Lightning
Porter Johnson

Typical Lightning Flash
Source:  http://www.chaseday.com/lightning.htm


Lightning, the thunderbolt from mythology, has long been feared as an atmospheric flash of supernatural origins; the great weapon of the gods.  The Greeks both marveled and feared lightning as it was hurled by Zeus. For the Vikings, lightning was produced by Thor as his hammer struck an anvil while riding his chariot across the clouds.  Thunder and lightning are mentioned several times in the Bible.  Here is one especially graphic reference:

And the LORD shall be seen over them, and his arrow shall go forth as the lightning; and the Lord GOD shall blow the trumpet, and shall go with whirlwinds of the south.
            Zechariah 9:14, Authorized King JamesVersion

Today, scientific rather than mystical techniques are used to explain lightning with experimental procedures and replacing intuitive concepts.  Yet, we remain in awe of lightning which still flashes in its mystery, and rightly so.

G W Richmann, a Swedish physicist working in Russia in 1753, proved that thunderclouds contain electrical charge, and was killed when lightning struck him. Benjamin Franklin performed the first systematic scientific study of lightning during the second half of the 18th century.  While others had noted the similarity between laboratory sparks and lightning, Franklin was the first to design an experiment that conclusively proved the electrical nature of lightning.  In addition to showing that thunderstorms contain electricity, by measuring the sign of the charge delivered through the kite apparatus, Franklin concluded that the lower part of the thunderstorm was generally negatively charged.  Lightning current measurements were made in Germany over a century ago by Pockels, who analyzed the induced magnetic field to estimate current values. Lightning research in modern times began with the measurement of the induced electric field by  C T R Wilson, who won the Nobel Prize in Physics for invention of the Cloud Chamber.

At any time, there are more than a thousand thunderstorms around the earth, producing some 6000 flashes of lightning per minute.  This continuous discharge occurs because of the build-up of negative charge high in the earth's atmosphere, and positive charge near the earth's surface.  During a thunderstorm the negative charges migrate to the bottom of the cloud, while positive charges go to the top. When the negative charge becomes large enough, it flows to the positively charged ground below.


Questions Concerning Lightning


What is Lightning?

Lightning is a transient discharge of static electricity that serves to re-establish electrostatic equilibrium within a storm environment. A thunderstorm is a very turbulent environment, with strong updrafts that transport water droplets up in the cloud, while ice particles descend from the frozen upper regions.  As they collide, electrons shear off the ascending water droplets and collect on descending ice particles. [A similar effect occurs when you rub your feet across a carpet before touching a door knob.]

As positive charge at the top of the cloud and negative charge at the bottom increase, the electric field inside the cloud grows in strength.  Because the atmosphere is a very good insulator, a tremendous charge must be built up before electric discharge [lightning] can occur.  Most lightning occurs within the cloud itself, because the electric fields within the cloud are typically much greater than those between the bottom of the cloud and the surface of the earth  Here is a Biblical reference to cloud-to-cloud lightning:.

For as the lightning, that lighteneth out of the one part under heaven, shineth to the other part under heaven; so shall the Son of man be in his day.
            Luke 10:18, Authorized King JamesVersion

Cloud-to-ground lightning can involve a transfer of positive charge to the ground (about 10% of the time).  Usually, there is only one stroke for positive flashes, which most often occur away from the central rain shaft, and which are generally more damaging than negative flashes.  It is believed that a large percentage of forest fires and power line damage is caused by positive flashes.

Lightning sometimes strikes the ground and tunnels downward into the earth.  The intense deposition of energy causes the sand particles to come together to form a tubular crust [the shape of the bolt's path] called a fulgarite, after the Latin word for lightning.  Some fulgarites are more than 3 meters long.  Lightning may flash-heat ferromagnetic materials in minerals above their Curie temperatures, effectively resetting the magnetic fields trapped in lava flows.  In addition, generates reactive chemicals in the atmosphere that enhance the production of acid rain.

For more information see the websites

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When and where is Lightning more likely to strike?.

You should avoid Florida, Texas, and North Carolina, the states with the greatest number of lightning deaths.  Especially avoid these states in June [worst month], followed by August, July, April, and September.  Central Florida is generally regarded as the Lightning Capital of the United States.  The most common situation for a lightning death or injury in Florida was found NOT to be in the heaviest rain area with lots of flashes, but before of after the time when rain and lightning was most intense.

Lightning Map of the United States

Avoid open spaces, fields, and ballparks, standing under trees, golf courses or heavy road equipment, telephone booths, as well as boating, fishing, and other water-related activities.  The deadliest lightning strike ever was of a Boeing 707 near Elkton MD on 08 December 1963.  The plane then crashed, killing 81 people.

Lightning occurs more frequently over land than over water.  According to NASA's Lightning Imaging Sensor, about 90 percent of lightning occurs over land, in spite of the fact that 75 percent of the earth is covered with water.  Increased lightning activity over land is most likely due to enhanced convection, by which water carries excess energy into the upper atmosphere.  Over land there is more ice production, and consequently more lightning.

For more information see these websites:

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What is the energy source of lightning?

The earth's charge would dissipate in less than an hour at this rate, except that lightning recharges the earth by delivering negative charge back to the earth.

Lightning is caused by a build-up of charge in the atmosphere, generating electric fields that cause the formation of an electrical channel through air.  The channel is actually begun with a leader carrying a relatively low charge.  This initial pulse [leader] usually fails to "even out" the charge imbalance.  The resulting unbalanced charge now flows preferentially along the same path because of the ions created by the leader, to form a "return stroke" and flow until the charge imbalance is reduced

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What determines the color of Lightning?

The curious red flashes of light nicknamed elves that flicker above thunderstorms are powered by lightning.  These rings of light appear at altitudes of about 90 kilometers and move outwards like ripples on a pond.  Elves are cousins of red sprites that occur lower in the stratosphere.  Sprites were identified in the following passage from William Shakespeare:

Now is the time of night
That the graves, all gaping wide
Every one let forth his sprite
In the church-way paths to glide
        - A Midsummer Night's Dream: Act 5, Scene 1

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What is Ball Lightning?

Ball lightning (boules de fue or foudre spherique in French and Kugelblitz in German) refers to mobile luminous spheres sometimes observed during thunderstorms.  Most observations of ball lightning are made during thunderstorm activity, within a few meters of the ground.  A typical ball lightning is about as big as an orange or grapefruit, and lasts a few seconds.  Visual sightings are often accompanied by sound, odor, and permanent material damage.  Here is an unusual incident reported by Morris (1936):

During a thunderstorm I saw a large, red hot ball come down from the sky.  It struck our house, cut the telephone wire, burnt the window frame, and then buried itself in a tub of water which was underneath.  The water boiled for some minutes afterwards, but when it was cool enough for me to search I could find nothing in it.

Very similar, if not identical phenomena, occur in submarines due to electric discharge of direct current across a circuit breaker, and in high-power electrical equipment.  Ball lightning is sometimes confused with St Elmo's Fire. St Elmo's fire is a corona discharge from a pointed conducting object, which must remain attached to a conductor, and which usually lasts much longer than ball lightning.

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Where do the sounds of Thunder come from?

Aristotle suggested that thunder preceded lightning, the lightning being a burning wind produced after the impact of the dry exhalation of a cloud, whereas Lucretius thought lightning and thunder were created simultaneously in a collision between clouds.  Both were aware that an observer sees lightning before he hears thunder. Descartes felt that thunder was due to higher clouds descending on lower clouds, the sound from the resonance of air between the clouds.  It was generally accepted in the mid-nineteenth century that a lightning discharge produced a vacuum along its path, and that thunder was caused by subsequent motion of air into the vacuum.  Mershon (1870) suggested that

The electricity in passing from one cloud to another, or to the earth, decomposes the water in the cloud into its component gases; and the great heat of the electricity ignites and explodes these gases, and reforms them into water.

The sound of the explosion was supposedly the thunder.  The modern view of thunder was first proposed by Hirn.

Thunder is the acoustic shock wave caused by the extreme heat generated by a lightning flash.  The temperature of air near a lightning bolt  is about 30,000o Celsius.  The air expands expands rapidly, and when its expansion rate exceeds the speed of sound, a sonic boom [thunder] results.  When the lightning strikes very close by, the thunder will sound like a loud bang, crack, or snap, with very short duration.  As the shock wave [thunder] propagates from its source, it stretches and becomes elongated.  At great distances, the continuous rumble of thunder may last for several seconds.  Thunder can typically be heard up to 15 kilometers, and even further on cool, calm, and quiet nights.

For more information see the website http://en.wikipedia.org/wiki/Thunder

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Can Lightning be produced by man?

Yes:  Try this easy experiment to make your own miniature version of a lightning bolt.

Here are some references on research programs to generate lightning strikes:

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What about poems and stories involving Lightning?

 In the East, early statues of Buddha show him carrying a thunderbolt with arrows at each end.  Indian tribes in North America believed that lightning was due to the flashing feathers of a mystical bird whose flapping wings produced the sound of thunder.

English folklore described a number of trees and the relative danger of being struck beneath them by this adage:

Beware of the oak, it draws the stroke.
Avoid the ash, it courts the flash.
Creep under the thorn (hawthorn), it will save you from harm.

The truth of proverbs such as these has been the subject of a number of studies which indicated that beech (smooth bark) is a poor conductor of electricity and oak (rough bark) a good conductor.  Of course there are other factors to consider.

During the middle ages, ringing of church bells was believed to diffuse lightning, and many medieval bells were engraved with the inscription Fulgura frango [I break up the lightning]. A medieval scholar discredited this theory, observing that over a 33 year period there were 386 lightning strikes on church towers and 103 fatalities among bell ringers. For details see the website Clap...Peal...Runble...and Roll... by meterologist Steve Horstmeyer:   http://www.shorstmeyer.com/wxfaqs/thunder/thunder2.html .

One of the most profound references to lightning in poetry is the following stanza written by the American poet Emily Dickinson.

The lightning that preceded it.
Struck no one but myself,
But I would not exchange the bolt
For all the rest of life.

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Lightning Trivia.

For more details, see these references:

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References

Lightning Images

While watching the storm from my house in Ealing, I would in several instances distinctly perceive a flickering appearance of a discharge, and in one particular case the repetitions were at least five or six in number, just sufficiently slow for the eye to detect the variations in brightness without removing the impression of a single flash. ... the camera was moved in a horizontal plane about the lens as a center at the rate of once to and fro in about three quarters of a second. ... I hoped, by the camera moving, to be able to separate the successive components of the flashes, and in this I was fortunately successful.
        H H Hoffert (1889)

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