere (A) equals a rate of one coulomb/second.

Electricity is usually described as the activity of electrons. However, electrical phenomena are not limited to free electrons. Many molecules can split into positively charged and negatively charged parts, called ions. These molecules are called ionic compounds and they include many compounds that dissolve easily in water, such as table salt, NaCl, which dissolves into positive Na+ ions (11 protons and 10 electrons) and negative Cl- ions (17  protons and 18 electrons). Ionic compounds are explored in my article, Atoms Part4B: Atoms and Chemistry - Ionic and Covalent Bonds. Lightning, as we're about to see, involves not just free electrons but ions as well.

Using a Water Analogy To Understand Electricity

Electricity itself is invisible (the bright flash of lightning comes from another process as you'll see) so sometimes it helps to understand electricity by using a water analogy. We can think of current (C/s) as the rate of charge flow, in the same way as we would measure the flow rate of water in litres/second.

In order to understand voltage, let's use a water stream analogy: Like the flow of water down a stream, electricity is the flow of electrons "down" an electrical conductor, for example, a wire. A conductor is any material that lets electrons move through it. What makes a material a conductor has to do with what kinds of atoms are in it and how they are arranged.

Another Analogy: Comparing Lightning To A Battery

Like water flowing down a stream, the current in a wire must be continuously replenished or it will stop. Somewhere "upstream," something must generate the current. This can be a battery, for example. A battery acts like higher elevation does for a stream. Water won't flow down the stream unless its starting point is higher than its ending point, unless gravitational potential energy is available in other words. Likewise, a battery "pumps" electrons "upstream" to a higher electric potential energy. This is also called electric potential for short, or voltage. A battery stores electric potential energy by building up two separate regions of different charge. One region contains more free electrons than the other region. These electrons want to move back across the two regions to equalize the charges but they can't. Keeping them separate adds potential energy to the system.

The simplest battery is composed of a single electrochemical cell, shown below.

When the two electrochemical half-cells (the two tubs of solution with strips of metal stuck in them) are connected by a wire, the potential difference (voltage) causes charge to move through the wire. The black arrows show this current. We can also call this current electrical discharge, which means any flow of charge through a gas, liquid or solid, in this case a solid metal wire, although the term discharge generally implies the current is temporary.

The potential difference is created as zinc metal dissociates into zinc (Zn2+) ions and free electrons in solution. Electrons (e-, negative charge) build up at the anode as positively charged zinc ions build up in the solution. At the cathode, copper atoms are deposited on the copper strip, consuming free electrons (and copper ions, Cu2+) from the solution. The overall number of electrons in each tub stays the same but the number of free electrons changes. Sulphate ions (SO42-) move from right to left through a semipermeable barrier to balance the electron flow through the wire. This current flows from the anode, where free electrons accumulate, to the cathode, where free electrons are in short supply.

Energy is needed to move electrons through the wire and light up the bulb. That energy ultimately comes from the two chemical reactions in the tubs. The oxidation reaction of zinc releases more energy than the reduction reaction of copper uses up. The electrochemical cell will keep making current until the chemicals needed for the reactions are used up. If the wire is removed, the reactions in the two half-cells will quickly reach equilibrium states as their products build up in each solution.

The thundercloud is a little bit like a battery that is not hooked up to a wire, with some exceptions. Rather than chemical reactions, the movement of water molecules within the cloud builds up separate pockets of different charge. Water molecules have a unique ability to attract or lose an electron, so they can form both positive and negative ions. Like the battery, this builds up potential difference. The cloud has no electrodes, but water molecules both supply and attract electrons. In the top of the cloud, positively charged water ions (possibly ice pellets) build up. In the bottom, negatively charged water ions (possibly rain droplets) and free electrons build up. Like a battery that is not hooked up to a wire, there is no current flow, at least for now. And like a battery, there is a build-up of potential difference (voltage) and an electric field is created (wherever there is potential difference, there is an electric field). Unlike a battery, with a limited supply of chemicals, there is a huge supply of charge-building water molecules in a thundercloud.

A Water Pipe Analogy Helps to Explain Current and Resistance

Let's move on to a water pipe analogy to understand current and resistance. A thin water hose has more resistance to water flow than a hose with a bigger diameter. The flow rate of water molecules through the second hose is higher. Put another way, decreasing the resistance of the hose increases the water flow. In the same way, decreasing the electrical resistance of a conductor by using a thicker wire increases the flow of electrons, or current.

A hose that's twice as long will have half the water flow rate because we've doubled the friction that the water experiences. If you double the length of wire in the electrochemical cell shown above, you double the resistance against the charge flowing through it, and you reduce the current by half.

What happens if we use two identical water pipes but connect one pipe to a water source that's higher up than the other one? Here, we add gravitational potential energy to the water source of one pipe. There will be higher water pressure through the higher pipe. All other things being the same, the rate of water flow depends on water pressure, so the higher-end pipe will have a higher water flow rate over the lower end one. If we compare two wires the same way (both identical), the amount of current now depends on the voltage applied across the wire. If we increase the voltage across one wire, it will have more current through it than the other wire.

This is how Ohm's law, current = voltage/resistance, works.

Air Is An Electrical Resistor

Some materials allow electrons to move through them more easily than others. That is why copper is commonly used in home wiring. It is a good electrical conductor, like most metals are. In these materials, the outermost electrons are shared among atoms, and move around easily. To learn more about how electrons move in metals, try my article, Atoms Part 4D: Atoms and Chemistry - Polar Covalent and Metallic Bonds, and scroll down to Metallic Bonding: A special Kind Of Covalent Bond. Other materials, called resistors, tend to resist the flow of current through them. Rubber, glass and air are all good resistors. Outer electrons in their atoms tend to stay fixed in place.

Air is an excellent resistor. It will not allow current to flow through it, and this leaves the storm cloud with its ever-building pockets of charge, and ever-increasing electric potential, in a quandary. The system must eventually find a way to release all this energy, but how? It can't be conveniently hooked up to a wire like a battery can.

Comparing Lightning To a Short Circuit

Lightning is a temporary flow of current, an electrical discharge. Electrons rush from where there are too many toward where there are too few. It is a bit like a short circuit between two differently charged bodies. A circuit is simply a path for electrons to flow. The wire connected to the electrochemical cell provides a path for electrons to move. A short circuit is technically an abnormal connection, or path, between two sections of an electrical circuit that are at two different electric potentials. That accidental path is usually short and it offers little or no resistance to the circuit.

An electric circuit requires a continuous path for electrons to flow, left.

If you connect a wire to any one of the two terminals on a battery, nothing will happen. This is an incomplete or broken electric circuit. The same thing happens when you flip off a light switch in your home. The switch opens the circuit, so the current stops flowing to the light bulb. The two circles and line at the bottom of the circuit below left is a symbol for an open (off) switch. A lightbulb is an example of an electrical load. Sometimes people think that the light switches off so fast because the electrons themselves travel near the speed of light through the wire. They actually travel quite slowly, on the order of centimetres per minute. A light shuts off almost instantly because the energy of the circuit travels at almost the speed of light. This effect is similar to that observed in earthquakes and tsunamis - the energy (the wave propagation itself) travels very fast but the energy-carriers (electrons, soil/rock particles and water molecules, respectively) do not.

While a battery maintains a constant voltage, an electrical load, such as a light bulb, resistor, or motor, increases the resistance and decreases the current through the circuit. If no load is present, the current could overload the wire.

We can think of a circuit with absolute minimum load, or resistance, as a very thick wire attached to a large battery. There is almost nothing to reduce current, so a very large flow of electrons goes through the wire and the battery itself. The wire will heat up and the battery will experience increased internal resistance, as too many electrons push into one electrode and out the other one.

This can cause the battery to heat up and possibly explode, shown right. This is the danger when you accidently cross the wires when you jumpstart a car battery. If you connect the electrodes positive-to-negative and negative-to-positive, you create a circuit of a high voltage battery connected by thick wire. If we use a too thin wire instead of a thick one, we still have a dangerous problem: The wire won't be able to handle all the current going through it. A smaller wire has higher resistance than a larger diameter wire, but too much current will cause it to break down and fail. It will create such intense friction, as too many electrons try to flow through it at once, that it may melt or explode, shown below.

A highly charged thundercloud will eventually discharge through any path of least resistance. As we will see, it will make its own "wire." And when it does, there will be very little resistance (load) to slow down the rate of discharge. The lightning channel that will eventually form is surprisingly small in diameter. It is like a very thin wire attached to a very high voltage battery with no load - a powerful, and potentially deadly, electrical phenomenon.

Lightning Is A Static Discharge

The thundercloud is building up charge but no lightning has struck yet. There is no electrical current. At this point we are really talking about the build-up of static electrical charge. Static electricity is a build-up of charge in a material. There is very little flow of electrons, or current, as the charge builds up. There is, however, significant voltage.

You have probably experienced a consequence of static electricity first-hand - an electrical discharge. When you walk along a carpet floor, rubbing your feet against it, and then touch someone else you might feel (and perhaps see) a tiny spark, especially in the winter when the air is dry. The spark is called an electrostatic discharge. Lighting is an example of a gigantic spark.

Static Electricity Is A Separation Of Electrical Charge

When you rub your feet in the carpet, some of the outermost electrons in the carpet atoms are removed from it and deposited onto your feet as your feet and carpet rub together. The rubbing increases the contact (electron exchange opportunity) between the two materials. Your feet (your whole body actually) build up negative charge while the carpet becomes positively charged. You are building up electric potential energy just like a battery. When you touch someone (who is neutrally charged) the electrons will travel out through the point of nearest contact (your finger for example) into that person because there is an electric potential difference between you two. You might even see a spark cross the tiny gap between your finger and the other person.

Air is a very good electrical insulator, a material whose electric charges do not flow freely, but the tiny bit of air between your finger and the other person's body reaches a point where it can no longer provide enough electrical insulation between the increasing potential difference between the two regions of charge. There is high enough electric potential energy to overcome the resistance of the air. Current (the spark) flows and returns the system to a state of electrical equilibrium (lowering the potential energy). Molecules of water vapour in air allow electrons to move through the air more easily. In humid air, your feet still build up negative charge but water is an excellent conductor so excess electrons on your body can enter microscopic water droplets in the air and be carried away, dissipating the charge before it builds up very much. This is why electronic equipment is more likely to be damaged by electrostatic discharge when the air is dry. When air is very dry, charge can build up to a damaging voltage level before it discharges. There is no "water droplet" release valve.

You might wonder then, why the humid air around a storm cloud doesn't allow the charge do dissipate in the same way, before lighting can form. Some charge certainly does dissipate this way, as it does around waterfalls. Water turbulence allows the mist around waterfalls to acquire a negative charge that dissipates as the mist spreads through the air. Water particle movement in the air around a thunderstorm is not sufficient to undo the enormous charge build-up inside the cloud. Powerful drafts around and within a developing storm promote charge build-up.

A thunderstorm dies when the charge-building updraft mechanism weakens and is overwhelmed by downdrafts. The thunderstorm loses energy and dissipates. Charged water particles carry off and dissipate any residual charge, as opposite charges recombine once again into neutral atoms and molecules.

Why some materials, like your feet, get negatively charged and other materials, like carpet, get positively charged has to do with how the electrons are arranged in the specific atoms and molecules in those materials.

Electrons in atoms are arranged in energy shells. Only electrons in the outermost shell can be exchanged between atoms. These electrons are furthest away from the positive nucleus so they are not so tightly electrostatically bound to it. Other nearby atomic nuclei may offer enough attraction to them to get them to "jump ship." In some materials (carpet), they can be removed, especially if another material, which tends to attract electrons (feet), is in contact. Some of the atoms in your feet have sparsely populated outermost electron shells that would be more stable if one or more electrons were added to them. They attract electrons from other materials. This effect is called the triboelectric effect.

The triboelectric series, shown left, tells you which materials tend to lose electrons (positive) and which materials tend to gain them (negative).

When your feet rub against the carpet, you increase your electric potential energy (voltage). Your body wants to shed its excess electrons and it will do that if something at a lower voltage gets close enough. A Van de Graaff generator, shown below right, present in many high school physics classrooms, generates electrostatic charge, just like your feet did.

(GDFL; Wikipedia)
A demonstration model like this one can attain a potential difference of hundreds of thousands of volts, but you can touch it harmlessly because the maximum current is very tiny. As a pulley drives a belt, a positively charged tiny metal comb at the bottom sloughs electrons off the belt (usually silk, which loses electrons easily), so that the belt is positively charged by the time it reaches the top. A tiny metal comb at the top allows electrons to flow away from the outer metal dome, leaving it positively charged. When a negatively charged wand is brought close enough to the dome, a spark will jump between it and the dome, as shown below.

Lightning is exactly the same phenomenon as the carpet and Van de Graaff generator examples, but on a much larger scale. In this case, charge separation occurs within just one kind of material - water molecules. Most researchers believe that as water drops or ice pellets fall through rising drops and pellets in a storm cloud, they experience a great deal of contact with one another. Although the mechanism(s) isn't fully understood, falling water molecules somehow gain electrons at the expense of rising water molecules. This builds up a large charge separation within the cloud, with the bottom of it becoming negatively charged and the top of it becoming positively charged.

Now that we have a handle on where electricity comes from and how it works, let's focus again on the thundercloud. Lightning is just about to strike.

Most researchers agree that water molecule-molecule contact is the root cause of charge build-up in a thundercloud. However, what kinds of movement are involved, and what kinds of objects (raindrops or rain against ice, snow or sleet) are involved, is not well understood. A scientific review paper by physicist and atmospheric physicist Clive Saunders (2008) offers several mechanism possibilities.

The top of a thunderstorm cloud, which is technically called a cumulonimbus cloud, climbs rapidly up to around 6000 m (here in Alberta) and to over 23,000 m in the tropics where the air column itself extends much higher in the atmosphere. A typical rising cumulonimbus cloud looks like the one below.


At this high altitude, any liquid water present freezes and the frozen portion at the top of the cloud tends to become positively charged versus the lower (liquid) portion. Water and ice molecules behave differently in a charged environment. Although water is just one material, two of its physical states - liquid water and ice - differ very slightly in their triboelectric nature. This means that water tends to accept or lose electrons depending on its physical state. This is well documented but it seems to be very complex. Researchers also know that the molecules in ice are less densely packed than they are in water. This is why water expands when it freezes. It also means that ice exhibits something called higher static charge permeability, between 10 and 100 times higher than water due to an effect called proton-hopping. This difference could be connected in some way to cloud charging, but the mechanisms involved are far from understood.

Cloud Charge Separation Generates An Electric Field

The entire thundercloud region contains an electric field, where the potential energy of the field at any point is the electric potential energy measured at that point. Charge separation always creates an electric potential and an electric field. The field is generally negative at the bottom of the cloud and positive at the top of the cloud, but it's not necessarily uniform or straight up and down. Cloud-to-cloud lightning may occur across a roughly horizontal electric field, for example. Electrical charge is concentrated around curved objects, as we'll see, and that may add curves to the electric field. Here we are focused on cloud-to-ground lightning so this field can be represented as a series of parallel lines, as shown below right (imagine the cloud superimposed on the diagram).

The direction of cloud's electric field is downward, shown by the arrows.
An electric field exerts a force on charged particles. The black arrows on the lines right represent the direction of the force on a (test) point of positive charge. As negative charge accumulates in the bottom of the thundercloud, electrons experience an upward force. If the cloud-charging mechanism suddenly stopped, they would simply migrate upward while positive ions migrate downward and the charges would neutralize. But the charging mechanism is going full-blast, so the electrons can't move upward. Eventually, however, these electrons are going to go somewhere. They will stream down to the ground instead. Why? The electric field is negative at the bottom of the cloud, so it induces a positive charge on the surface of the ground, as shown below left. Electrons here rush deeper underground, leaving an over-abundance of positive ions at the ground surface.

Beneath the cloud, the electric field direction is reversed. Electrons want to go downward. The strength of the field's force directly depends on the amount of charge build-up. Under a storm cloud, the electric field and the electric potential become very powerful. This is why you might feel the hairs on your arms standing up. Your hair is like the carpet mentioned earlier. In an electric field, it tends to become positively charged, so it will "repel" the ground and be attracted to the underside of the cloud above you, and lift up. This is also a warning sign that you are about to be struck! It means you are in a region of extreme positive charge and lightning could strike you within seconds.

Air Resists Lightning

When charge separation builds up, the electric potential energy grows. Like any system in physics, there is a built-in push toward finding the lowest energy state possible, and nature wants to find that now. It can do it by creating an electrical discharge from the cloud to the ground. All the excess electrons in the bottom of the cloud can simply flow down into the ground, neutralizing both the cloud and the ground and eliminating the electrical potential. All this would be far less dramatic (and there would be no lightning) if air cooperated with this plan, but it doesn't.

The gas molecules in air strongly resist any movement of charge through them. The reason they resist is because the molecules created when they bond exhibit very stable electron configurations. 99% of air is composed of the diatomic gases nitrogen (N2) and oxygen (O2). Electrons are not free to move between these molecules, and they are not easily transferred between them, as they are between water molecules.

An electrical discharge is a temporary flow of electrons (or current) through a material. There is no way for electrons to flow through air when its electrons stay stubbornly fixed in place within the molecules. This makes air an excellent electrical insulator. Resistivity is a measure of how strongly a material resists electric current flowing through it.

This picture can change, however. Like any insulator, air can experience electrical breakdown if it is subjected to a sufficiently intense electric field.

The figure shown below illustrates what happens when a block of Plexiglass ® is subjected to intense voltage (an intense electric field).

(Berk Hickman;Wikipedia)

Plexiglass ® is normally a good insulator. It has a resistivity of 1 x 1013 ohm metres, versus air which varies between 1 and 3 x 1016 ohm metres. Plexiglass ® strongly resists any movement of charge through it. When high enough voltage (a strong enough electric field) is applied to it, it begins to break down. The electrical breakdown of the Plexiglass ® creates a beautiful hair-like discharge pattern that is thought to grow finer and finer, extending all the way to the molecular level. All the white "hairs" are where the Plexiglass ® has broken down into conductive material. It is also where an electrical discharge eventually flowed. Notice the similarity between this pattern and the fine branching of many lightning bolts.

When air is subjected to a sufficiently intense electric field, finely branched paths within it break down and become conducting paths. It does this through a series of remarkable steps.

Changes in Air Set The Stage For A Lightning Strike

Air is an electrical insulator and it exhibits a high dielectric strength. The dielectric strength of a material is similar to, but different from, its resistivity. It is a measure of the maximum electric field strength a material or gas can withstand before it breaks down and its electrical insulating properties fail. The dielectric strength of air is affected by dust, temperature, pressure and water vapour. The dielectric strength of the typical warm moist air under a thundercloud is about 3 million volts/m. Increased air pressure, for example, increases this value, while humidity (water vapour content) decreases it (remember that water is a good conductor). Tiny differences in the dielectric strength of air due to dust, humidity fluctuations, etc., might account for the intricate jagged path that lightning often takes, as if it is stepping from region to region across the sky (while generally traveling in the direction of the electric field) where resistance to its discharge flow is minimized.

We can call the cloud and the ground - two charged bodies separated by distance - electrodes. The shape of the two electrodes involved affects electric discharge. Pointy objects, like your finger and someone else's sharp elbow, make good electrodes, while flat surfaces don't. The cloud bottom and the ground are both generally poor electrodes due to their flatness, but tall pointy trees, lightning rods and the wing tips of planes, for example, change the picture. They encourage electric discharge by allowing charge to concentrate in one spot, as shown right.

This rough diagram demonstrates how a pointy or curved surface locally enhances the strength of the electric field, and the voltage, in that region and it encourages electric discharge - a spark or lightning - to take place there.

Now we're ready to get to the most exciting part,  in Lightning Part 3: The Lightning Bolt.

Monday, January 21, 2013

Lightning Part 1: Lightning Begins With a Thundercloud

Lightning is a source of endless wonder.


It has been studied scientifically for centuries and yet mysteries about the mechanism of lightning remain. This makes lightning a fascinating tool we can use to explore the ins and outs of how electricity originates from the movement of subatomic particles called electrons. This subject extends quite well from the "Atoms" series in Scientific Explorer (scroll down on the right to access these articles).

There are many different kinds of lightning, some of which are rare with unworldly names like sprites, elves and blue jets. Here we will focus on one familiar type of lightning - negative strike cloud-to-ground lightning. It is not the most common form, cloud-cloud lightning is, but the cloud-to-ground lightning mechanism is better understood among experts.

Where Does Lightning Come From?

In order to understand lightning we need to start at the very beginning of a thunderstorm. To predict one, meteorologists look for three key ingredients: warm moist air, an unstable air mass and something that will trigger a rapid movement of air upward. This trigger could be an uplift of air over mountains, converging winds, or an uplifting weather front. Daytime heating tends to trigger upward air movement so most, but not all, storms occur in the late afternoon or evening.

Why is upward-moving warm moist air important? Atmospheric temperatures get colder as one travels upward. As temperatures change, materials tend to undergo a physical change, a process called phase transition. You already know this process when it comes to water: When ice is placed in a pot over high heat, it melts into water and eventually evaporates into steam. The energy of the water molecules themselves, called enthalpy, increases as water is heated, as shown in the diagram below left.

This process happens not just with water, but with many other compound materials and every elemental material as well. Even gold, which we know as a solid, melts and, if it's hot enough, evaporates into a gas. If you are curious, Wikipedia makes it easy to compare the temperatures at which various materials melt into liquid and evaporate into gas. Just look at the right-hand side of the element's page. For example, you'll see that gold melts at 1064°C and evaporates into gas at its boiling point, 5173°C. That's much hotter than lava, which is usually around 1000°C. Water can exist as liquid (water droplets), solid (ice pellets, hail, sleet, snow) and as gas (water vapour) in air.

When warm moist air lifts upward, it cools and the water vapour in it begins to condense into tiny droplets as the air reaches its dew point. The graph below right compares dew point temperature with air temperature and humidity.

To understand dew point, think about dew on the lawn in the early morning. Let's say yesterday's daytime temperature was 20°C with 60% relative humidity. The dew point temperature would be 12°C, so as the overnight temperature dropped below 12°C, the relative humidity increased to 100% and water vapour condensed out of the air and fell as droplets onto the grass. Water vapour in air is continuously evaporating and condensing. When the evaporation rate equals the condensation rate, the air is at 100 % relative humidity. This means also that when the dew point of air approaches its temperature, the air has 100% relative humidity (see the darkest blue line right). Like dew on grass, clouds are regions of air where the rate of condensation of water vapour approaches the rate of evaporation.

Dew point is a good storm indicator. Warm air with a high dew point means it's humid and it has a lot of water molecules in it. Humid air is an essential ingredient in building up a thunderstorm. As this air rises, the water in it condenses and it releases energy, called enthalpy of vapourization. This energy allows the water droplets condensing out in an air pocket to cool less than the surrounding air, and this means the droplets can rise even higher. The water condensation process adds energy to a building thundercloud.

As air is swept high up in a thundercloud, the temperature can drop so fast that water vapour can change directly into tiny ice pellets, a process called deposition. This contributes even more energy to the growing storm cloud, because it releases additional energy - enthalpy of vapourization and enthalpy of fusion (the energy released when a water molecule freezes).

Gravity causes the ice pellets to fall. When they do, they collide with and sometimes stick to other pellets, growing in size on the way down. Water drops and ice pellets are carried upward in pockets of rising air and downward by gravity. This up and down raindrop/pellet motion marks the moment when the lightning-making machinery switches on. High humidity means there are lots of these drops and pellets moving in the air.

Not all lightning storms may involve the movement of ice pellets. Water droplet motion alone may be sufficient. Some researchers believe that ice formation is essential while others are not as certain. Most researchers, however, agree that the up/down motion is required to form lightning. The lifecycle of a typical thunderstorm is shown below.

The mature stage, above center, is marked by turbulence created by updrafts and downdrafts, resulting in winds, severe downpours and lightning. Next, we'll explore how water and ice movement in a thundercloud make lightning, in Lightning Part 2: Lightning Is Electricity.