Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements

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CHAPTER 22

ATMOSPHERIC ELECTRICITY AND

LIGHTNING

WALTER A. LYONS AND EARLE R. WILLIAMS

1 GLOBAL ELECTRICAL CIRCUIT

The atmosphere is the thin veneer of gas surrounding our planet and is one of the

most highly electrically insulating regions in the universe. The electrical conductiv-

ity of air near Earth’s surfa ce is some 12 orders of magnitude smaller than both

Earth’s crust and the conductive ionosphere. In such a medium, electric charges may

be physically separated by both air and particle motions to create large electric ﬁelds.

The laws of electrostatics apply in this situation, and, though the charges involved

are seldom static, the application of these laws in the atmosphere (in fair weather

regions, in electriﬁed clouds which may produce lightning, and in the upper atmo-

sphere where sprites are found) is the science of atmospheric electricity.

Earth itself, whose surface is composed of seawater and water-impregnated

crustal rock, is a good electrical conductor. This conducting sphere is known to

carry a net negative charge of half a million coulombs. The corresponding down-

ward-directed electric ﬁeld at the conductor’s surface in fair weather regions is of the

order of 100 V=m. The conductivity of highly insulating air near Earth’s surface is of

the order of 10

14

mho=m and, according to Ohm’s law, invoking a linear relation-

ship between electric ﬁeld and current density, a downward-directed current is

ﬂowing with a current density of order 1 pA=m

. Integrated over the globe, this

amounts to a total current of about 1 kA.

The origin of Earth’s persistent fair weather electriﬁcation remained a mystery for

more than 100 years. In the 1920s, C.T.R. Wilson proposed that the electriﬁcation

manifest in fair weather regions was caused by the action of electriﬁed clouds and

Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,

and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.

ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.

407

thunderstorms worldwide. The fair weather region provided the return current path

for electriﬁed weather and is the collective battery for the global circuit. Nominally,

1000 storms are simultaneously active worldwide, each supplying a cur rent of the

order of 1 A, which ﬂows upward from the cloud top into the conductive upper

atmosphere where the current spreads out laterally and then turns downward in fair

weather regions to the conductive earth to complete the circuit.

The ﬁnite conductivity of the lower atmosphere is primarily the result of ioniza-

tion of neutral nitrogen and oxygen molecules by energetic cosmic radiation from

space. The atmosphere attenuates the incoming ionizing radiation, and this causes

the conductivity to increase exponentially with altitude toward the highly conductive

ionosphere, with a scale height of about 5 km. Since the fair weather return current is

uniform with altitude, Ohm’s law guarantees that the electric ﬁeld will decline

exponentially with altitude from its maximum nominal value of 100 V=m at the

surface, with the same scale height. The integral of the fair weather electric ﬁeld

with altitude from the conductive earth to the conduct ive upper atmosphere is

referred to as the ionospheric potential, and amounts to some 250,000 V. This quan-

tity is a global invariant and the standard measure of the global electrical circuit. The

ionospheric potential and the net charge on Earth are not constant but vary by

approximately 15% over the course of every day in response to the systematic

modulation of electriﬁed weather by solar heating, centered in the tropics. Three

major zones of deep convection—the Maritime Continent (including the Indonesian

archipelago, Southeast Asia, and Northern Australia), Africa, and the Americas—

exhibit strong diurnal variations, and the integrated effect lends a systematic varia-

tion to the electriﬁcation of Earth and the ionospheric potential in universal time.

The consistent phase behavior between the action of the three tropical ‘‘chimney’’

regions and Earth’s general electriﬁcation is generally regarded as key evidence in

support of C.T.R. Wilson’s hypothesis.

According to this general idea, the negative charge on Earth is maintained by the

action of electriﬁed clouds. One direct agent is cloud-to-ground lightning, which, as

explained below, is most frequently negative in polarity. The less frequent but more

energetic positive ground ﬂashes, which are linked with sprites in the mesosphere

should actually deplete the negative charge in Earth. Possibly more important

(though more difﬁcult to evaluate) agents for maintaining the negatively charged

Earth are point discharge currents that ﬂow from plants, trees, and other elevated

objects in the kilovolt-per-meter level electric ﬁelds beneath electriﬁed clouds, and

precipitation currents carried to Earth by rain and graupel particles charged by

microphysical processes within storms.

The global electrical circuit outlined here provides a natural framework for moni-

toring electriﬁed weather on a worldwide basis. The expectation that lightning

activity is temperature-dependent has spurred interest in the use of the global circuit

as a monitor for global temperature change and for upper atmospheric water vapor.

A second manifestation of the global circuit, sometimes referred to as the ‘‘AC’’

global circuit, is provided for by the same insulating air layer between two conduc-

tors. This conﬁguration is an electromagnetic waveguide for a phenomenon known

as Schumann resonances and maintained continuously by worldwide lightning

408 ATMOSPHERIC ELECTRICITY AND LIGHTNING

activity. The wavelength for the fundam ental resonant mode at 8 Hz is equal to the

circumference of Earth. One advantage afforded by Schumann resonances over

ionospheric potential as a measure of the global circuit is the capability for contin-

uous monitoring from ground level. The signatures of the three major tropical zones

referred to earlier are distinguishable in daily recordings from a single measurement

station. Furthermore, variations in Schumann resonance intensity on longer time

scale (semiannual, annual, interannual), for which global temperature variations

are recognized, have also been documented.

The evidence that giant positive lightning can simultaneously create sprites in the

mesosphere and ‘‘ring’’ Earth’s Schumann resonances to levels higher than the

contribution of all other lightning combined has also spurred interest in this

aspect of the global circuit.

2 WHAT IS LIGHTNING?

Lightning is a transient, high-current atmospheric discharge whose path length is

measured in kilometers (Uman and Krider, 1989). It is, in effect, a big spark.

Benjamin Franklin’s legendary (and incredibly risky) kite ﬂying experiment in

1752 documented that, indeed, Thor’s bolts were ‘‘merely’’ electricity. The number

of scientists killed or injured in subsequent years attempting to duplicate Franklin’s

experiment attests to the threat to life and property inherent in a lightning discharge.

Lightning represents an electrical breakdown of the air within a cloud between

separated pockets of positive and negative electrical space charge. What actually

causes the formation and separation of the electrical charges within the thunde r-

storm? While considerable progress is being made in this research area, a concise

theory has yet to emerge (Williams, 1988; MacGorman and Rust, 1998). Sufﬁce it to

say that in almost all cases of natural lightning, charge separation results from the

interactions between ice particles, graupel, and supercooled droplets within thunder-

storm clouds (Fig. 1). The complex updrafts and downdrafts separate the positive

and negative charge pools allowing the electric ﬁeld to attain ever-higher values until

dielectric breakdown occurs. Storm updrafts penetrate well above the freezing (0



level. It is generally thought that signiﬁcant updrafts (>8m=s) in the layers where

supercooled water droplets and ice particles coexist (0



to 40



C) are necessary

though not sufﬁcient for most ligh tning-generating scenarios. Lightning discharges

also occur within the stratiform precipitation regions of large mesoscale convective

systems. Though the sources of the charge centers have been thought caused by

advection from the convectively active parts of the storm, there are also strong

arguments favoring in situ charge generation. Occasional reports of lightning

occurring in warm clouds (entirely above 0



C) have never been convincingly

substantiated.

Several terminologies are employed to describe lightning. Discharges occurring

within the cloud are called intracloud (IC) events. A cloud-to-ground event is called

a CG. In most storms ICs greatly outnumber CGs. The typical ratio may be 4:1, but

the ratio may approach 100:1 or more in some especially intense storms. The IC:CG

2 WHAT IS LIGHTNING? 409

ratio also shows geographical variations. It has been maintained that the ratio is

lowest at more northerly latitudes. Recent satellite measurements have shown that,

within the United States, certain regions, notably the High Plains and West Coast

have much higher IC:CG ratios than other parts of the country such as Florida.

Cloud-to-ground events typically lower negative charge to earth (the negative CG,

or CG). CG discharges are sometimes initiated by leaders that are positively

charged, and the resulting return stroke essentially lowers positive charge to

ground (the þCG). Once thought to be very rare, the þCG is now thought to

constitute about 10% of all CGs. For reasons not well understood, many severe

local storms tend to have higher percentages of þCG ﬂashes. Negative CGs typi-

cally lower on the order of 5 to 20 C of charge to ground, while þCGs can often b e

associated with much larger amounts of charge transfer.

When viewed with the naked eye, a CG ﬂash often appears to ﬂicker. This is a

consequence of the fact that CG ﬂashes are often composed of a sequence of multi-

ple events called strokes. While ﬂashes can consist of a single stroke (which is the

norm for þCGs), the average stroke multip licity for CGs is around 4. Flashes with

greater than 10 strokes are not uncommon, and there have been up to 47 strokes

documented within a single ﬂash. Strokes within the same ﬂash tend to attach

themselves to the same point on the surface, but this is not necessarily always the

case. One Florida study (Rakov and Uman, 1990) found that about 50% of the

ﬂashes showed mul tiple terminations per ﬂash to the ground. The multiple attach

Figure 1 Cumulonimbus cloud over the U.S. High Plains unleashes a thunderstorm. See ftp

site for color image.

410

ATMOSPHERIC ELECTRICITY AND LIGHTNING

points within single ﬂashes were separated by 0.3 to 7.3 km, with a mean of 1.3 km

(Thottappillil et al., 1992). A very small percentage of ﬂashes are initiated from the

tops of tall towers, buildings, or mountains. Unlike conventional CG discharges,

their channels branch upwards and outwards.

The CG ﬂash is part of a complex series of events (Golde, 1977; Holle and

Lopez, 1993; Uman, 1987; MacGorman and Rust, 1998). A typical ﬂash begins

with one or more negatively charged channel(s), called stepped leaders, originating

deep within the cumulonimbus cloud and may emerge from the base of the cloud.

Depending on the electrical charge distribution between cloud and ground, the leader

proceeds erratically downward in a series of luminous steps traveling tens of meters

in around a microsecond. A pause of about 50 ms occurs between each step. As the

stepped leader approaches the ground, it is associated with electrical potentials on

the order of 100 million volts. The intense electric ﬁeld that develops between the

front of the leader and the ground causes upward-moving discharges called strea-

mers from one or more objects attached to the ground. When one of these streamers

connects with the leader, usually about 100 m above the ground, the circuit is closed,

and a current wave propagates up the newly completed channel and charge is

transferred to the ground. This last process is called the return stroke. This is the

brilliant ﬂash seen by the naked eye, even though it lasts only tens to perhaps a few

hundred microseco nds. The peak current, which is typically on the order of 30 kA, is

attained within about 1 ms. After the current ceases to ﬂow through the leader-created

channel, there is a pause ranging from 10 to 150 ms. Another type of leader, called a

dart leader, can propagate down the same ioni zed channel, followed by a subsequent

return stroke. The entire lightning discharge process can last, in extreme cases, for

over 3 s, with the series of individual strokes comprising the CG ﬂash sequence

sometimes extending over a second. Certain phases of the lightning discharge,

particularly the return stroke, proceed at speeds of more than one half the speed

of light, while other discharge processes travel through the clouds up to two orders

of magnitude more slowly.

While many CG return strokes are very brief (sub-100 ms), additionally many are

followed by a long lasting (tens to many hundreds of milliseconds) continuing

current. This behavior is particularly true of þCGs. This, along with the high initial

peak cur rents often found in some þCGs, explains why such ﬂashes are thought to

ignite a substantial number of forest ﬁres and cause other property damage (Fuquay

et al., 1972). Since the temperature within the lightning channel has been estimated

to reach 30,000 K, extending the duration of the current ﬂow allows for g reater

transfer of heat energy and thus combustion.

There is great variability in the amount of peak current from stroke to stroke.

While a typical peak current is in the 25 to 30 kA range, much smaller and larger

currents can occur. Recent data suggest that peak currents of less than 10 kA may be

more common than once believed. One survey of 60 million lightning ﬂashes found

2.3% had peak currents of >75 kA; the largest positive CG reaching 580 kA and the

largest negative CG 960 kA (Lyons et al., 1998). It has long been assumed that

þCGs on the average had larger peak currents than CGs, but again recent studies

in the United States suggest that, while there are certainly many large peak current

2 WHAT IS LIGHTNING? 411

þCGs, that for all classes between 75 and 400 kA, negative CGs outnumbered the

positives (Lyons et al., 1998). It has generally been assumed that the ﬁrst stroke in a

ﬂash contains the highest peak current. While on the average this is true (typi cally

the ﬁrst stroke averages several times that of subsequent strokes), recent research has

found that subsequent strokes with higher peak currents are not that unusual

(Thottappillil et al., 1992).

There are many reports of lightning strikes out of an apparently clear sky over-

head. This is the so-called bolt from the blue. Powerful lightning discharges can leap

for distances of 10 km (and sometimes more) beyond the physical cloud boundary.

Under hazy conditions or with the horizon obscured by trees or buildings, the

perception of a bolt from the blue is entirely understandable. Lightning tracking

equipment at NASA’s Kennedy Space Center has documented one discharge

that came to ground more than 50 km from its point of origin within a local

thunderstorm.

Lightning has also been associated with smoke plumes from massive forest ﬁres,

likely from the thunderstorm clouds induced by the intense local heat source

(Fuquay et al., 1972). Plumes from volcanic eruptions have also produced lightning

discharges (Uman, 1987). Lightning has also been associated with large underwater

explosions as well as atmospheric thermonuclear detonations. Lightning is not

conﬁned to Earth’s atmosphere. Space probes have detected lightning within the

clouds of Jupiter, and possibly in Saturn and Uranus. Considerable additional infor-

mation on the physics and chemistry of lightning can be found in Uman (1987),

Golde (1977), Uman and Krider (1989), the National Research Council (1986), and

Williams (1988).

3 HUMAN COST OF LIGHTNING

Lightning is a major cause of severe-weather-related deaths in the United States.

According to the U.S. Depar tment of Commerce’s ‘‘Storm Data,’’ during the period

1940–1981, lightning killed more people in the United States (7583) than hurricanes

and tornadoes combined (7071). Between 1959 and 1995, the toll was 3322 reported

deaths and 10,346 injuries. Thus while there was a downward trend in lightning

casualties during the later p art o f the twentieth century in the United States, in many

years lightning deaths still continue to outnumber those associated with tornadoes

and hurricanes. Floods, however, are the number one killer. Between 1959 and 1987,

the most lightning casualties were recorded in Florida, the nation’s ‘‘lightning capi-

tal.’’ The states of Michigan, North Carolina, Pennsylvania, Ohio, and New York

followed in the rankings. Though having lower lightning frequencies, the large

populations of these states translated into large numbers of casualties. Lightning

casualties are less well documented in other parts of the world. It has been estimated

by Andrews et al. (1992) that the annual worldwide lightning casualty ﬁgures are

about 1000 fatalities and 2500 injuries.

Many of the published U.S. and worldwide losses from lightning may be signiﬁ -

cant underestimates (Lopez et al., 1993). In Colorado, a region with an efﬁcient

412 ATMOSPHERIC ELECTRICITY AND LIGHTNING

emergency response and reporting system, detailed examinations of hospital records

found that U.S. government ﬁgures still underestimated lightning deaths by at least

28% and injuries by 40%. It appears many deaths ultimately caused by lightning (by

heart attacks or in lightning-triggered ﬁres) are not included within the casualty

totals. Cooper (1995) suggests actual U.S. lightning deaths are probably 50%

higher than reported, with as many as 500 injuries, many of them unreported or

erroneously classiﬁed. Thus the lightning threat tends to be underestimated by public

safety ofﬁcials. This may be in part also due to the tendency of lightning to kill or

injure in small numbers. It rarely receives the press notice accorded more ‘‘specta-

cular’’ weather disasters such as hurricanes, tornadoes, and ﬂash ﬂoods. In recent

years, however, there have been increasing reports of ‘‘mass casualties.’’ In July

1991, at least 22 people were injured when lightning struck a crowded beach in

Potterville, Michigan. In 1980 a high school football team practicing in Wickliffe,

Ohio, was struck by lightning. All the players were knocked over and one was

injured. In the Con go, 11 members of one soccer team were killed as a ﬂash

struck the playing ﬁeld. In Nigeria, 12 were killed as a stor m struck an outdoor

funeral ser vice. In the Colorado Rockies, a herd of 56 elk were found dead within a

radius of 100 yards, apparently the result of a lightning strike.

4 ECONOMIC COSTS OF LIGHTNING

While some summaries of lightning property damages in the United States have

listed relatively modest annual losses (under $100 million) recent research suggests

the damages are many times greater. One study of insurance records (Holle et al.,

1996) estimated about 1 out of every 55 Colorado CG ﬂashes resulted in a damage

claim. A major property insurer, covering about 25% of all U.S. homes, reports

paying out 304,000 lightning-related claims annually. The average insurance light-

ning damage claim was $916. This suggests that U.S. homeowner losses alone may

exceed $1 billion annual ly. Lightning has been responsible for a number of major

disasters, including a substantial fraction of the world’s wildﬁres in forests and

grasslands. During the 1990s in the United States, some 100,000 lightning-caused

forest ﬁres consumed millions of acres of forests. Lightning is the leading cause of

electric utility outages and equipment damage throughout the world, estimated to

account for 30 to 50% of all power outages. On July 13, 1977, a lightning strike to a

power line in upstate New York triggered a series of chain-reaction power outage s,

ultimately blacking out parts of New York City for up to 24 h. The resulting civil

disturbances resulted in over $1 billion in property losses. Though the actual total of

lightning’s economic effects are unknown, the National Lightning Safety Institute

estimates it could be as high as $4 billion to $5 billion per year in the United States

alone. Given the substanti al noninsured and poorly documented losses occurring in

forestry, aviation, data processing, and telecommunications plus the lost wages due

to power disruptions and stoppages of outdoor economic activities, this estimate

seems plausible.

4 ECONOMIC COSTS OF LIGHTNING 413

On December 8, 1963, a lightning strike ignited fuel vapors in the reserve tank of

a Pan American airliner while the plane was circling in a holding pattern during a

thunderstorm. The Boeing 707 crashed in Elkton, Maryland, with a loss of 81 lives.

Engineering improvements make such deadly airliner incidents much less likely

today. Spacecraft launches, however, still must contend with the lightning hazard.

The 1969 ﬂight of Apollo 12 almost ended in tragedy rather than the second moon

landing when the vehicle ‘‘triggered’’ two lightning discharges durin g the initial

moments of ascent. The event upset much of the instrumentation and caused

some equipment damage, but the crew fortunately was able to recover and proceed

with the mission. On March 26, 1987, the U.S. Air Force launched an Atlas=Centaur

rocket from the Kennedy Space Center. Forty-eight seconds after launch the vehicle

was ‘‘struck’’ by lightning—apparently triggered by the ionized exhaust plume

trailing behind the rocket (Uman and Krider, 1989). The uninsured cost to U.S.

taxpayers was $162 million. Ground facilities can fare poorly as well. On July 10,

1926, lightning struck a U.S. Navy ammunition depot. The resulting explosions and

ﬁres killed 19 people and caused $81 million in property losses. A lightning strike to

a Denver warehouse in 1997 resulted in a $50 million ﬁre.

Blaming lightning for equipment failure has become commonplace, and perhaps

too much so. One estimate suggests that nearly one third of all insurance lightning

damage claims are inaccurate or clearly fraudulent. Damage claim veriﬁcation using

data from lightning detection networks is now becoming commonplace in the

United States.

5 CLIMATOLOGY OF LIGHTNING

An average of 2000 thunderstorm cells are estimated present on Earth at any one

time (Uman, 1987). Over the past several decades, the worldwide lightning ﬂash rate

has been variously estimated at between 25 and 400 times per second. Based upon

more recent satellite monitoring, the consensus is emerging that global lightning

frequency probably averages somewhat less than 50 IC and CG discharges per

second. Lightning is strongly biased to continenta l regions, with up to 10 times

more lightning found over or in the immediate vicinity of land areas.

Before the advent of the National Lightning Detection Network (NLDN), various

techniques were used to estimate CG ﬂash density (expressed in ﬂashes=km

year

unless otherwise stated). One measure frequently employed is the isokeraunic level,

derived from an analysis of the number of thunderstorm days per year at a point. A

thunderstorm day occurs when a trained observer at a weather station reports hearing

at least one peal of thunder (typically audible over a range of 5 to 10 km). Changery

(1981) published a map of thunder days across the United States using subjective

reports of thunder from weather observers at 450 stations over the period 1948–

1977. A very rough rule of thumb proposed that for each 10 thunderstorm days there

were between 1 and 2 ﬂashes=km

year.

Needed, however, was a more accurate determination of the CG ﬂash density, for

a variety of purposes, including the design of electrical transmi ssion line protection

414 ATMOSPHERIC ELECTRICITY AND LIGHTNING

systems. During the 1980s, as discussed below, lightning detection networks capable

of locating CG events began operating in the United States. By the mid-1990s, a

reasonably stable annual pattern had begun to emerge (Orville and Silver, 1997).

Approximately 20 to 25 million ﬂashes per year strike the lower 48 states. The

expected ﬂash density maximum was found in central Florida (10 to

15 ﬂashes=km

year). Other regional maxima include values around 6 ﬂashes=km

year along the Gulf Coast and around 5 ﬂashes=km

year in the Midwest and Ohio

River region. More than half the United States has a ﬂash density of 4 ﬂashes=km

year or greater. In any given year the highest ﬂash densities may not be found in

central Florida. For instance, the highest ﬂash densities during 1993 were found in

the Mississippi and Ohio valleys, in association with the frequent and massive

thunderstorms leading to the great ﬂoods of that summer. Two annual maxima

occurred in 1995, in southern Louisiana and n ear the Kentucky–Illinois border. In

the intermountain west, values around 1.0 ﬂashes=km

year are common with West

Coast densities being less than 0.5 ﬂashes=km

year.

Different thunderstorm types can produce lightning at very different rates. A

small, isolated air mass shower may produce a dozen IC discharges and just one

or two CGs. At the other extreme, the largest convective system, aside from

the tropical cyclone, is the mesoscale convective complex (MCC). These frequent

the central United States during the warm season as well as many other parts of the

world including portions of South America, Africa, and Asia. The MCC, which can

cover 100,000 km

and last for 12 h or more, has been known to generate CG strikes

at rates exceeding 10,000 per hour. Goodman and MacGorman (1986) have noted

that the passage of the active portion of a single MCC can result in 25% of a region’s

annual CG total.

There are distinct regional differences in such param eters as the percentage of

ﬂashes that are of positive polarity and also of peak current. The U.S. region with the

largest number of positive polarity CGs (>12.5% of the total) is concentrated in a

broad belt in the interior of the United States, stretching from Texas north to

Minnesota. A study of the climatology of CGs with large peak currents (deﬁned

as >75 kA) found powerful þCGs were similarly clustered in a band stretching from

eastern New Mexico and Colorado northeastward into Minnesota (Lyons et al.,

1998). By contrast, large peak current CGs were largely conﬁned to the overocean

regions of the northern Gulf of Mexico and along the southeastern U.S. coastline and

the adjacent Atlantic ocean. The implications of these regional differences in ﬂashes

with large peak current upon facility design and protection as well as hazards to

human safety are yet to be explored.

The geographic distribution of lightning ﬂash density on a global basis is becom-

ing better known. A combination of proliferating land-based CG detect ion networks

and satellite observations should allow a more robust global climatology to emerge

over the next decade or so. For now we must rely on observations of total lightning

(IC þ CG events) made by polar orbiting satellites such as NASA’s Optical Transient

Detector (OTD). Figure 2 shows the elevated lightning densities over parts of North

America, the Amazon, central Africa, and the Maritime Continent of Southeast Asia.

5 CLI MATOLOGY OF LIGHTNING 415