air conductivity See conductivity

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.

air--earth conduction current (Also called fair-weather current)

That part of the air-earth current contributed by the electrical conduction of the atmosphere itself. It is represented as a downward current in storm-free regions all over the world. The conduction current is the largest portion of the air-earth current, far outweighing the contributions made by the precipitation current and convection current which are zero in storm-free regions.. Its magnitude is approximately 3 x 10^-12+ amp/m^2+, or about 1800 amperes for the entire earth. Such observations of the vertical variation of the conduction current as have been made indicate that it is approximately uniform throughout the troposphere, a condition that is consistent with the generally accepted view that the conduction current flows from a positively charged conducting region in the lower ionosphere downward to the negatively charged earth. Only in areas of temporarily disturbed weather does the conduction current become replaced by reverse flow. Accumulating evidence points to the conclusion that the conduction current continues to exist only because of the action of thunderstorms scattered at all times over the earth, which supply the positive charge to the upper atmosphere and negative charge to the earth ( see supply current ).

air--earth current

The transfer of electric charge from the positively-charged atmosphere to the negatively-charged earth. This current is made up of the air-earth conduction current ,a point-discharge current, a precipitation current , a convection current , and miscellaneous smaller contributions. Of these, the first is by far the largest. The existence of this quasi-steady current in fair weather and the observed maintenance of the earth's net negative charge are both better established than the nature of the supply current which must replenish the positive charge in the upper atmosphere and the negative charge on the earth.

aircraft electrification

The accumulation of a net electrical charge on the surface of aircraft; or the separation of charge into two concentrations of opposite sign on distinct portions of the aircraft surface. Net charges appears as a result of autogenous electrification when aircraft fly through clouds of ice crystals or dust. Charge separation occurs by induction when aircraft fly through regions of strong atmospheric electrical field, as in thunderstorms. Charging may also occur by the engine exhaust carrying away a net charge leaving the aircraft charged. Development of large local charges on aircraft parts may lead to appearance of corona discharge or St. Elmo's fire and is almost always accompanied by poor radio communication due to so-called precipitation static .

arc discharge

A luminous electrical discharge in a gas where the current flows more of less continuously along a narrow channel of high ionization. An arc discharge requires a continuous source of electric potential difference across the terminals of the arc. This steady-state high current discharge is to be distinguished from the low current and visually diffuse corona discharge and point discharge, as well as from the transient, high luminosity, high-current spark discharge.

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.

atmospheric electricity - 1

Electrical phenomena, regarded collectively, which occur in the earth's atmosphere. These phenomena include not only such striking manifestations as lightning and St. Elmo's fire , but also less noticeable but more ubiquitous effects such as atmospheric ionization , the air-earth current , and other quiescent electrical processes. The existence of separated electric charges in the atmosphere is a consequence of many minor processes (spray electrification, dust electrification, etc.) and a few major processes (cosmic ray ionization, radiactive-particle ionization, and thunderstorm electrification). The details of thunderstorm charge separation are poorly understood at the present time, which is unfortunate in that the maintenance of the prevailing atmospheric electric field is now widely believed to be due to thunderstorm effects.

atmospheric electricity - 2

The study of electrical processes occurring within the atmosphere.

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 with frequencies of the order of 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.

autogenous electrification

The process by which net charge is built up on an object, such as an airplane, moving relative to air containing dust or ice crystals. The electrification is produced by frictional effects ( triboelectriciation ) accompanying contact between the object and the particulate matter.

ball lightning

A relatively rarely seen form of lightning, generally consisting of an orange or reddish ball of the order of 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 lighting is not understood. Similar phenomena occasionally occur in high current industrial equipment, but the conditions for such production are not reproducible.

beaded 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.

Bishop wave

A striking example of an atmospheric lee wave , formed in the lee of the Sierra Nevada range near Bishop, California. The phenomenon includes a rotor cloud and a series of lenticular clouds parallel to the crest of the range. [Note that this is not an atmospheric electricity term. It was assigned by mistake to R. Orville. Please reassign this word to someone in cloud dynamics]

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.

breaking-drop theory

A theory of thunderstorm charge separation based upon the suggested occurrence of the Lenard effect in thunderclouds, that is, the separation of electric charge due to the breakup of water drops. This theory, advance by Sir George C. Simpson in 1927, was initially intended to account for a bipolar charge distribution within a thundercloud having the main positive charge center near the base of the cloud and the main negative charge center higher up. Simpson's theory, however, does not explain this phenomenon well because temperatures are below the freezing point in this region of the cloud. Evidence does remain to support the weak positive charge center that lies slightly below the lower main negative charge concentration in many, if not all, thunderclouds where the temperature is above the freezing point. Hence the breaking-drop theory is best ascribed to this localized secondary positive charge center. The mechanism was also proposed as an explanation for waterfall electricity.

chain lightning

Same as bead or beaded lightning.

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.

Clayden effect

When a photographic emulsion is given a very brief exposure to light of high intensity, it is desensitized towards a subsequent longer exposure to light of moderate intensity. That is, the second exposure produces less effect than if the pre-exposure had not been given. This phenomenon was observed originally by Clayden when photographing lightning flashes, but it can be produced equally well by any type of light source, provided the intensity is sufficiently high and the duration short enough.

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

(also called cloud flash, intracloud flash, cloud-to-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.

cloud-to-cloud flash

A type of cloud flash. A lightning discharge that occurs between two electrified clouds.

cloud-to-ground flash

A lightning flash occurring between a charge center (either negative or positive) in the cloud and the ground. Negative discharges are about 10 times more common than positive. The cloud-to-ground discharge is initiated by a downward propagating leader followed by an extremely intense upward-moving return stroke that is the principal source of luminosity. A cloud-to-ground discharge is usually a composite event, called a flash, composed of several distinct leader-return-stroke sequences, the majority using the same channel following each other at intervals of a few hundredths of a second (see return stroke, stepped leader dart leader).

columnar resistance

In atmospheric electricity, the electrical resistance of a column of air one meter square, extending from the earth's surface to some specified altitude. Measurements extending to an altitude of 18 km indicate that the atmospheric columnar resistance to that height amounts to about 10^17 ohm per m^2. Probably, this is only slightly less than the total columnar resistance from earth to ionosphere. In fact, roughly half of the total columnar resistance from earth to 18 km is contributed by the lowest 3 km of the column where, in addition to the greater density of the air, the high concentration of atmospheric particulates leads to a relatively high population of poorly conducting large ions rather than the more mobile small ions. Total columnar resistance does not vary greatly with either time or locality. In contrast to this is the wide variation in columnar resistance of the lowest fraction of a kilometer. It is chiefly because of the latter variations that the sea-level atmospheric electric field fluctuates, especially in industrial areas of highly variable atmospheric pollution.

conduction current

The migration of charged particles in a gaseous medium acted upon by an external electric field.

conductivity current

same as air-earth conduction current

continuing current

A sustained current in the lightning stroke that flows to the ground after the return stroke. Continuing currents can have a duration in excess of 100 ms with magnitudes of typically 100 amperes. Continuing currents occur in negative and positive cloud-to-ground flashes.

continuous leader

same as dart leader

convection current

Any net transport of electric charge effected through mass motions of some charged medium; any electric current induced by other than electrical forces. In atmospheric electricity, the convection current is part of the air-earth current of charge transfer vertically between the earth's surface and the upper atmosphere. The term includes not only eddy diffusion currents existing in regions of net space charge but also currents due to fall of charged precipitation particles (precipitation current ).

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 (e.g., rain) carrying net charge. Eddy diffusion of air containing a net charge gradient may also yield a convection current.

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 m. 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. ( See St. Elmo's fire )

corposant

same as corona discharge

current

Any movement of electric charge in space, by virtue of which a net transport of charge occurs as, for example (in atmospheric electricity), in a conduction current , convection current , an precipitation current .

dart leader

The leader which, after the first stroke, typically initiates each succeeding stroke of a multiple-stroke flash lightning. (The first stroke is initiated by a stepped leader.) The dart leader derives its name from its appearance on photographs taken with streak cameras. The dart leader's brightest luminosity is at its tip which is tens of meters in length, propagating downward at about 10^7 m/s. In contrast to stepped leaders, dart leaders do not typically exhibit branching because the pre-established channel's low gas density and residual ionization provide a more favorable path for this leader than do any alternative ones.

dielectric strength

A measure of the resistance of a dielectric to electrical breakdown under the influence of strong electric fields; usually expressed in volts per meter. The dielectric strength of dry air at sea-level pressures is about 3,000,000 volts per meter. The exact value for air depends upon geometry of the electrodes between which the electric field is established, upon the humidity, and upon whether or not water drops are present in the air ( Macky effect ).

direction finder

An instrument consisting of two orthogonal magnetic loop antennas and associated electronics for the purpose of detecting azimuth to a cloud-to-ground lightning flash.

dissipation constant

In atmospheric electricity, a measure of the rate at which a given electrically charged object loses its charge to the surrounding air. If the object bears a charge [s] at time [s], then according to a law established by Coulomb, IFI, where [s] is the object's dissipation constant expressed in reciprocal time units. It is found that [s] depends not only upon the geometry of the charged object but also upon the density of the surrounding air, its humidity, and its motion relative to the object.

dust-devil effect

In atmospheric electricity, a rather sudden and short-lived change of the vertical component of the atmospheric electric field that accompanies passage of a dust devil near an instrument sensitive to the vertical gradient. Such changes may be either positive or negative and the charge is probably produced by triboelectrification .

dynamo theory

The hypothesis, first proposed by Balfour Stewart, which explains the regular daily variations in the earth's magnetic field in terms of electrical currents in the lower ionosphere , generated by tidal motions of the ionized air across the earth's magnetic field.

earth current

A large-scale surge of electric charge within the conductive eartht, associated with a disturbance of the ionosphere . Current patterns of quasi-circular form and extending over areas the size of whole continents have been identified and are known to be closely related to solar-induced variations in the extreme upper atmosphere.

earth--air current

see air-earth conduction current

earth-current storm

Irregular fluctuations in an earth current, often associated with electric field strengths as large as several volts per kilometer, in the earth's crust, superimposed on the normal diurnal variation of the earth currents. Such storms are closely related to magnetic storms.

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 currents in the atmosphere

see air-earth current

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 intensity

same as electric field strength

electric field strength

The electrical force exerted on a unit positive charge placed at a given point in space. The electric field strength is expressed, in the mks system of electrical units, in terms of volts per m and is a vector quantity. The electric field strength of the atmosphere is commonly referred to as the atmospheric electric field .

electric field

same as electric field strength

electric intensity

same as electric field strength

electric potential gradient

Same as electric field strength except of opposite sign.

electric storm

same as electrical storm

electrical breakdown (Term Reference: electrical breakdown or, simply, breakdown)

The sudden decrease of resistivity of a substance when the applied electric field strength rises above a certain threshold value (the substance's dielectric strength). For air at normal pressures and temperatures, experiment has shown that the breakdown process occurs at a field strength of about 3,000,000 volts per m. This value decreases approximately linearly with pressure, and is dependent upon humidity and traces of foreign gases. In the region of high field strength just ahead of an actively growing leader in a lightning stroke, breakdown occurs in the form of a rapidly moving wave of sudden ionization (electron avalanche). The dielectric strength in a cloud of water drops is less than that in cloud-free humid air, for the water drops are distorted as a result of the Macky effect .

electrical discharge

see electric discharge

electrical storm

Name sometimes applied to a relatively rare condition of disturbed atmospheric electric field in the lower atmosphere that arises when strong winds are blowing and much dust is in the air, but involving no thunderstorm activity. Triboelectrification due to the blowing dust may charge fences and other metallic objects to such an extent that slight shocks are felt upon touch.

electrical storm

same as earth-current storm

electricity of precipitation

see precipitation current

electrode effect

The accumulation of an excess of ions of positive sign in the neighborhood of a negative electrode, and vice versa, when ions are continuously produced in the space above the electrode and move under the influence of the electrode's field. The electrode effect is observed over water surfaces where radioactivity is not present.

electron avalanche

The process in which a relatively small number of free electrons in a gas that is subjected to a strong electric field accelerate, ionize gas atoms by collision, and thus form new electrons to undergo the same process in cumulative fashion. All streamers in a lightning discharge propagate by formation of electron avalanches in the regions of high electric field strength that move ahead of their advancing tips. Particularly in the case of the intense return streamer , avalanche processes are enhanced by formation of photoelectrons as a result of ultraviolet radiation emitted by the excited molecules in the region just behind the tip. An avalanche cannot possibly begin until the local electric field strength is high enough to accelerate a free electron to the minimum ionizing speed in the space and time interval corresponding to one mean free path of the electron, for upon collision, the electron usually loss its forward motion in the direction of the field. Maintenance of an avalanche requires a large reservoir of charge, such as accumulates more or less periodically in active thunderstorms.

exogenous electrification

The separation of electric charge in a conductor placed in a preexisting electric field. This is especially applied to the charge separation observed on metal-covered aircraft. It is the result of induction effects, and does not by itself create any net total charge on the conductor. It is to be distinguished, therefore, from autogenous electrification .

fair-weather current

same as air-earth conduction current

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.

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.

gaseous discharge

same as electric discharge

gaseous electric discharge

same as electric discharge

ground discharge

same as cloud-to-ground discharge

ground streamer

An upward advancing column of high ionization (a streamer or arc) which typically ascends from a point on the earth's surface toward a descending stepped leader. The ground streamer usually joins the stepped leader about fifty meters above the ground, after which the upward-propagating light and current of the return stroke begin. Ground streamers occur because of the very high electric field intensities that build up directly below the descending, charged stepped leader. Often, more than one ground streamer starts up from the general area under a descending leader, but usually only one makes contact with the leader.

ground-to-cloud discharge

A lightning discharge in which the original leader process starts upward from some object on the ground; the opposite of the more common cloud-to-ground discharge. Ground-to-cloud discharges most frequently emanate from very tall structures which, being equipotential with the earth, can exhibit the strong field intensities near their upper extremities necessary to initiate leaders.

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.

igneous meteor

In United States weather observing practice, a visible electrical discharge in the atmosphere. Lightning is the most common and important type, but types of corona discharge are also included.

intercloud discharge

same as cloud-to-cloud discharge

intracloud discharge

same as cloud discharge

ion concentration

same as ion density

ion density

In atmospheric electricity, the number of ions per unit volume of a given sample of air; more particularly, the number of ions of given type (positive small ion, negative small ion, positive large ion, etc.) per unit volume of air.

ion mobility

In gaseous electric conduction, the average velocity with which a given ion drifts through a specified gas under the influence of an electric field of unit strength. Mobilities are commonly expressed in units of m/sec per volt/m. In a vacuum, a single gaseous ion subjected to any non-zero potential gradient would accelerate indefinitely; but in the midst of a gas the ion continually experiences collisions with gas molecules. These encounters tend to break up its trajectory into a series of short intervals of acceleration punctuated by deflections. The net result is that the ion's gross motion resembles drift at a uniform velocity. The mobility depends not only upon the nature of the ion and gas but also upon the density of the gas, for the latter controls the mean free path of the ion. In atmospheric electricity, the mobilities of small and large ions weight their relative importance in atmospheric conduction. Small ions have mobilities of about 1.3 x 10[-4] m/sec per volt/m in air at sea level with negative small ions exhibiting slightly greater values than do the positive small ions. High humidities suppress small ion mobilities slightly. Large ions have mobilities of only about 4 x 10[-7] m/sec per volt/m at sea level, their sluggishness being due to their great mass.

ion

In atmospheric electricity, any of several types of electrically charged submicroscopic particles normally found in the atmosphere. Atmospheric ions are of two principal types, small ions an large ions , although a class of intermediate ions has occasionally been reported. The ionization process which forms small ions depends upon two distinct agencies, cosmic rays and radioactive emanations. Each of these consists of very energetic particles which ionize neutral air molecules by knocking out one or more planetary electrons. The resulting free electron and positively charged molecule (or atom) very quickly attach themselves to one or, at most, a small number of neutral air molecules, thereby forming new small ions. In the presence of Aitken nuclei , some of the small ions will in turn attach themselves to these nuclei, thereby creating new large ions. The two main classes of ions differ widely in mobility. Only the highly mobile small ions contribute significantly to the electrical conductivity of the air under most conditions. The intermediate ions and large ions are important in certain space charge effects, but are too sluggish to contribute much to conductivity. The processes of formation of ions are offset by certain processes of destruction of the ions ( see recombination ).

ion-capture theory

A theory of thunderstorm charge separation advanced by C. T. R. Wilson (1916). According to this theory, the lower negative charge of a thundercloud is generated by the accumulation there of raindrops which have captured predominantly negative ions in their descent through the cloud. The preferential capture of negative ions by such drops is said to be due to the polarization of the drops in the normal atmospheric electric field existing between the negatively charged earth and positively charged ionosphere. The lower halves of the falling drops therefore would attract and capture negative charges while their upper halves would be unable to draw in positive charges with comparable efficiency; so a net negative charge builds up on the drops. This theory is generally regarded today as incapable of accounting for any important portion of thunderstorm charge separation, for it is quantitatively inadequate in view of typical ion densities.

ionic conduction

Any electrical conduction where the current is sustained by the motion of ions (as opposed to electrons) within the conductor. All electrical conduction in the atmosphere is of this type.

ionic mobility

see ion mobility

ionization

In atmospheric electricity, the process by which neutral atmospheric molecules ( small ions ) or other suspended particles (mainly large ions ) are rendered electrically charged chiefly by collisions with high-energy particles. Cosmic rays and emanations from radioactive gases are the main sources of atmospheric ionization. In the lower atmosphere, decay electrons of mu-mesons plus alpha particles from radioactive gases, as well as beta particles and gamma rays, serve to ionize air molecules. The rate at which these agencies ionize the air is expressed in units of one ion pair per cubic centimeter per second, symbolized by I. Cosmic rays at sea level yield about 2 I, both over land and at sea. Radioactive gases contribute about 5 I over land areas at sea level, while radioactive materials in the soil and rocks themselves yield about 4 I. At heights above about 5 km, only cosmic rays provide significant ionization, and this contribution finally reaches a maximum at about 13 km, above which the rate decreases, due to decreasing air density and consequent lack of target molecules for the cosmic rays.

junction streamer

The process by which negative charge centers at successively more distant locations in a thundercloud are "tapped" for discharge by successive strokes of cloud-to-ground lightning.

leader

The electric discharge which initiates each return stroke in a cloud-to-ground lightning discharge. It is a channel of high ionization which propagates through the air by virtue of the electric breakdown at its front produced by the charge it lowers. The stepped leader initiates the first stroke in a cloud-to-ground flash and establishes the channel for most subsequent strokes of a lightning discharge. The dart leader initiates most subsequent strokes. Dart-stepped leaders begin as dart leaders and end as stepped leaders. The initiating processes in cloud discharges are sometimes also called leaders but their properties are not well measured.

Lenard effect

The separation of electric charges accompanying the aerodynamic breakup of water drops, first studied systematically by the German physicist P. {Lenard}. Experiments have shown that the degree of charge separation in spray processes depends upon the drop temperature, presence of dissolved impurities, speed of the impinging air blast, and contact with foreign surfaces. The largest fragments of the broken drops are observed to carry positive changes and the fine spray of drops carried off in the impinging air current carries a net negative charge. Distilled water drops of 4 mm diameter, broken after a five centimeter free fall into an updraft of 1 m/sec, were found by {Chapman} to yield about 10[-10] coulombs of separated charge per drop. The Lenard effect was incorporated by {Simpson} into his breaking-drop theory of thunderstorm charge generation, but many critical details are but poorly understood.

lightning

Lightning is a transient, high-current electric discharge whose path length is measured in kilometers. The most common sources of lightning is the electric charge 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 (i.e., 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 clout-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 bi-directional leader although at different stages of the process uni-directional 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. (See also cloud-to-ground, intracloud, and air discharges) 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.) 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 important 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.

lightning discharge

The series of electrical processes taking place within one second by which charge is transferred along a discharge channel between electric charge centers of opposite sign within a thundercloud (intracloud discharge) between a cloud charge center and the earth's surface (cloud-to-ground discharge or ground-to-cloud discharge), within two different clouds (intercloud or cloud-to-cloud discharge), or between a cloud charge and the air (air discharge). It is a very large-scale form of the common spark discharge. A single lightning discharge is called a lightning flash.

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 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 he 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 silveer 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.

mobility

Quantitative measure of the velocity of ions in an electric field of unit strength. The mobility can be expressed as Dmu+3Dv/E where v is the velocity of the ion and E is the electric field strength. The units are m^2 Volts^(-1) s^(-1). Heavier ions will have lower mobilities than lighter ones and values of mobility vary inversely with density producing slower mobilities near the surface.

negative rain

Rain which exhibits a net negative electrical charge.

pearl lightning

same as beaded lightning

pearl necklace lightning

same as beaded lightning.

pilot streamer

A relatively slow-moving, non-luminous lightning streamer, the existence of which has been postulated but not verified, to provide a physical explanation for the observed intermittent mode of advance of a stepped leader as it initiates a cloud-to-ground lightning discharge. Whereas the stepped leader descends at an average speed of the order of 10^5 m/s during its downward motion, it advances only about fifty meters at a time with higher speed and then pauses for fifty to one hundred microseconds before resuming its downward movement. The average downward speed has been associated with an invisible streamer, the pilot streamer, which is postulated to descend at a uniform speed only slightly in excess of ionizing speed of electrons in air and lay down a trail of weak residual ionization along which the stepped leader moves very rapidly in a pulsating manner. The idea of a pilot leader has been supplanted by more modern theory based on laboratory measurements of long spark generation.

point discharge

A silent, non-luminous, gaseous electrical discharge from a pointed conductor maintained at a potential which differs from that of the surrounding gas. In the atmosphere, trees and other grounded objects with points and protuberances may, in disturbed weather, be sources of point discharge current . Close to a pointed and grounded conductor that extends above surrounding objects, the local electric field strength may be many times greater than that existing at the same level far from the elevated conductor. When this local field reaches such a value that a free electron, finding itself acted upon by this field, can be accelerated (in one mean free path ) to a sufficiently high velocity to ionize neutral air molecules, point discharge will begin. Different structures will yield point discharge under quite different gross field conditions, for geometry is critically important. Point discharge is recognized as a major process of charge transfer between electrified clouds and Earth, and is a leading item in the charge balance of the global electrical circuit.

point discharge current

The electrical current accompanying any specified source of point discharge . In the electrical budget of the earth-atmosphere system, point discharge currents are of considerable significance as a major component of the supply current . Estimates made by B. F. J. {Schonland} of the point discharge current from trees in arid Southwest Africa suggest that this process accounts for about twenty times as much delivery of negative charge to the earth as do lightning discharges during typical thunderstorms. Although the great height of thundercloud bases in arid regions, such as that referred to in Schonland's study, tends to favor point discharge over lightning charge transfer, point discharge still seems more significant than lightning even in England, where {Wormell} found for Cambridge a ratio of about five to one in favor of point discharge over lightning charge transfer.

polarization

With respect to particles in an electric field , the displacement of the charge centers within a particle in response to the electric force acting thereon.

positive rain

Rain having a net positive electrical charge.

potential gradient

In general, the local space rate of change of any potential , as the gravitational potential gradient or the velocity potential gradient. In atmospheric electricity, the electric potential gradient ( electric field strength ) of the atmosphere is commonly referred to as the atmospheric electric field . The electric field is the negative of the potential gradient.

precipitation current

The downward transport of charge, from cloud region to earth, that occurs in a fall of electrically charged rain or other hydrometeors; a particular case of a convection current . Observations of the charge on individual raindrops during thunderstorms have revealed a complex picture. On the average, more positive than negative charge is brought to earth by precipitation currents, but wide deviations occur both within individual storms and from one storm to another. The reasons for these wide fluctuations are not understood. Precipitation currents in continuous rain generally vary from about 10^[-12] to 10^[-10] amp/(m^2), while thunderstorm currents become as large as 10^[-8] amp/(m^2).

precipitation electricity _ 1

That branch of the study of atmospheric electricity concerned with the electrical carried by precipitation particles and with the manner in which these charges are acquired.

precipitation electricity - 2

The electrical charge borne by precipitation particles. A very complex and highly variable picture is obtained when charges are measured on individual raindrops or snow crystals and no present theory approaches a complete explanation of all details. In general, more raindrops are positively than negatively charged. Sometimes the prevailing sign of the charges even shifts in the course of a given storm's lifetime.

radius of protection

The radius of the circle within which a lightning discharge will not strike due to presence of an elevated lightning rod at the center. A rule of thumb is that this radius is equal to the height of the rod, though cases of lightning damage inside this distance are well established.

return stroke

The intense luminosity which propagates upward from earth to cloud base in the last phase of each lightning stroke of a cloud-to-ground discharge . In a typical flash, the first return stroke ascends as soon as the descending stepped leader makes electrical contact with the earth, often aided by short ascending ground streamers . The second and all subsequent return strokes differ only in that they are initiated by a dart leader and not a stepped leader. It is the return stroke which produces almost all of the luminosity and charge transfer in most cloud-to-ground strokes. Its great speed of ascent (about 1 X 10^8 m/sec) is made possible by residual ionization of the lightning channel remaining from passage of the immediately preceding leader, and this speed is enhanced by the convergent nature of the electrical field in which channel electrons are drawn down toward the ascending tip in the region of the streamer's electron avalanche . Current peaks as high as 300,000 amp have been reported, and values of 30,000 amp are fairly typical. The entire process of the return stroke is completed in a few tens of microseconds, and even most of this is spent in a long decay period following an early rapid rise to full current value in only a few microseconds. Both the current and propagation speed decrease with height. In negative cloud-to-ground flashes the return stroke deposits the positive charge of several coulombs on the preceding negative leader channel, thus charging earth negatively. In positive cloud-to-ground flashes, the return stroke deposits the negative charge of several tens of coulombs on the preceding positive leader channel, thus lowering positive charge to ground. The entire return stroke process is completed in a few tens of microseconds. In negative cloud-to-ground flashes, multiple return strokes are common. Positive cloud-to-ground flashes, in contrast, typically have only one return stroke. The return streamer of cloud-to-ground discharges is so intense because of the high electrical conductivity of the ground, and hence this type of streamer is not to be found in air discharges , cloud discharges , or cloud-to-cloud discharges.

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.

sferics fix

The determination of the bearing to the lightning source usually based on the measurement of the horizonatal magnetic field with orthogonal coils or loop antennae.

sferics observation

The detection of electromagnetic radiation from lightning generally in the frequency range 10-30 kHz. The physical measurement can include the electric field, the magnetic field, or both. Sferics are generally attributed to the high current phases of lightning, i.e., to return strokes and K-changes.

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 as 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 .

spark discharge

That type of gaseous electrical discharge in which the charge transfer occurs transiently along a relatively constricted path of high ion density , resulting in high luminosity. It is of short duration and to be contrasted with the non-luminous point discharge and corona discharge , and also with the continuous arc discharge . The exact meaning to be attached to the term ``spark discharge'' varies somewhat in the literature. It is frequently applied to just the transient phase of the establishment of any arc discharge. A lightning discharge can be considered a large scale spark discharge.

spray electrification

A process of charge separation associated with the mechanical disruption of liquid drops and believed to be based on the electrical double layer at the air-water interface. Spray electrification is the common explanation for the electric field generation in the vicinity of waterfalls.

stepped leader

The initial leader of a lightning discharge; an intermittently advancing column of high ionization and charge which establishes the channel for a first return stroke. The peculiar characteristic of this type of leader is its step-wise growth at intervals of about fifty to one hundred microseconds. The velocity of growth during the brief intervals of advance, each only about one microsecond in duration, is quite high (about 5 x 10^7 m/s), but the long stationary phases reduce its effective speed to only about 5 x 10^5 m/s. To help explain its mode of advance, the concept of a pilot streamer was originally suggested but has been supplanted by analogy to recent work on long laboratory sparks.

magnetic storm

A worldwide disturbance of the earth's magnetic field. Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normalcy, which may take several days. Magnetic storms are caused by solar disturbances, though the exact nature of the link between solar and terrestrial disturbances is not understood. They are more frequent during years of high sunspot number. Sometimes a magnetic storm can be linked to a particular solar disturbance. In these cases, the time between solar flare and onset of the magnetic storm is about one or two days, suggesting that the disturbance is carried to the earth by a cloud of particles thrown out by the sun. When these disturbances are observable only in the auroral zones, they may be termed polar magnetic storms.

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

see lightning stroke, heat stroke

stroke density

The areal density of lightning discharges over a given region during some specified period of time, as number per square mile or per square kilometer per year.

supply current

The electrical current in the atmosphere which is required to balance the observed air-earth current of fair-weather regions by transporting positive charge upward or negative charge downward The problem of accounting for the supply current has been for many years a key problem of the field of atmospheric electricity and has received much attention. A quasi-steady current of about 1800 amperes for the earth as a whole is estimated to be required to balance the air-earth current. C. T. R. {Wilson} suggested in 1920 that the thunderstorms present in widely scattered regions of the earth at any one time might be responsible for the supply current. Although this suggestion has not been fully confirmed, there is growing conviction that this is correct. When one considers an average over many storms, thunderstorm lightning transports negative charge downward to earth, as does point discharge in the regions below thunderstorms. Also, positive ions flow upward above active thunderstorms.

surface wave

A gravity wave formed on the free surface of a fluid. In classical hydrodynamics, to distinguish surface waves from tidal waves, the condition is imposed that vertical accelerations are not negligible. Dynamically this wave is similar to that on an interface separating two fluids, becoming identical in the case of zero density in the upper fluid.

surge current

A short-duration, high-amperage, electric current wave that may sweep through an electrical network, as a power transmission network, when some portion of it is strongly influenced by the electrical activity of a thunderstorm. This activity may take the form of a direct lightning strike, or it may be simply the release of previously induced charge on the line when a thundercloud overhead suddenly discharges itself. Such current surges flow rapidly through the lines until they find a path to the ground through arc-over at a weak insulator or by entering terminal equipment at the end of the line. Use of grounded guard wires above the power lines of a transmission system reduces the frequency of surge current difficulties, and installation of lightning arresters at sensitive terminal equipment protects the system against damage there.

surge

A short-duration, high-amperage, electric current that may sweep through an electrical network, such as a power transmission network, when some portion of it is strongly influenced by the electrical activity of a thunderstorm. This may take the form of a direct lightning stroke, or or may be smply the release of previously induced charge on the line when a thundercloud overhead suddenly discharges itself. Such current surges rapidly through the lines until they find a path to the ground through arc-over at a weak insulator or by entering terminal equipment at the end of the line. Use of grounded guard wires above the power lines of a transmission system reduces the frequency of surge current difficulties, and installation of lightning arresters at sensitive terminal equipment protects the system from damage there.

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.

total conductivity)

In atmospheric electricity, the sum of the electrical conductivity of the positive and negative ions found in a given portion of the atmosphere.

triboelectrification

A process of charge separation that involves the rubbing together of dissimilar material surfaces. The triboelectric series is a classification scheme for the ordering of the tendency for positive charge acquisition in rubbing. The detailed physical mechanism in triboelectrification is a long unsolved problem.

type-[alpha] leader

A stepped leader that exhibits very little branching and whose individual steps are short and so weakly luminous as to be difficult to discern on high-speed streak photographs.

type-[beta] leader

A stepped leader in which the upper portion of the channel is characterized by longer and brighter steps than those found in the lower portion of the channel. Many researchers report that a preliminary breakdown precedes the stepped leader in electric field records. The beta leader may be this preliminary breakdown.

waterfall effect

same as Lenard effect

Workman--Reynolds effect

A mechanism for electric charge separation during freezing of slightly impure water, discovered by E. J. Workman and S. E. Reynolds (1948). When a very dilute solution of certain salts freeze rapidly, a strong potential difference is established between the solid and liquid phases. For some salts, the ice attains negative charge, for others, positive. This mechanism was thought to play a role in the charging of thunderstorms but it is now known that it cannot account for the electrification of riming graupel by the shedding of charged surface water because the droplets freeze faster than the time taken for a substantial ice-water freezing potential to be set up. This mechanism has been suggested as one possible mode of thunderstorm charge separation in those portions of a thunderstorm downdraft where snow-pellet or hail particles sweep out supercooled water drops. Partial freezing and partial blow-off of a liquid film could lead to charge separation. The acknowledged predominance of dry growth of graupel in New Mexico thunderclouds led Reynolds to question the viability of this mechanism in the atmosphere. = This should not be confused with the Reynolds effect .

zigzag lightning

Ordinary lightning of a cloud-to-ground discharge that appears to have a single, but very irregular, lightning channel.

blue jet

A narrowly collimated beam of mostly blue, with some green, light that appears to propagate upward from the tops of thunderstorms. The typical blue jet is observed to appear from the apparent top of the anvil and propagate upwards, at speeds of about 100 km/s, in a narrow cone and to flare out as it reaches maximum altitude, approximately 40-50 km, such that it resembles a trumpet. Currently, there is no satisfactory theory of this phenomenon. One possible mechanism may be that jets are discharge following some sort of collimated quasi-straight ion trail emanating from the thunderstorm. Any mechanism producing blue jets will have to continue for at least 200 ms.

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. (See return stroke, stepped leader and dart leader)

dark lightning

The present definition for dark lightning is ok. (R. Orville)

breakdown field

The electric field necessary to produce breakdown.

double layer

On the surface of a substance, a layer of electric dipoles whose axes have an average orientation normal to the surface.  Double layers may appear on interfaces of solid and gas, liquid and gas, liquid and liquid, etc. They arise whenever media with different electron affinities (forces of attraction, or work function) are contiguous, and if dipoles are available. A net potential difference, the electrokinetic potential exists across a double layer.

earth--ionospheric waveguide

The physical structure formed by the conductive Earth and the conductive lower ionosphere (D region) which together sandwich the highly resistive atmosphere between them. This waveguide sustains global electromagnetic wave propagation from the lower end of the Schumann resonance band (5 Hz) to the waveguide cutoff frequency near 1500 Hz.

electromagnetic radiation

I do not understand why the current definition does not appear in this space. The current definition is ok.

electrosphere

A layer beginning a few tens of kilometers above the surface of the earth and extending to the ionosphere in which the electrical conductivity is so high that the layer is essentially at a constant electric potential. This layer is called the electrosphere.

EOSO

End-of-Storm-Oscillation; a characteristic in the electric field signature at the ground near the end of a thunderstorm. The electric field changes from upward-directed (foul-weather-polarity) to downward-directed (fair-weather-polarity) and often retains the latter polarity for several tens of minutes before returning momentarily to foul-weather-polarity. Positive ground flashes and spider lightning are occasionally observed during the period of reversed field polarity. The origin of the EOSO is only partly understood.

fair-weather electricity

The distribution of ions and currents in the atmosphere and at the surface of the earth that occur during "Fair Weather," or in areas where there is no thunderstorm activity. This distribution sets up a downward directed electric field referred to as a fair-weather field.

FEAWP

(Field Excursion Associated with Precipitation) A characteristic feature in electric field records directly beneath a thundercloud in which the electric field momentarily reverses polarity during a burst of intense precipitation. The time scale for these events is of the order of minutes and of shorter duration than the EOSO. The physical origins of the FEAWP are still unresolved.

first-hop wave

The first ionospheric reflection from a lightning stroke which arrives after the original ground-wave.

global circuit

The structure and combination of processes set up by the conductive Earth, the conductive ionosphere, and all agents of electrification within the resistive troposphere. The so-called 'DC' global circuit is characterized by the ionosphere potential and the 'AC' global circuit refers to the Schumann resonances within the Earth-ionosphere waveguide.

induction charging mechanism

A physical process for particle charging involving the collision of pairs of particles in an ambient electric field. Electric charge induced on the particle surfaces by the ambient electric field is made available for transfer when the two particles come into contact. Subsequent differential particle motions under gravity is postulated to result in large scale charge separation. The specific role of induction charging in the electrification of thunderclouds has not been resolved.

Q burst

An electromagnetic transient launched by a lightning discharge of large amplitude within the Earth-ionosphere cavity. Originally named by Toshio Ogawa, the 'Q' connotes 'Quiet' and characterizes a transient in which the fundamental 8 Hz mode of the Schumann resonances is the dominant contributor. In general, a mix of Schumann modes makes up a Q-burst.

Q noise

Quasi-continuous sequences of VHF (Very High Frequency) radiation emanating from lightning that is associated with high-speed K-change activity along pre-ionized lightning channels. The process was named by David Proctor.

radio source

radioactive decay (in reference to conductivity near ground)

The primary source of ions in the lowest one or two kilometers of the atmosphere above land (cosmic radiation is dominant over the oceans and at higher altitudes). The ionizing radiation can take the form of (1) emission of [alpha], [beta], or [gamma] radiation by radioactive materials in the soil or (2) from gaseous radioactive daughter products (radon) that emanate from the soil. The [alpha] radiation emitted by materials in the ground is absorbed in the lowest few centimeters of the atmosphere, [beta] radiation penetrates a few meters and [gamma] radiation a few hundred meters into the atmosphere. The ionization produced by radioactive gases in the atmosphere is highly variable and depends on the rate of emission from the soil and also on atmospheric dispersion. [^ 222 Rn], which has a half-life of 3.8 days, is produced by the uranium decay series. [^220 Rn] (thoron) is produced by the thorium decay series and has a half-life of 54 seconds.

Schumann resonance

Electromagnetic waves in the ELF range trapped in the spherical cavity formed by the conductive earth and the conductive ionosphere. The fundamental resonance mode represents one wavelength around the Earth with a corresponding frequency of approximately 8 Hz. The resonances are maintained continuously by global lightning activity. Five to seven higher order modes are frequently discernible.

sferics source

That portion of a lightning discharge that radiates strongly in the frequency interval 10-30 kHz. The physical source is generally identified with the return stroke in flashes to ground and the K-change in the case of intracloud flashes.

sky wave

In radio terminology, or for lightning transients, electromagnetic waves received after they have been reflected by the ionosphere.

slow tail

The extremely low frequency (ELF) component of some sferics that lags the initial very low frequency (VLF) arrival because of the lower phase velocity at low frequencies. The cause of the slow tail is still controversial. One explanation attributes it to the continuing current. The lightning radiation arrives after the initiating ground stroke because a) it has components below about 3 k Hz in frequencies (necessary for the propagation in the earth-ionospheere waveguide cavity) and b) because it suffers dispersion as a result of the waveguide propagation which broadens the original vlf impulse making it look like a "tail." See J. R. Wait, "On the Theory of the Slow-tail Portion of Atmospheric Waveforms," J. Geophys. Res. v 65, pp 1939, July 1960.

spider lightning

Lightning with extraordinary lateral extent near cloud base where its dendritic structure is clearly visible. This lightning type is prevalent beneath the stratiform anvil of mesoscale convective systems and is often associated with positive ground flashes. This discharge form is also referred to as 'sheet' lightning.

sunspot cycle

A cycle with an average length of 11.1 years, but varying between about 7 and 17 years, in the number and area of sunspots, as given by the relative sunspot number. This number rises from a minimum of 0-10 to a maximum of 50-140 about four years later, and then declines more slowly. Records of sunspot counts have been kept since the early seventeenth century. A period of anomalously low activity was observed between about 1645-1700 and is known as the Maunder minimum. An approximate 11-year cycle has been found or suggested in geomagnetism, frequency of aurora, and other ionospheric characteristics. The n index of geomagnetic intensity variation shows one of the strongest known correlation to solar activity. Eleven-year cycles have been suggested for various tropospheric phenomena, but none of these has been substantiated.

thunder

(The current definition looks ok.)

thunderstorm dipole

The simplest representation of the electrostatic structure of an electrified cloud with overall charge neutrality. Ordinary thunderstorms are characterized by upper positive charge and lower negative charge.

thunderstorm tripole

A refinement of the simpler dipole representation for the electrostatic structure of isolated electrified clouds. The tripole structure includes the lower positive charge center that appears in many observations. The tripole picture is generally attributed to C. C. Simpson.

aircraft charge and/or charging

Same as aircraft electrification

bipolar pattern

A lightning pattern revealed in cloud-to-ground lightning locations in which areas of predominantly negative flash locations are separated horizontally from positive flash locations. This pattern has been identified in midlatitude mesoscale convective systems (MCS). The convective regions of the MCS (those with radar echoes 35 dBZ) typically have negative lightning flashes, whereas the positive flashes are typically located in the stratiform region (<35 dBZ) adjacent to the convective region. The bipolar pattern is hypothesized to arise from (1) the advection of positively charged particles from the upper portions of the convective region to the stratiform region due to vertical wind shear; (2) in situ charging of ice particles that acquire positive charge in an environment of supercooled liquid water and mesoscale ascent in the stratiform cloud layer; or (3) some combination of the two processes.

blue jets

Weakly luminous upward propagating discharges, blue in color, emanating from the tops of thunderstorms. Following their emergence from the top of the thundercloud, they typically propagate upward in narrow cones of about 15 degrees full width at vertical speeds of roughly 100 km/s (Mach 300), fanning out and disappearing at heights of about 40-50 km. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m^3. Blue jets are not aligned with the local magnetic field.

conductivity

A unit measure of electrical conduction; the facility with which a substance conducts electricity, as represented by the current density per unit electrical potential gradient in the direction of flow. Electrical conductivity is the reciprocal of electrical resistivity and is expressed in units such as mhos (reciprocal ohms) per meter. It is an intrinsic property of a given type of material under given physical conditions (dependent mostly upon temperature.)

continuing current

A sustained current in the lightning stroke that flows to ground after the return stroke. Continuing currents can have durations in excess of 100 milliseconds with magnitudes of typically 100 amperes. Continuing currents occur in negative and positive cloud-to-ground lightning flashes.

electric field

same as electric field strength

electric potential

In electrostatics, the work done in moving unit positive charge from infinity to the point whose potential is being specified.

elve

Transient laterally extensive illumination of the airglow layer, about 90 km, over thunderstorms and associated with the electromagnetic pulse from the return stroke of a lightning flash to ground.

glow discharge

ground flash

same as cloud-to-ground flash or cloud-to-ground discharge

hydrometeor charge

The net charge, for example, on a raindrop or a water drop.

intracloud flash

A lightning discharge occurring between a positive charge center and a negative charge center, both of which lie in the same cloud. Starts most frequently in the region of the high electric field between the upper positive and lower negative space charge regions. In summer thunderstorms, intracloud flashes precede the occurrence of cloud-to-ground flashes; they also outnumber cloud-to-ground flashes. Intracloud lightning develops bidirectionally like a two-ended tree: one end of the tree is a branching negative leader, the other is a branching positive leader. Later in the flash, fast negative leaders similar to dart leaders (also called K-changes) appear in the positive end region and propagate toward the flash origin. = In weather observing, this type of discharge is often mistaken for a cloud-to-cloud flash , but the latter term should be restricted to true intercloud discharges, which are far less common than intracloud discharges. Cloud discharges tend to outnumber cloud-to-ground discharges in semiarid regions where the bases of thunderclouds may be several kilometers above the earth's surface. 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 .

ionospheric potential

The potential difference between the conductive Earth and the conductive upper atmosphere, whose value is approximately +250 kilovolts. This voltage difference is maintained by worldwide cloud electrification. Most of the 250 kV is realized well below the formal ionospheric height because of the rapid increase of electrical conductivity with height in the atmosphere.

K changes

The K-process is generally viewed as a "recoil streamer" or small return stroke that occurs when a propagating discharge within the cloud encounters a pocket of charge opposite to its own. In this view, the J-process represents a slowly propagating discharge that initiates the K-process. This is the case for K-changes in cloud discharges. It is reasonable to expect that cloud discharge K-changes are similar to the in-cloud portion of ground discharges.

lightning current

the current flowing in a component of the lightning flash. It is usually considered to be the current in the return stroke.

lightning mapping system

A network of lightning detection equipment for locating the electromagnetic sources of a lightning flash. The flash, both intracloud and cloud-to-ground, is mapped in three-dimensional space using equipment with a time resolution of less than a microsecond.

M component

M-component is the name given to an increase in channel luminosity accompanied by a rapid electric field variation, itself called a M-electric field changes. M-components occur when the channel is already faintly luminous. Downward-moving leaders have not been observed to precede M-components. M-components may be confused with branch components, the increases in channel luminosity that occur between each branch and ground when the upward-propagating return stroke reaches that branch, since the higher branches are obscured by the cloud. The M-component is a minor surge of current that re-illuminates the channel in a negative cloud-to-ground flash. It may occur within microseconds or up to a few milliseconds of a return stroke. Evidences exists that M-components are a result of the sequence of fast in-cloud negative leader (K-changes) contacting a conducting ground channel and renewed ground potential wave that re-illuminates the channel.

negative cloud-to-ground lightning

A lightning flash or stroke between a cloud and ground that lowers negative charge to ground.

negative ground flash

same as negative cloud-to-ground lightning

noninductive charging mechanism

A charging mechanism whereby the electric field increase is independent of the existing electric field.

ohmic current

A current that is proportional to the voltage and inversely proportional to the resistance.

particle charge

The electric charge, associated with a particle, e.g. a rain drop, ice crystal, hail stone, or aerosol.

peak current

Usually refers to the maximum current in a lightning return stroke.

positive cloud-to-ground lightning

a positive cloud-to-ground lightning flash lowers positive charge from the cloud to the ground.

positive discharge

a positive discharge lowers positive charge to ground via a lightning flash. The flash may be initiated in the cloud or from the ground.

positive ground flash

a positive ground flash lowers positive charge to ground from the cloud above.

reversal temperature

The temperature at which the charge transferred to riming graupel particles during collisions with ice crystals in thunderstorms reverses sign. The reversal temperature is affected by the cloud liquid water content such that graupel charges positively (negatively) at high (low) liquid water contents and temperatures above (below) the reversal temperature. The process can account for the positive charging of ice crystals carried to the top of the clouds, for the negative charge region typically at temperatures between -10C and -20C and for the lower positive charge center.

rocket-triggered lightning

A form of artificial lightning discharge initiated with a rocket trailing wire which may or may not be connected to the ground. The first phase of the discharge is a unidirectional leader starting from the tip of the wire. When the low end of the wire is not connected to ground, bi-directional leader development occurs from both ends of the wire, similar to lightning initiation from aircraft. In the case of the negative space charge overhead (usual summer thunderstorm condition), a triggered lightning may only be a positive leader or become a sequence of dart leader/return stroke processes following the initial positive leader. The latter is analogous to the subsequent return stroke process in a negative cloud-to-ground flash with the initial positive leader being analogous to the first return stroke. In case of the positive space charge overehead (usual winter storm condition), the triggered lightning is a single negative leader.

screening layer  on the surface of a cloud opposite in polarity to the main charge inside  the cloud. This can lead to the underestimation of remotely measured main charge magnitudes.

sprite

weak luminous emissions that appear directly above an active thunderstorm and are coincident with cloud-to-ground or intracloud lightning flashes. Their spatial structures range from small single or multiple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings which extend from the cloud tops to altitudes up to about 95 km. Sprites are predominantly red. The brightest region lies in the altitude range 65-75 km, above which there is often a faint red glow or wispy structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary structures often extend downward to as low a s 40 km. High-speed photometer measurements show that the duration of sprites is only a few ms. Current evidence strongly suggests that sprites preferentially occur in decaying portions of thunderstorms and are correlated with large positive cloud-to-ground flashes. The optical intensity of sprite clusters, estimated by comparison with tabulated stellar intensities, is comparable to a moderately bright auroral arc. The optical energy is roughly 10-50 kJ per event, with a corresponding optical power of 5-25 MW. Assuming that optical energy constitutes 1/1000 of the total for the event, the energy and power are on the order of 10-100 MJ and 5-50 GW, respectively. Early research reports for these events referred to them by a variety of names, including "upward lightning," "upward discharges," "cloud-to-stratosphere discharges," and cloud-to-ionosphere discharges." Now they are simply referred to as sprites, a whimsical term that evokes a sense of their fleeting nature, while at the same time remaining nonjudgmental about physical processes that have yet to be determined. (See also blue jets)

superbolt

A lightning discharge of extraordinary peak luminosity when observed from space.

thunderstorm

(The current definition in the Glossary is acceptable. R. Orville)

thunderstorm charge

The existence of regions of net charge in a thunderstorm. During transient collisions of ice crystals with riming graupel pellets charge is transferred. The separating particles then carry equal and opposite charges, the larger (often negative) particles fall while the smaller ones (often positively charged ice crystals) are carried up in the updraft to produce a vertical electric field that eventually produces lightning. There is no complete understanding of the charge transfer process, but possible processes include charges on the surface layers of the particles, charges on dislocations in the ice lattice, temperature differences along surface features that may be broken off during collisions, and contact potential differences between the surfaces of the interacting particles.

time-of-arrival technique

the time-of-arrival technique refers to locating the source of an emitted signal from a precise recording of the time that a signal is observed. For example, the time interval between an observed lightning flash and the arrival of the thunder can be used to estimate the distance to the lightning flash.

VHF source

typically, a source of very high frequency radiation. Lightning discharges are one example of VHF sources in the atmosphere.