ABOUT LIGHTNING

Lightning Physics

Professor Martin A. Uman, Fellow, IEEE Chief Scientist, BOLT, Inc.

I. INTRODUCTION

Benjamin Franklin, more than 200 years ago, proved that lightning was an electrical discharge and measured the sign of the cloud charge that produced it. Modern research on the physics of lightning began in the early 20th century with the work of C.T.R. Wilson, the same scientist who received the Nobel Prize for his invention of the cloud chamber. Wilson, by making and analyzing remote measurements of thunderstorm electric fields, was the first to infer the charge structure of the thundercloud and the amount of charge involved in lightning. In the 1930's, lightning research was motivated primarily by the need to reduce the effects of lightning on electric power systems and by the desire to understand an important meteorological process. The pace of that research was fairly steady until the 1960s when there was renewed interest because of the generally unexpected vulnerability of solid state electronics to damage from lightning-induced voltages and currents with the resultant hazard to both modern ground-based and airborne systems.

II. SOURCES OF LIGHTNING

Most research on the electrical structure of clouds has focused on the cumulonimbus, the familiar thundercloud or thunderstorm, because this cloud type produces most of the lightning. There have been limited studies of the electrical properties of other types of clouds such as stratus, stratocumulus, cumulus, nimbostratus, altocumulus, altostratus, and cirrus clouds that might potentially produce lightning.

The classic model for the charge structure of a thundercloud was developed in the 1920's and 1930's from ground-based measurements of both thundercloud electric fields and the electric field changes that are caused when lightning occurs. In this model, the thundercloud forms a positive electric dipole as shown in Figure 1.1 and Figure 1.2; that is, a primary positive charge region is found above a primary negative charge region. By the end of the 1930's, this overall structure had been verified from measurements made with sounding balloons inside clouds and had also identified a small localized region of positive charge at the base of the cloud. Subsequent measurements of electric fields both inside and outside the cloud have confirmed the general validity of this double-dipole structure. However, in any given cloud the charge distribution can be more complex, and there is often a negative screening layer above the primary positive charge region.

Figure 1.1 Thundercloud charge distribution and categorization of the four types of lightning between cloud and ground.

III. NATURAL LIGHTNING

Lightning is a transient, high-current discharge whose path length is measured in kilometers. Well over half of all flashes occur wholly within the cloud and are called intracloud (IC) discharges. Cloud-to-ground (CG) lightning has been studied more extensively than other forms of lightning because of its practical importance (for instance, as the cause of injuries and death, disturbances in power and communication systems, and the

ignition of forest fires) and because lightning in the clear air below the cloud base is more easily studied with optical techniques. Cloud-to-cloud and cloud-to-air discharges occur less frequently than either IC or CG lightning. All discharges other than CG are often combined under the general term cloud discharges.

Four different types of lightning between cloud and Earth have been identified, the ways by which these are initiated being shown in Figure 1.1. CG flashes initiated by downward-moving negatively-charged leaders probably account for about 90% of the CG discharges worldwide (Figure 1.1, category 1), while less than 10% of lightning discharges are initiated by a downward-moving positive leaders (category 3). Ground-to-cloud discharges are also initiated by leaders of either polarity that move upward from the Earth (categories 2 and 4). These upward-initiated flashes are relatively rare and usually occur from mountain peaks and tall man-made structures.

The number of cloud-to-ground flashes per square kilometer per year in the U.S. has a maximum in Florida of 15 to 20, and a typical over-land value of 2 to 5. About 20 million CG flashes strike the continental United States annually. Worldwide there are about 100 total (cloud and ground) flashes per second for a worldwide average flash density of about 6 per square kilometer per year.

IV. NEGATIVE CG LIGHTNING

A negative CG discharge (Figure 1.1, category 1) begins in the cloud and effectively lowers some tens of Coulombs of negative charge to Earth. The total discharge is termed a flash (as is the total discharge for other types of lightning). Flash durations are typically about half a second. A flash has several components, the most significant being three or four high-current pulses called strokes. Each stroke lasts about a millisecond, and the separation between strokes is typically several tens of milliseconds. Lightning often appears to "flicker" because the human eye can just resolve the individual pulses of luminosity that are produced by each stroke.

The sequence of luminous processes that are involved in a typical negative CG flash is shown in Figure 1.2. The stepped leader initiates the first return stroke after it propagates downward in a series of discrete steps. The stepped leader is itself initiated by a preliminary breakdown within the cloud, although there is no agreement about the exact form and location of this process. High-speed photographs show that leader steps are typically 1 microsecond in duration, tens of meters in length and that the pause time between steps is 20 to 50 microseconds. A fully developed stepped leader can effectively lower 10 Coulombs or more of negative charge toward the ground in tens of milliseconds. The average downward speed of propagation is about 2105 meters/second. The average leader current is between 100 and 1000 Amperes. The leader steps have peak pulse

currents of at least 1 kiloAmperes. During its progression toward ground, the stepped leader produces a downward-branched geometrical structure.

Figure 1.2 Various processes that make up a negative CG lightning discharge.

The potential difference between the lower portion of the negatively charged leader and the earth has a magnitude in excess of 107 Volts. As the tip of the leader nears ground, the electric field at sharp objects on the ground or at irregularities on the surface increases until it exceeds the breakdown strength of air. At that time, one or more upward-moving discharges are initiated from those points, and the attachment process begins. When one of the upward-moving discharges contacts the downward-moving leader, some tens of meters above the ground, the leader is effectively connected to ground potential. The leader channel is then discharged by an ionizing wave of ground potential that propagates up the previously charged leader channel. This process is the first return stroke. The electric field across the difference in potential between the return stroke, which is at ground potential, and the channel above, which is near cloud potential, is what produces the additional ionization. The upward speed of a return stroke is typically one-third to one-half the speed of light near the ground, and the speed decreases with height. The total transit time between ground and cloud is on the order of 100 microseconds. The first return stroke produces a peak current of typically 30 kiloAmperes at the ground, with a time from zero to peak of a few microseconds. Currents measured at the ground decrease to half the peak value in about 50

microseconds, and currents of the order of hundreds of amperes may flow for times of a few to several hundred milliseconds. We will discuss these long-duration, low-amplitude currents later in this section.

The rapid release of return-stroke-energy heats the leader channel to a peak temperature above 30,000 K and produces a high-pressure channel that expands and creates the shockwaves that eventually become thunder. The return stroke effectively lowers to ground the negative charge originally deposited on the stepped leader channel including all the branches, as well as other negative cloud charge that may become available at the top of the channel.

When the return-stroke current ceases, the flash, including various discharge processes within the cloud, may end. In that case, the lightning is called a single-stroke flash. On the other hand, if additional cloud charge is available, a continuous dart leader can propagate down the residual first-stroke channel and initiate another return stroke. During the time between the end of the first return stroke and the initiation of a dart leader, so-called J- and K-processes occur in the cloud. The J-process involves charge motion in the cloud on a tens-of-milliseconds time scale, while the K-process moves charge on a time scale ten times shorter. The dart leader has a peak current of 1 kiloAmperes or more and lowers a total charge on the order of 1 Coulomb at a speed of about 3106 meters/second. Some dart leaders become stepped leaders toward the end of their progression toward ground and do not follow the previous return stroke channel. Dart leaders and return strokes subsequent to the first are usually not branched.

The time between successive strokes in a flash is usually several tens of milliseconds, but can be tenths of a second if a continuing current persists in the channel after a return stroke. Continuing currents are of the order of l00 Amperes and represent a direct transfer of charge from cloud to ground. Between 25 and 50% of all CG flashes contain a continuing current component.

The highest maximum rate-of-rise of current measured to date is 400 kiloAmperes/microsecond with a typical value of 150 kiloAmperes/microsecond.

V. POSITIVE CG LIGHTNING

Positive flashes to ground (category 3 in Figure 1.1) are of considerable practical interest because their peak currents and total positive charge transfer to ground can be much larger than the more common negative flashes. Positive flashes to ground are initiated by leaders that generally do not exhibit as distinct steps as their negative counterparts. Rather, they exhibit a more or less continuous luminosity that is modulated in intensity. Positive flashes usually contain only a single return stroke followed by a period of continuing current. Positive flashes are probably initiated by the upper positive charge in thunderclouds (Figure 1.1) where this charge has

been separated horizontally from the lower negative charge by wind shear, but this may not always be a necessary condition. Positive flashes are the majority of flashes to ground in winter thunderstorms (and snowstorms) even though these storms produce few flashes overall, and they are relatively rare in summer thunderstorms, only 1 to 15% of the flashes, although storms with predominantly negative lightning often end with positive discharges. The fraction of positive discharges in summer thunderstorms apparently increases with increasing geographic latitude and with increasing height of the local terrain; that is, the closer the cloud charge is to the ground, the more probable is positive lightning, but again, not enough is known about positive lightning to be able to say that this is always a necessary condition.

VI. UPWARD LIGHTNING

The leaders in upward-initiated lightning are usually positive (Figure 1.1, category 2). Positive upward leaders show a continuous luminosity that is modulated in a fashion similar to positive downward-stepped leaders. Negative upward leaders (category 4) exhibit a stepped behavior that is similar to negative downward-stepped leaders.

Positive upward leaders often enter the cloud and produce only a more or less continuous flow of current, of the order of 100 to 1000 Amperes, at ground. In about half of the upward-initiated events, however, the continuous current is followed by a sequence of dart leaders and return strokes that are similar to those following first strokes in natural CG discharges that are initiated by negative downward-moving leaders.

VII. CLOUD DISCHARGES

Cloud discharges can be subdivided into IC, intercloud, and cloud-to-air flashes, but there are no experimental data at present to distinguish between these three types. Indeed, on the basis of electric field records, there is considerable similarity between these discharges. The term cloud discharge could also be applied to those portions of a flash to ground that take place within the cloud. In some cases, flashes that are primarily within the cloud, and are best characterized as cloud flashes, produce a channel to ground, seemingly as an unimportant byproduct.

Intracloud flashes typically occur between positive and negative charge regions or represent discharges away from concentrated regions of positive or negative charge and have total durations that are nearly the same as ground flashes, about half a second. A typical cloud discharge effectively moves tens of Coulombs of charge over a distance of 5 to 10 kilometers. The discharge process is thought to consist of a breakdown phase followed by a continuously propagating leader that generates weak return stroke-like processes called recoil streamers when the leader contacts pockets of space charge opposite to its own. The electric field changes that are associated with recoil streamers are termed K-changes. K-changes are

thought to be similar, but usually of opposite polarity, to the K-changes that occur in the intervals between return strokes in CG discharges.

VIII. ABOVE-THE-CLOUD AND CLEAR AIR LIGHTNING

Discharges called blue jets, sprites, and elves occur in the region between the cloud top and the ionosphere in response to lightning charge dissipation. Lightning has also been reported when there is a clear blue sky, commonly referred to as a "bolt from the blue." Most of these reports, however, refer to a situation where there is blue sky overhead and the thunderstorm is 10 or more kilometers away, out of viewing range, from where the lightning originates. However, there are photographs and supporting charge locations that show that a triggered discharge (see below) can occur entirely in clear air near a thunderstorm. In the case cited, there was, a thunderstorm about 10 kilometers away, and the lightning was artificially initiated by firing a small rocket upward that trailed a grounded wire. There were high electric fields, but the sky overhead where the charge appears to have been located was mostly clear with broken altocumulus and altostratus clouds at higher altitudes.

IX. ARTIFICIALLY-INITIATED (TRIGGERED) LIGHTNING

We can define artificially-initiated lightning as discharges that occur because of the presence of man-made structures or events. Such lightning is characterized by an initial upward-moving leader if it is triggered below the N charge region of the thundercloud, or the equivalent charge region of other cloud types, as is the case for small rockets trailing grounded wires. Discharges initiated by upward-moving leaders also occur naturally, for example, from mountain tops, and artificially-initiated lightning via an upward leader is expected to be similar to those natural upward events. Upward-initiated lightning has no "first return stroke" of the type always observed in normal downward-initiated lightning. Rather its place is taken by an upward-moving leader and any continuous current that may follow when that leader reaches the cloud, followed sometimes by combinations of downward-moving dart leaders and upward-moving subsequent return strokes that appear to be very similar to subsequent strokes in normal cloud-to-ground flashes. The physical processes that occur in discharges that are initiated artificially within the cloud or otherwise relatively far above ground by aircraft or space vehicles and are not attached to ground are not as well understood as are discharges initiated by objects below the cloud that are attached to ground.

In general, lightning can be initiated artificially by rapidly introducing a long electrical conductor into a region of relatively high electric field. In this case the conductor serves to enhance the existing electric field to a value sufficient for electrical breakdown. Small balloons continuously flown on metal wires of several kilometers length do not get struck, even during periods of active lightning. Further, in the laboratory, artificial initiations of a spark occur with the rapid introduction of a conductor into an electric field

when the steady presence of that conductor does not result in a spark discharge.

A practical means of triggering a CG strike is to fire a rocket with a trailing wire upward below a cloud when the ground electric field exceeds a predetermined amplitude. This rocket-triggered lightning allows the flash to be attached to an instrumented experiment: an experimental method of fundamental importance in developing effective lightning protection systems.

X. LIGHTNING DAMAGE, LIGHTNING AVOIDANCE, AND LIGHTNING PROTECTION

In addition to being esthetically beautiful and scientifically fascinating, lightning is a formidable natural event often causing costly damage to businesses and tragic harm to people. Lightning damage is the subject of the next menu item of this dropdown menu (About Lightning). When one is faced with the possibility of a lightning hazard, two methods of protection are available: 1) identify and avoid the hazard or 2) harden the threatened systems to withstand the effects of nearby and direct strikes. Lightning avoidance often appropriate for mobile or interruptible operations but seldom appropriate or desirable for expensive fixed industrial facilities and continuous operations is not explored in this website. However, links to lightning identification companies and information can be found in the menu item "General Lightning Links" of this dropdown menu. Hardening of the systems (also commonly called lightning protection) and its economic benefits to you are the subject of this website and the information is presented left to right in the dropdown menus: About Lightning, Conventional Protection, Modern Protection, BOLT Services, About BOLT, and BOLT Contacts. Conventional Protection explores the capabilities and limitations of the types of lightning protection that have been commonly used to date. Modern Protection presents an emerging and dramatically superior lightning protection method using "elemental Faraday cages." This method provides fire protection comparable to conventional protection methods and is the only method to protect satisfactorily the interior of struck structures from lightning induced arcs that can cause explosions and/or damage or disrupt electronic systems. BOLT is the only commercial company providing this new and superior class of lightning protection, and BOLT, its services, and their significant benefits to you are discussed in the final three dropdown menus: BOLT Services, About BOLT, and BOLT Contacts.

This material has been adapted from numerous of my publications (see References in this menu item) for this website. These general references contain extensive references to detailed original scientific publications supporting this brief overview. (M.A.U.)

Lightning Damage

In addition to being beautiful and scientifically fascinating, lightning can be destructive to buildings and to numerous systems critical to daily life, and it can be lethal to people. We are all aware of stories of lightning striking people (Lee Trevino twice) or causing forest fires. Less often but occasionally spectacularly in the news are incidents of extremely costly and tragic fires or explosions caused by lightning at industrial or military sites. This brief overview primarily considers lightning damage to structures, not people.

A typical lightning stroke is a dramatically powerful natural event capable of damaging even intentionally protected structures. The lightning stroke reaches temperatures of several tens of thousand degrees Kelvin, clearly sufficient to initiate combustion in many common materials. Indeed, when lightning current dissipates into the earth it often melts sand, creating glassy channels called fulgurites that can be tens of feet long. The cloud-to-ground voltage of thunderheads can be many tens of million volts, and the current in a lightning stroke can exceed 200 thousand amperes (>1000 times the typical household wiring capacity). When lightning strikes a building, it can cause internal electric fields in excess of 100 thousand volts per meter and can cause internal arcing across rooms. The energy released by a lightning strike can be of the order of 1010 Joules, more than the energy in 1000 gallons of gasoline or more than the energy of some bombs. Fortunately, only a small portion of this energy couples to the building to produce damage.

Among the damaging effects of lightning are the following:

Fire Lightning is a major cause of forest and range fires. It presents a daily fire threat to buildings and other commercial structures. The existing lightning protection industry primarily addresses the fire threat (see menu item Conventional Protection), and significant expenditures have historically been made to mitigate this threat.

Fracture and spalling In many common materials lightning current causes a rapid localized expansion that causes the material to fracture or split apart. This effect can be observed in trees that have been split by lightning or in masonry buildings that have bricks "blown" off.

Voltage surges Lightning strikes to power lines cause a transient over-voltage pulse to be transmitted for miles. You have probably installed power strips with over-voltage clamps in them to protect your PC, TV and Stereo. This works well for distant strikes.

High electric fields and arcing When lightning directly strikes a building it can cause electric fields inside the building that can damage or disrupt electronics and can cause internal arcing. In industrial sites, disruption of controlling electronics can have extremely costly consequences including long down time and destroyed equipment. Arcing can also damage or disrupt electronics.

Explosions For industrial sites housing volatile compounds and military sites housing explosives, lightning induced arcs can directly

initiate explosions, or lightning caused fires can subsequently cause explosions.

Fires and spalling cause damage to the structures or buildings that lightning strikes. Voltage surges on power lines cause damage or disruption to electronic systems. You will soon understand that both conventional and modern lightning protection mitigate damage from these causes. High electric fields and arcing cause damage or disruption to electronic systems, and you will soon understand that only modern lightning protection mitigates this damage. (That is, only modern protection mitigates all types of lightning damage.) Of course, explosions can damage both the structures and their contents, including electronic systems.

To understand better how lightning causes these effects, consider the typical lightning current waveforms in Figure 1.3. As described in the previous menu item (Lightning Physics), here lightning current is seen in the two example waveforms to consist of an initial stroke typically followed by several subsequent strokes on the average about four subsequent strokes but occasionally perhaps 10 - 20. Each stroke is seen to consist of a very rapid rise (often

Peak currents of 200 kiloamperes and the peak rate-of-rise of current of 400 kiloamperes per microsecond are accurate estimates for extreme (99 percentile) lightning characteristics.

For nominal (50 percentile) lightning flash attachment, peak currents of 30 kiloamperes and the peak rate-of-rise of current of 150 kiloamperes per microsecond are accurate estimates.

The total charge transferred (the integrated current) and the "action integral" are important in causing heating and thus in causing fires and spalling. (The action integral is the time integral of I2(t)R, where R = 1 ohm.) Both of these measures are dominated by the low frequency components of lightning. Typically, spalling results from the rapid conversion of moisture to steam. The steam creates pressure, which fractures the material, causing rapid and violent spalling.

As stated, the rapid current rise and peak current (the high frequency components of lightning) result in high electric fields and subsequent arcing in a building struck by lightning. To understand this effect, consider that many buildings, particularly modern buildings, can be modeled as a series resistor, R, and inductor, L (see Modern Protection). Given this simple model, the voltage, V, between the floor and ceiling that results from the current, I, is the following:

As a very good approximation of the upper bound of the maximum voltage, the peak current and peak rate of current rise stated above can be used:

An upper bound for Vmax occurs when the current parameters from extreme lightning are used. The maximum voltages and consequent electric fields often exceed the breakdown strength of air; thus, arcs are generated. The current parameters for the slower current decay and for continuing current generate much lower maximum voltages and thus generate much lower electric fields, which typically do not produce arcs, but can sustain the arcs once started.

In summary, the dominant cause of fires and spalling is the low frequency energy in the current decay and continuing current. The dominant cause of over-voltage, high electric fields and arcing is the initial peak current and its rise time; this region is very short in duration with high frequency energy content. Remember these causes for lightning damage; they will be revisited when we consider conventional lightning protection and modern lightning protection in subsequent menu items.

In the US on average, lightning strikes to ground occur about 4 times per square kilometer per year. Florida, the lightning capital of the US, has about

15 to 30 strikes to ground per square kilometer annually, and Nevada only a few strikes per square kilometer annually. Some regions of the world have more frequent strikes than Florida, particularly tropical regions. From lightning density statistics one can calculate the expected strikes per year to facilities based on their footprints. Many industrial systems have surprisingly large footprints for lightning attachment. For example, pipelines can transmit lightning energy for miles, and lightning can attach to pipelines from about 30 feet away. In Florida there are estimated to be up to one lightning attachment per pipeline mile per year.

The total loss caused to US property by lightning damage probably exceeds $5 billion per year. Many instances of lightning damage go unreported so an accurate total estimate of cost is difficult. Some interesting categories of damage cost are the following:

Half the forest fires are lightning caused, costing about $100 million annually.

About 5% (~$1 billion) of annual insurance claims are lightning related.

About 30% (~$1 billion) of annual power outages are caused by lightning.

Over 100 thousand PCs (~$100 million) annually are destroyed or damaged by lightning.

Gallery of Photographs

Lightning strikes fascinate all of us.

Lightning strikes to industrial sites can result in severe financial loss.

Fulgurites demonstrate the extreme temperatures of lightning and the 10s of feet lighting can travel through the earth to couple to a conductor (power cables or pipes).

Rocket-triggered lighting is a practical means to test lightning protection systems with strikes similar to natural cloud-to-ground strikes.

General Lightning Links

1. www.lightning.ece.ufl.edu 2. www.glatmos.com 3. www.weatherstock.com

References

1. Uman, M., The Lightning Discharge, Dover Publications, 2001. 2. Bazelyan and Raizer, Lightning Physics and Lightning Protection,

Institute of Physics Publishing, 2000. 3. Uman. M., All About Lightning, Dover Publications, 1986. 4. Uman, M., Lightning, Dover Publications, 1984. 5. Golde, R. H., Lightning, Volume 1 and 2, Academic Press, New

York, 1977. 6. Golde, R. H., Lightning Protection, Edward Arnold, London, 1973.