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This article describes how a lightning strike forms and moves between the earth's surface (or a church steeple or tree) and a storm cloud
We describe the types and process of formation of groundstroke lightning. Lightning bolts also occur within storm clouds moving even horizontally as well as vertically.
This article series describes common lightning protection systems, certification, installation, and lightning protection system inspection. We provide information about lightning strikes, lightning hazards, related equipment, sources of lightning protection system installers, and lightning strike risk assessment
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This article series, in two parts, describes first how a lightning strike forms and moves between a storm cloud and objects on the earth's surface, and second, how a lightning protection system installed on a church, tree, tower or other tall object reduces the chance of lightning damage by conducing the lightning strike more-safely to earth.
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During a thunderstorm a cloud becomes statically charged with electricity. (The exact physics of how this static charge is formed are widely discussed but not perfectly understood.) The upper area of the storm cloud develops a positive charge, while the lower area of the cloud becomes negatively-charged.
Lightning bolts are static electricity discharges that can occur within a single cloud, between storm clouds, or of most-interest to earth-dwellers, as a lightning strike occurring between a storm cloud and the ground.
Our photograph shown here illustrates a lightning bolt moving between clouds or cloud areas well above ground. I photographed this lightning bolt from an aircraft flying over northern Minnesota.
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Researching for this article series and zooming past Ben Franklin, kites and keys, we found a contemporary cloud of articles and opinions offering explanations of the phenomenon of lightning strikes including assertions that lightning bolts always move "up" from the ground, that lightning always moves "down" towards the ground from a cloud, horizontal lightning bolts, downward lightning leaders from a cloud that meet upward leaders from the ground, and return stroke lightning.
Here we'll sort out those terms, excerpting from or paraphrasing lightning experts. We focus exclusively on lightning strikes that make contact with the earth, and we ignore for now horizontal or other forms of lightning strikes that occur inside and stay within storm clouds.
Modern research on lightning and how its electrical current moves really began in the 1930's when storage oscilloscopes became available, making it possible for researchers like McEachron to actually measure lightning currents (at the Empire State Building in New York, in the U.S.) and for other experts to make lightning measurements in Switzerland, Italy, South Africa, and Russia. Heidler (2008) offers a lucid clarification of the types of lightning and the direction that lightning strike currents move.
The most-important, most-informative modern lightning research was performed by Berger who performed lightning current measurements in Switzerland between 1943-1971.
Heidler (2008) in PARAMETERS of LIGHTNING CURRENT GIVEN in IEC 62305-background, experience and outlook." [PDF] and cited in more detail at REFERENCES, writing with Zischank, Flisowski, Bouquegneau, and Mazzetti have provided a lucid description of the types of lightning that actually occur. Portions of this article include my interpretation of that article - interpretation for which Heidler et als. bear no blame. Abstract extract:
The paper reviews the various types of lightning ground flashes, the current components relevant for protection and the current parameters obtained from measurements. On the basis of these measurements, some background information is given how TC 81 selected the parameters published in IEC 62305-1. Finally, methods to simulate lightning currents in laboratory are presented which allow the verification of lightning protection measures or components.
There are four key lightning strike terms, two describing the charge type and two describing the direction of movement of the initial lightning leader:
Negative downward lightning begins with a negative downwards leader. Positive downward lightning begins with a positive downwards leader.
Of significance to most building owners and builders, those authors note that small buildings that are 100m or less in height are almost exclusively struck by downward lightning.
Our photo, edited to portray positive and negative areas of charge accumulating in a cloud over San Miguel de Allende in Guanajuato illustrates the formation of negative lightning in which an accumulation of negative charge in the bottom of the storm cloud forms a leader that moves downwards from the cloud bottom to earth.
A lightning strike to ground is basically a discharge of static electricity between the negatively-charged lower area in the storm cloud and the positively-charged earth's surface.
This discharge can occur when the electrical charge within the cloud exceeds the insulating property of the air between the cloud and the ground surface.
If we consider that during a thunderstorm a stormcloud may be as much as 15 KM in vertical thickness and may have its bottom just about one KM from the ground surface, you can picture the storm cloud as a huge electrical device whose static charge (developed within its 15 KM height) has but to overcome the electrical resistance of just 1 KM of air between the cloud's bottom and the earth's surface.
What follows is a more detailed look at how this static charge develops and how its discharge occurs between the storm cloud and the earth's surface.
Upward lightning strikes originate at the upper portions of very tall towers, over 100 meters in height. Such tall structures develop an electrical field that exceeds a critical minimum, and that increases further with increases in building height.
When the electrical field at the top of a tall tower is strong enough, it can initiate an upwards lightning strike leader that moves towards a storm cloud overhead. Upwards lightning happens mostly during severe winter thunderstorms when the bottom of the storm cloud is closer to the top of the tower.
Upward lightning strikes are very severe, occurring principally at very tall towers like the CN-tower in Canada or the San Chrischona tower in Switzerland.
Image of (c) the initial leader of a negative upward lightning strike between a stormcloud and a tall tower top and (d) the initial leader of a positive upward lightning strike between a stormcloud and a tall tower top is provided by and used courtesy of Heidler et als (2008).
Hold on to your chair while we report that:
What the heck? In the two points above I'm paraphrasing lightning experts (Heidler 2008). Below is my interpretation that explains why the charge polarity of the initial upwards leader is not what you might have assumed:
Luckily very-tall tower upward lightning strikes are rare, and it's possible to protect tall towers by sending a rocked-propelled wire into the cloud to discharge the cloud's electrical field before the lightning strike forms to hit the tower.
Return strokes of downwards lightning follow the initial upwards leader. We discussed return strokes as Step 8 - secondary strike formation in our description of downstroke lightning. Multiple return strokes discharge additional current from cloud to ground and may be categorized as
The static electrical charge within a storm cloud becomes polarized: a positive area of static charge forms at the upper area or top of the storm cloud while the lower area of the cloud develops an area of negative static charge. How does this happen?
Scientists explain this charge polarization by two mechanisms. First is frictional charging that occurs as a result of churning movement of tiny water and ice particles whirling about in the cloud colliding with rising ground moisture.
Image of (a) the initial leader formation of a negative downward lightning strike and (b) the initial leader formation of a positive downward lightning strike formation is provided by and used courtesy of Heidler et als (2008).
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Additional moisture rising upwards from the ground as evaporating water forms clusters of droplets that collide with droplets within the cloud. As the ground moisture is rising it naturally will first-impact the lower area of the clouds above.
The collision between these droplets causes separation of and accumulation of (negatively-charged) electrons from droplets (or water molecules or maybe other molecules in the air?) in the bottom area of the cloud.
As water molecules continue to rise, stripped of an electron, they form positive ions that accumulate higher in the cloud while the stripped-electrons accumulate as a negative charge in the lower are of the cloud.
Several mechanisms have been offered to explain how freezing contributes to static charge polarization within a storm cloud. Of these, A 2016 Wikipedia article offers a palatable explanation:
During wind-driven collisions, ice crystals tend to develop a positive charge, while a heavier, slushy mixture of ice and water (called graupel) develops a negative charge.
Updrafts within a storm cloud separate the lighter ice crystals from the heavier graupel, causing the top region of the cloud to accumulate a positive space charge while the lower level accumulates a negative space charge. - Wikipedia (2016) cited at REFERENCES
In sum, the added effect of freezing and movement of lighter, positively-charged ice crystals upwards in the storm cloud, leaving more negatively-charged heavier slush lower in the cloud adds to the polarization of the static electrical charge within the cloud.
Experts note that the frequency of lightning strikes is not uniform over the earth's surface, as climate and weather are important factors. For example, ground strike lightning will be more frequent in colder climates where the contribution of to in-cloud static charge polarization will be greater.
The result of these two stages of electrical charge formation and separation results in an upper-positively-charged area and lower-negatively-charged area in the storm cloud.
Actually our steps 1 and 2 of polarization within the storm cloud are probably occurring more or less at the same time, and at varying rates depending on local conditions such as wind velocity, turbulence, temperature gradients from earth surface to cloud interior, and available moisture on the earth's surface.
Our annotated photo (above) of dark storm clouds low over San Miguel de Allende in Guanajuato, Mexico illustrates how the static charge may move between a high point on the positively-charged earth surface and the negatively-charged lower area of a storm cloud.
According to physics stack exchange,
... thunderstorms maintain a voltage of around +300kV at the electrosphere with respect to the Earth's surface, with a current of around 1 kA slowly discharging around 500 kC of total charge separation. - retrieved 2016/07/25, original source: http://physics.stackexchange.com/questions/3955/what-is-the-net-charge-of-the-earth
The negatively-charged lower area of the storm cloud in turn disturbs the earth's natural electrical field, repelling negative electrons in the space between the cloud's lower area and the earth's surface.
The result of the repelled electrons closer to the earth's surface creates an opposing electrical charge. A positive static charge builds up on the earth's surface including things on that surface that project upwards: trees, mountain tops, people, radio towers, church steeples.
While most lightning experts agree that most cloud-to-ground lightning occurs between a negatively-charged cloud bottom and a positive charge on or close-to the earth's surface, at least one lightning expert disagrees, pointing out that the earth's surface is generally charged negatively.
However that author adds that the air is charged positively, being about 100 Vm-1 near the earth's surface. The effects described here in the formation of lightning bolts may increase the positive charge near high points on the earth's surface, thus continuing to support the layman's explanation of groundstroke lighting described here. (Rakov 2003).
A more comprehensible article at english Wikipedia in 2016 helps sort this dilemma:
... there is always some amount of unbound positive and negative, but net positive, electric charge in the atmosphere closest to the surface of the negatively charged Earth on a 'fine day'. When days are not so 'fine', the net unbound charge that exists in the clouds of thunderstorms can be exceedingly negative. - "Atmospheric electricity", Wikipedia 2016.
The electrical charge developing in the storm cloud and in the air between the cloud and the earth's surface is strong enough to ionize gas molecules that make up air in the area of the storm. (Ions are particles (really molecules) that have lost electrons (negative ions) or that have gained electrons (positive ions).)
Photo: lightning forming within storm clouds over northern Minnesota, photographed by the author from the air above the storm.
Below a storm cloud, gas molecules in the air have electrons stripped away, becoming positive ions.
Thus the air between the storm cloud and the earth's surface now contains a mixture of both free electrons and positive ions of gases that comprise the air.
Air is abut 78% nitrogen, 21% oxygen, 1% argon and other gases.
This charged air is referred to by physicists as a conductive plasma because the air becomes increasingly capable of conducting an electrical charge.
The accumulated electrons in the lower, negatively-charged area of the cloud begin moving through the cloud and the air below downwards towards the earth, following complex pathways probably affected by the presence of other airborne particulates such as dust comprised of earth, pollen, fungal spores, perhaps smoke or other very small particulates. Some areas in the plasma are more conductive than others.
The downwards movement of electrons form a step leader that pushes more electrons in the air down towards the earth's surface. (Remember that two negatively-charged particles tend to repel one another.) This electron push causes the positive charge at the earth's surface below the storm cloud to increase.
The step leader may produce a bright purple glow as it forms a potential pathway between the cloud and the earth's surface.
As the descending step leader from the storm cloud pushes electrons downwards and a positive charge accumulates on the earth's surface, the positive charge reaches a level that causes it to begin to move upwards as a rising, positively-charged streamer moving up towards the descending step leader.
When the downward propagating leader approaches ground the electric field at grounded objects increases due to the charge contained in the downward leader channel. As soon as the electric field exceeds a certain level, connecting leaders start from the grounded objects making the final connection between the objects at ground and the downward leader. This is the beginning of the return stroke phase, where the return stroke current flows through the struck object. - Heidler (2008)
At a height above ground that varies widely but may typically be about 100 meters the upper end of the rising streamer (red arrow in our illustration) contacts the lower end of the step leader (yellow arrow in our photo).
When these two areas of electrical charge make contact the conductive pathway between the storm cloud and the earth's surface has been completed and the lightning strike occurs.
At about 80,000 kilometers per second the area of contact between the top of the positively-charged streamer and the bottom of the negatively-charged step leader zooms upwards towards the storm cloud.
In less than a millisecond (1/1000th of a second), the contact point moves between the point of initial contact (about 100 meters above the ground) to the area of strong negative charge in the bottom of the cloud (height varies).
This is most of what you see when you "see" a lightning strike.
In a typical lightning strike, the rapid rise of the contact point between step leader and streamer - step 7 - is followed (in milliseconds or less) by some secondary electrical surges that follow the same pathway (more or less) between earth and cloud underside.
Lightning strike steps 7 and 8 occur so rapidly and so closely together that to the naked eye these all appear as a single bolt of lightning.
The enormous electrical charge (around 100,000 amps) moving through the air during the actual lightning strike described in steps 7 and 8 heats the air around that upwards passage of energy.
That extremely-rapid and very hot (up to 50,000 deg. F - five times hotter than the surface of the sun) heating of air around the strike passage causes the air in that area to expand so rapidly that it creates a shock wave, producing thunder, the tremendous sound that we hear moments after seeing the lightning bolt.
Many of us learn as kids to count second between the time we see the lightning bolt and the time we first hear the thunder produced by that bolt. Divide your seconds count by 4 to get the approximate distance in miles between yourself and where the lightning strike occurred.
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