Lightning Glossary
air discharge
A
form of lightning discharge probably similar to a cloud discharge in which
the lightning channel propagates away from a cloud charge center into
apparently clear air where it terminates. Thus, cloud charge is moved away
from its original location and space charge of opposite sign outside the
cloud may be neutralized.
atmospherics
The radio frequency electromagnetic
radiation originating, principally, in the irregular surges of charge in
thunderstorm lightning discharges. Atmospherics are heard as a
quasi-steady background of crackling noise (static) on certain radio
receivers, such as AM radio. Since any acceleration of electric charge
leads to emission of electromagnetic radiation, and since the several
processes involved in propagation of lightning lead to very large charge
accelerations, the lightning channel acts like a huge transmitter, sending
out radiations in a very wide band of frequencies with a broad peak of
about 10 kHz. Atmospherics may occasionally be detected at distances in
excess of 2000 miles from their source. Advantage has been taken of this
in using radio direction-finding equipment to plot cloud-to-ground
lightning locations, to locate active thunderstorm areas in remote
regions, and in-between weather reporting stations.
atmospheric electric field
A
quantitative term, denoting the electric field strength of the atmosphere
at any specified point in space and time. In areas of fair weather, the
atmospheric electric field near the earth's surface typically is about 100
volts per meter and is directed vertically in such a sense as to drive
positive charges downward to the earth. In areas of fair weather this
field decreases in magnitude with increasing altitude, falling, for
example, to only about 5 volts per meter at an altitude of about 10 km.
Near thunderstorms, and under clouds of vertical development, the surface
electric field varies widely in magnitude and direction, usually reversing
its direction immediately beneath active thunderstorms. In areas of
minimum local disturbance, a characteristic diurnal variation of electric
field strength is observed. This variation is characterized by a maximum
which occurs at about 19 hr UTC for all points on the earth, and is now
believed to be produced by thunderstorms which, for geographic regions,
are more numerous for the world as a whole at that universal time than at
any other. It is now believed that thunderstorms, by replenishing the
negative charge to the earth's surface, provide the supply current to
maintain the fair-weather electric field in spite of the continued flow of
the air-earth current that tends to neutralize that field.
ball lightning
A
relatively rarely seen form of lightning, generally consisting of an
orange, reddish, or dazzling blue-white ball a few cm to 30 cm in diameter
and of moderate luminosity, which may move up to 1 m/s horizontally with a
lifetime of a second or two. Hissing noises sometimes emanate from such
balls, and they can explode noisily or disappear noiselessly. The physical
nature of ball lightning is not understood. Similar phenomena occasionally
occur in high current industrial equipment, but the conditions for such
production are not reproducible.
bead lightning
A
particular visual variation of the end of a normal lightning flash where
periodic sections of the channel appear to die out slowly because they
have greater radius and hence lose heat more slowly, are seen end-on, or
for other unknown reasons.
breakdown
The process by which electrically-stressed air is transformed from an
insulator to a conductor. Breakdown involves the acceleration of electrons
to ionization potential in the electric field imposed by the thundercloud,
and the subsequent creation of new electrons which avalanche and expand
the scale of enhanced conductivity. Breakdown precedes the development of
lightning or higher current-carrying processes during lightning flashes.
charge separation
The physical process causing cloud electrification. The process can
include particle collisions with selective charge transfer and particle
capture of small ions at the particle scale. The process can include
gravity-driven differential particle motions and convective transport of
charged air parcels at the cloud scale.
cloud
electrification
The process by which clouds become electrified. This process separates
positive and negative electric charge and develops potential differences
occasionally sufficient to produce lightning.
cloud flash
A
lightning discharge occurring between a positively charged region and a
negatively charged region, both of which may lie in the same cloud. The
most frequent type of cloud discharge is one between a main positive
charged region and a main negative charged region. Cloud flashes tend to
outnumber cloud-to-ground flashes. In general, the channel of a cloud
flash will be wholly surrounded by cloud. Hence the channel's luminosity
typically produces a diffuse glow when seen from outside the cloud and
this widespread glow is called sheet lightning. Also called intracloud
flash, and cloud-to-cloud flash.
cloud-to-ground
flash
A
lightning flash occurring between a charge center in the cloud and the
ground. On an annual basis, negative charge is lowered to ground in about
95% of the flashes, the remaining flashes lowering positive charge to
ground. This type of lightning flash, which can be contrasted with an
intracloud flash or cloud-to-cloud flash, consists of one or more return
strokes. The first stroke begins with a stepped leader followed by an
intense return stroke which is the principal source of luminosity and
charge transfer. Subsequent strokes begin with a dart leader followed by
another return stroke. Most of the strokes use the same channel to ground.
The time interval between strokes is typically 40 milliseconds.
convection
In
atmospheric electricity, a process of vertical charge transfer by
transport of air containing a net space charge, or by motion of other
media (for example, rain) carrying net charge. Eddy diffusion of air
containing a net charge gradient may also yield a convection current.
corona
A
faint glow enveloping the high-field electrode in a corona discharge,
often accompanied by streamers directed toward the low-field electrode.
corona current
The electrical current that is equivalent to the rate of charge
transferred to the air from a pointed object (or array of objects)
experiencing corona discharge. Ordinarily, the corona current form
terrestrial objects at times of thunderstorm passage constitutes a
transfer of negative charge from air to object.
corona discharge
A
luminous, and often audible, electric discharge that is intermediate in
nature between a spark discharge (with, usually, its single discharge
channel) and a point discharge (with its diffuse, quiescent, and
non-luminous character). It occurs from objects, especially pointed ones,
when the electric field strength near their surfaces attains a value near
100,000 volts per meter. Aircraft flying through active electrical storms
often develop corona discharge streamers from antennas and propellers, and
even from the entire fuselage and wing structure. So-called precipitation
static results. It is seen also, during stormy weather, emanating from the
yards and masts of ships at sea. Known among mariners as St. Elmo's Fire.
electric charge
A
fundamental property of matter exhibiting two states, positive and
negative, that results in the action of electric forces in the presence of
an electric field. These two states were identified and named by Benjamin
Franklin. The positive charge on the proton and the negative charge on the
electron represent the fundamental charge, 1.6 x 10^-19 Coulombs.
electric
discharge
The flow of electricity through a gas, resulting in the emission of
radiation which is characteristic of the gas and of the intensity of the
current.
Electric Field Mill (EFM)
Device used to measure the surrounding
electric field. On a clear day, Electric Field Mill measurements normally
range from +500 to -500 volts per meter (V/m). In areas near thunderstorm
activity, the electric field intensity increases as the potential for
lightning increases. Electric field measurements of +/- 2000 V/m indicate
a high potential for lightning
Electrical Storm Identification Device
(ESID)
An omni-directional lightning detection
device that detects lightning discharges within a range of 30 miles (48
Km) and offers advanced warning of nearby storms. The device can sound an
internal alarm or relay a signal to trigger external equipment. The ESID
is solar powered.
field changes
The rapid variations of the electrical
field at the earth's surface, beneath, within and above thunderclouds.
Used to determine quantitative estimates of the charge transferred during
a lightning discharge, heights of the charge centers, and many other
features of thunderclouds.
flash
The entire cloud-to-ground electrical
atmospheric discharge. A flash may occur within clouds, between clouds, or
between clouds and earth. A flash includes one or more strokes and high
current pulses called return strokes. Duration of a flash is usually less
than one second.
forked lightning
The common form of cloud-to-ground
discharge always visually present to a greater or lesser degree which
exhibits downward-directed branches from the main lightning channel. In
general, of the many branches of the stepped leader only one is connected
to ground defining the primary, bright return stroke path, and the other
incomplete channels decay after the ascent of the first return stroke.
heat lightning
Non-technically, the luminosity observed
from ordinary lightning too far away for its thunder to be heard. Since
such observations have often been made with clear skies overhead, and
since hot summer evenings particularly favor this type of observation,
there has arisen a popular misconception that the presence of diffuse
flashes in the apparent absence of thunderclouds implies that lightning is
somehow occurring in the atmosphere merely as a result of excessive heat.
lightning
Lightning is a transient, high-current
electric discharge whose path length is measured in kilometers. The most
common source of lightning is the electric charges separated in ordinary
thunderstorm clouds (cumulonimbus). Well over half of all lightning
discharges occur within the thunderstorm cloud and are called intracloud
discharges. The usual cloud-to-ground lightning, sometimes called streaked
or forked lightning, has been studied more extensively than other
lightning forms because of its practical interest, as the cause of
injuries and death, disturbances in power and communicating systems, and
the ignition of forest fires, and because lightning channels below cloud
level are more easily photographed and studied with optical instruments.
Cloud-to-cloud and cloud-to-air discharges are less common than intracloud
or cloud-to-ground lightning. All discharges other than cloud-to-ground
are often lumped together and called cloud discharges.
Lightning is a self-propagating and
electrodeless atmospheric discharge that transfers through the induction
process the electrical energy of an electrified cloud into electrical
charges and current in its ionized and thus conducting channel. Positive
and negative leaders are essential components of the lightning. Only when
a leader reaches the ground, the ground potential wave (return stroke)
affects the lightning process. Natural lightning starts as a bidirectional
leader although at different stages of the process unidirectional leader
development can occur. Artificially triggered lightning starts on a tall
structure or from a rocket with a trailing wire. Most of the lightning
energy goes into heat, with smaller amounts transformed into sonic energy
(thunder), radiation, and light. Lightning, in its various forms, is known
by many names such as the common streak lightning, forked lightning, sheet
lightning, heat lightning and the less common air discharge; also, the
rare and mysterious ball lightning and rocket lightning. For some detailed
explanation of lightning processes, see lightning discharge and related
terms. (continued)
An abrupt, discontinuous natural electric
discharge in the atmosphere. Lightning produces a sound wave that is heard
as thunder.
An important effect of world-wide
lightning activity is the net transfer of negative charge from the
atmosphere to the earth. This fact is of great importance in one problem
of atmospheric electricity, the question of the source of the supply
current. Existing evidence suggests that lightning discharges occurring
sporadically at all times in various parts of the earth, perhaps 100 per
second, may be the principal source of negative charge that maintains the
earth-ionosphere potential difference of several hundred thousand volts in
spite of the steady transfer of charge produced by the air-earth current.
However, there also is evidence that point discharge currents may
contribute to this more significantly than lightning.
lightning channel
The irregular path through the air along
which a lightning discharge occurs. The lightning channel is established
at the start of a discharge by the growth of a leader, which seeks out a
path of least resistance between a charge source and the ground or between
two charge centers of opposite sign in the thundercloud or between a cloud
charge center and the surrounding air or between charge centers in
adjacent clouds. See also cloud-to-ground, intracloud, and air discharges.
lightning flash
The total observed lightning discharge
generally has a duration less than one second. A single flash is usually
composed of many distinct luminous events (strokes) that often occur in
such rapid succession that the human eye cannot resolve them.
lightning stroke
In a cloud-to-ground discharge, a leader
plus its subsequent return stroke. In a typical case, a cloud-to-ground
discharge is made up of three or four successive lightning strokes, most
following the same lightning channel.
lightning suppression
Procedures to prevent the occurrence of
lightning. Seeding below cloud base with 10 cm fiber chaff in a Colorado
study resulted in corona discharges that caused a discharging current to
flow within developing or active thunderstorms. Electric fields below
thunderstorms seeded with chaff decayed much faster than electric fields
below nonseeded storms, and chaff seeding of existing thunderstorms
greatly reduced cloud-to-ground flashes in non-seeded storms. Recent
evidence suggests that chaff releases may result in a significant decrease
in downwind cloud-to-ground lightning. Another experimental approach is to
use lasers to discharge lightning in an overhead cloud in order to divert
the flash from striking people or highly sensitive equipment on the
ground; more research is needed to make this a realistic method of
lightning suppression. In the 1960's, seeding with silver iodide was
considered in order to produce an excess of ice crystals to cause numerous
coronal discharges within the thunderstorm and reduce the need for the
flash to reach the ground, but the test results were complex and difficult
to identify. Finally, electric space charge was released into the
atmosphere from a network of high voltage wires on the ground to produce
corona discharge, but a field test showed minimal effects on suppressing
lightning.
peak current
The maximum current in kiloamperes (kA) of
the lightning discharge.
polarity
Lightning may be of a negative or positive
polarity depending on the direction of the current flow between a cloud
and the ground. Most lightning flashes are negative in polarity−negative
cloud to positive ground. Broadly defined polarity describes voltage as
positive or negative in reference to the originating point.
ribbon lightning
Ordinary cloud-to-ground lightning that
appears to be spread horizontally into a ribbon of parallel luminous
streaks when a very strong wind is blowing at right angles to the
observer's line of sight. Successive strokes of the lightning flash are
then displaced by small angular amounts and may appear to the eye or
camera as distinct paths. The same effect is readily created artificially
by rapid transverse movement of a camera during film exposure.
rocket lightning
A form of cloud discharge, generally
horizontal and at cloud base, whose luminous channel appears to advance
through the air with visually resolvable speed, often intermittently.
sheet lightning
A diffuse, but sometimes fairly bright,
illumination of those parts of a thundercloud that surround the path of a
lightning flash particularly a cloud discharge or cloud-to-cloud
discharge. Thus, sheet lightning is no unique form of lightning but only
one manifestation of ordinary lightning types in the presence of obscuring
clouds.
space charge
Any net electrical charge that exists in a
given region of space. In electronics, this usually refers to the
electrons in the space between the filament and plate of an electron tube.
In atmospheric electricity, space charge refers to a preponderance of
either negative or positive ions within any given portion of the
atmosphere. A net positive space charge is found in fair weather at all
altitudes in the atmosphere, and is largest near the earth's surface. The
general downward flux of this positive space charge is known as the
air-earth conduction current.
streak lightning
Ordinary lightning, of a cloud-to-ground
discharge that appears to be entirely concentrated in a single, relatively
straight lightning channel.
streamer
A sinuous channel of very high ion-density
which propagates itself though a gas by continual establishment of an
electron avalanche just ahead of its advancing tip. In lightning discharge
the stepped leader, dart leader, and return stroke all constitute special
types of streamers.
stroke
A singular electrical discharge or high
current pulse identified as a flash or as one of several strokes that make
up a flash of lightning. The transfer of electrical charge may be within
or between clouds or between clouds and earth.
The second stage of a lightning event is
called a "stroke." It creates a discharge of electrical energy, along with
a blinding white flash. These blinding white flashes are followed by the
crash of thunder due to the supersonic expansion of the air as it heats
up. The first leader/stroke is often followed by another leader/stroke
combination within a few hundredths of a second. Up to 20 or more such
sequences can occur, creating the flickering appearance of lightning.
Strokes can connect to the ground at more than one location, up to 7
kilometers (about 4.3 miles) apart! The combined strokes occurring in a
second around the first stroke location are called a "Flash".
subsequent stroke
The strokes that occur after the first
stroke of a flash.
thunderbolt
In mythology, a lightning flash
accompanied by a material bolt or dart; this is the legendary cause of the
damage done by lightning. It is still used as a popular term for a
lightning discharge accompanied by thunder.
zigzag lightning
Ordinary lightning of a cloud-to-ground
discharge that appears to have a single, but very irregular, lightning
channel.
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