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IEEE

TRANSACTIONS ON INDUSTRY APPLICATIONS. VOL. 30,

NO.

3, MAY/JUNE 1994

Natural Lightning

Martin A. Uman,

Fellow IEEE

785

Abstruct- The present understanding of natural lightning is

reviewed. Recent research on lightning has been motivated, in

part, by the

need

to protect advanced ground-based and airborne

systems that utilize low voltage, solid-state electronics; by the

desire

to

prevent spectacular accidents, such

as

occurredin 1 9

during the launch of Apollo 12 and in 1987 during the launch of

Atlas-Centaur 67; and by the desire to elucidate the physics of

one

of

natures most impressive phenomena.

I. INTRODUCTION

ENJAMIN Franklin, more than 200 years ago, proved

B hat lightning was an electrical discharge and measured

the sign of the cloud charge that produced it [l]. Modem

research on the physics of lightning began in the early 20th

century with the work of

C T R

Wilson [2], [3], 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 1930s, 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 modem ground-based and airborne systems.

A

good deal of the recent lightning research has been motivated

by three spectacular accidents involving aircraft or spacecraft:

(i) In 1963, a Boeing 707 jetliner flying near Elkton, MD,

at an altitude of about 5000 feet was struck and, via a fuel

explosion, was destroyed by lightning, killing all occupants

[4]. (ii) In 1969, the Apollo 12/Satum

V

space vehicle, during

launch through a weak cold front that was not producing

natural lightning, artificially initiated (or triggered) two

lightning flashes, one to ground and one intracloud discharge

[5], [6]. The vehicle and its crew survived various major

system upsets and minor permanent damage and were able

to complete successfully their mission to the moon. (iii) In

1987, an unmanned Atlas-Centaur space vehicle

AC167)

was

launched into weather conditions similar to those present at

the launch of Apollo 12 and triggered a lightning discharge

that compromised the vehicles control system, resulting in

the destruction of the Atlas-Centaur [7].

Paper ICPSD 93-1, approved by the Power Ssytems Engineering Depart-

ment of the IEEE Industry Applications Society, for presentation at the

1993 IEEE PROCEEDINGSand the 1993 I&CPS Dept. Technical Conference.

Manuscript released for publication October 7,1993. This work was supported

by the National Science Foundation under Grant Am-901 4084 .

M. A. Uman is with the Department of Electrical Engineering, University

of Florida, Gainesville,

FL

32611 USA.

IEEE

Log

Number 9400166.

d

A

1

3

2

4

Fig.

1.

Thundercloudcharge distribution and categorization of the four types

of lightning between cloud and ground. (Reprinted from

[26]

with permission.)

In this article, we will survey our present knowledge of

natural lightning. A companion paper in this issue will address

artificially initiated lightning.

11. 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, stratocu-

mulus, cumulus, nimbostratus, altocumulus, altostratus, and

cirrus clouds that might potentially produce lightning or, at

least, long (1 to 100 m) sparks [8], [9].

The classic model for the charge structure of a thundercloud

was developed in the 1920s and 1930s from ground-based

measurements of both thundercloud electric fields and the

electric field changes that are caused when lightning occurs

[2], [3], [lo], [ll]. In this model, the thundercloud forms

a

positive electric dipole as shown in Fig. 1 and Fig. 2; that

is, a primary positive charge region is found above a primary

negative charge region. By the end

of

the 1930s, Simpson and

co-workers [121, [131 had verified this overall structure from

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IEEE

TRANSACTIONS ON

IN D U S T R Y

APPLICATIONS, VOL 30, NO

3.

MAY/JLI\IE

1994

P

CLOUDCHARGE

DETRIBUTION

t O

19.00

ms

,-

.

;. K A N D J

.? PROCESSES

.

40 00 ms

PRELIMINARY STEPPED

BREAKDOWN LEADER

1 00

m s 1.10

m s

1.15 rns 1.20 ms

RETURN

20.00 m s 20.10 m s 20.15 ms 20.20

ms

60.00 m s

61.00 m s

{

62.00 m s

62.05

ms

Fig. 2.

Various processes that make up

a

negative

CG

lightning discharge. (Reprinted from 261 with permission.)

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 [ 141-[24].

111. NATURALLIGHTNING

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 Fig.

1. [25]. CG

flashes initiated by downward-

moving negatively-charged leaders probably account

for

about

90

of the

CG

discharges worldwide (Fig.

1 ,

category

I ,

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 has a maximum of 30 to 50, and a typical over-land

value of 2-5. Between

10 and 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 km-2yr-' [26].

IV. NEGATIVEG

LIGHTNING

A negative CG discharge (Fig.

I ,

category I 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

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U M A N NATURAL LIGHTNING I8

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 Fig. 2.The

stepped leader initiates the first return stroke after i t 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

ps

in duration, tens of meters in

length, and that the pause time between steps is

20

to 50

ps.

A

fully developed stepped leader can effectively lower 10 C or

more of negative charge toward the ground in tens of ms. The

average downward speed of propagation is about 2

x

lo5m/s

The average leader current is between 100 and lo00 A. The

leader steps have peak pulse currents of at least 1

kA.

During

its progression toward ground, the stepped leader produces a

downward-branched geometrical structure.

The potential difference between the lower portion of the

negatively charged leader and the Earth has a magnitude in

excess of

lo7

V. As the tip of the leader nears ground, the elec-

tric 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 ps.

The first return stroke produces a peak current of typically

30

kA 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

ps 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 shock

waves 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 kA or more and lowers a total charge on the order

of 1 C at a speed of about

3 x lo6

m / s 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

100

A and represent

a direct transfer of charge from cloud to ground

[27]-[31].

Between

25

and

50

of all CG flashes contain a continuing

current component

[27]-[3

13.

A comprehensive list of lightning current parameters has

been derived from tower measurements in Switzerland during

strikes initiated by downward leaders

[32], [33].

In these tower

studies the measured maximum rate-of-rise of current and the

duration of the current front are limited by the bandwidth of

measuring apparatus. The best data for these parameters are

probably derived from triggered lightning studies, where the

highest maximum rate-of-rise of current were found to be 4

x

10l1

A / s with a typical value of

1011

A / s and typical rise

times in the 100ns range

[34], [35].

Additional discussion of

these parameters is found in the last section of this paper.

V.

POSITIVECG LIGHTNING

Positive flashes to ground (category

3

in Fig.

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

[36].

Positive flashes

to ground

are

initiated by leaders that 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

[37], [38].

Positive

flashes are probably initiated by the upper positive charge in

thunderclouds (Fig. 1) where this charge has been separated

horizontally from the lower negative charge by wind shear

[36], [39],

but this may not always be a necessary condition

[40]-[42].

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

[43],

[44],

although storms with predominantly negative lightning

often end with positive discharges. The fraction of positive

discharges in summer thunderstorms apparently increases with

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O N IN D U S T R Y

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MA Y /J U N E

1Y93

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.

[46]-[52]. Lightning has also been reported when there is a

clear blue sky

[531-[58),

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 10or

more km away, out of viewing range, from where the lightning

originates. However, there are photographs and supporting

charge locations that show that a triggered discharge can

I. UPWARD LIGHTNING

The leaders in upward-initiated lightning are usually pos-

itive (Fig. 1 category 2). Positive upward leaders show a

continuous luminosity that is modulated in a fashion similar to

positive downward-stepped leaders. Negati1.e 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 A, at ground. In about half of the upward-

initiated events, however, the continuous current is followed

by a sequence of dart leaders and retum strokes that are similar

to those following first strokes in natural

CG

discharges that

are initiated by negative downward-moving leaders. A detailed

review of the measurements that have been made prior

to

1987

on upward-initiated lightning is found in

[26].

Lightning that is artificially initiated, as discussed

i n

a

companion paper in this issue, begins with an upward leader

from the triggering rocket.

VII.

C L O U D

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 [4SJ .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

by-product.

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

I O

km. The

discharge process is thought to consist of a breakdown phase

followed by a continuously propagating leader that generates

weak return strokes 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 Kch an ge s that occur

in the intervals between retum strokes in

CG

discharges. A

detailed review of the literature on cloud discharges is found

in [26j.

VIII.

TOP-OF-THE-1 2 0 U D A N D CLEAR AIRLIGHTNING

There are numerous reports of lightning propagating up-

ward from the tops of clouds and perhaps to the ionosphere

occur entirely in clear air near a thunderstorm 1581. In the

case cited, there was a thunderstorm about

10

km 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 aky overhead where the charge appears to have

been located was mostly clear with broken altocumulus and

altostratus clouds at higher altitudes.

1x.

LIGHTNING AVOIDANCEN D PROTECTION

When one is faced with the possibility of a lightning hazard,

there are two methods of protection:

i )

identify and avoid the

hazard and (ii) harden tile system of interest to withstand the

effects of nearby and direct strikes.

In

the last

10

years, exceptional progress has been made

in the technology for identifying and locating natural

CG

lightning, and commercial nationwide networks of lightning

sensors with a wideband magnetic direction-finding technol-

ogy that produce lightning locations in real time are now

operational in the United States and in about

IS

other countries

[59]-[63]. Many networks are being used both for research

and for providing warnings of natural lightning in a variety

of applications. Two addition techniques for lightning location

have recently become available commercially, time-of-arrival

and interferometry, with these technologies now being used in

the U.S. and in several other countries

1641-[691.

In

order to design better lightning protection systems to

improve the capability of ground-based facilities and of aircraft

and space vehicles to withstand the effects of nearby or direct

strikes, it is essential to understand the nature

of

the lightning

environment near and at the point of a direct strike. Among the

most important parameters responsible for coupling lightning

signals to electronic systems is the maximum current and the

maximum rate of change of current with respect to time.

Return stroke current measurements made on towers with

wideband transient recorders 701-1

72 1

currents measured

during lightning strikes to a F-l06B research aircraft

731.

and retum stroke current measurements in rocket-initiated

discharges

[

341,

13.51

show that the peak current rate-of-rise

is considerably larger than was believed to be the case a

decade or two ago. A maximum value of

3.8

x 101A/s has

been measured on the F-106

B

aircraft in flight, and values

above

2

x

10

A/s occurred frequently

1731.

Only a few

measurements

of

current rate of rise have been made with

sufficient bandwidth

on

natural retum strokes. but from this

small sample a peak value of 1.8 x

lO1]

A/s has been observed

1701-[72]. Finally, the rocket-triggered discharges to saltwater

at the NASA Kennedy Space Center have produced a peak

current rate-of-rise for return strokes of 4.1 x

lo1

A/s

[

341,

[ 3 S ] . hus, from the limited data that are available, we expect

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that a peak current rate-of-rise near 5 x 10I1 A / s is possible,

even at flight altitudes.

Although considerable progress has been made recently in

understanding lightning vis-a-vis lightning protection, we still

need to understand more thoroughly the following important

aspects of lightning: the total wave shapes

of

the currents that

are associated with all

of

the salient lightning processes in both

natural and triggered events, the physical processes by which

natural lightning attaches

to

ground-based structures and to

aircraft, and the role

of

the structures and aircraft in initiating

lightning.

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Martin A. Uman, SM73, F88) received the

bachelors, masters and doctoral degrees from

Princeton University, Princeton, NJ, USA, the latter

in 1961.

He is currently a Professor of Electrical

Engineering and Chairman of the Department

of Electrical Engineering at the University of

Florida, Gainesville, FL. He has also headed that

Departments Lightning Research Laboratory for

more than

20

years. He has written three

books

on

lightning (two in second editions) as well as a book

on plasma physics, and has published more than 125 reviewed journal papers

on

lightning and long sparks. He was an associate professor of Electrical

Engineering at the University

of

Arizona, Tucson, AZ, from

1961

to

1964.

He joined the University of Florida faculty in 1971 after working as a fellow

physicist at Westinghouse Research Labs in Pittsburgh. He co-founded and

served as president of Lightning Location and Protection, Inc. from 1975

to

1983.

Dr. Uman was named the 1990 Florida Scientist of the Year by the Florida

Acadetny of Sciences. Honored as 1988-89 University of Florida Teacher-

Scholar of the Year (the highest faculty award at UF), he is a Fellow of three

professional societies : the American Geophysical Union AGU) . American

Meteorological Society (AMS), and the

IEEE.