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63 APPENDIX B Glossary of Lightning Terms The text presented in this research study includes a wide range of often unfamiliar words and specialized terminology. For the convenience of the reader, the glossary of lightning terms is presented on the following pages. This glossary is extracted from the American Society of Meteorology Glossary of Meteorology, 2nd ed., and is used with permission (1).

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64 Airearth current--The transfer of electric charge from the originating, principally, in the irregular surges of charge in positively charged atmosphere to the negatively charged earth. thunderstorm lightning discharges. This current is made up of the air-earth conduction cur- Atmospherics are heard as a quasi-steady background of rent, a point-discharge current, a precipitation current, a crackling noise (static) on certain radio frequencies, such convection current, and miscellaneous smaller contri- as those used to broadcast AM radio signals. Since any butions. Of these, the airearth conduction current is by acceleration of electric charge leads to emission of electro- far the largest. This is not just true locally, but through- magnetic radiation, and since the several processes in- out the world where there are no thunderstorms occur- volved in propagation of lightning lead to very large charge ring, which is estimated to be 80%90% percent of the accelerations, the lightning channel acts like a huge trans- earth. The existence of this quasi-steady current in fair mitter, sending out radiation with frequencies of the order weather and the observed maintenance of the earth's net of 10 kHz. Atmospherics may occasionally be detected at negative charge are both better established than the nature distances in excess of 3500 km (2000 mi) from their source. of the supply current, which must replenish the positive Advantage has been taken of this characteristic by using charge in the upper atmosphere and the negative charge on radio direction-finding equipment to plot cloud-to-ground the earth. lightning locations, and to locate active thunderstorm Gish, O. H., 1951: Compendium of Meteorology, p. 113. 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 Ball lightning--(Also called globe lightning) A rare and ran- point in space and time. domly occurring bright ball of light observed floating or mov- ing through the atmosphere close to the ground. In areas of fair weather, the atmospheric electric field near the earth's surface typically is about 100 volts (V) m-1 and Observations have widely varying identifying characteris- is directed vertically in such a sense as to drive positive tics for ball lightning, but the most common description is charges downward to the earth. In areas of fair weather this that of a sphere having a radius of 1550 cm, orange or field decreases in magnitude with increasing altitude, falling, reddish in color, and lasting for only a few seconds before for example, to only about 5 V m-1 at an altitude of about disappearing, sometimes with a loud noise. Most often ball 10 km. Near thunderstorms, and under clouds of vertical lightning is seen in the vicinity of thunderstorms or a re- development, the surface electric field (the electric field cent lightning strike, which may suggest that ball lightning measured at the surface of the earth) varies widely in is electrical in composition or origin. Considered contro- magnitude and direction, usually reversing its direction versial due to the lack of unambiguous physical evidence immediately beneath active thunderstorms. In areas of for its existence, ball lightning is becoming more accepted minimal local disturbance, a characteristic diurnal varia- due to recent laboratory recreations resembling ball light- tion of electric field strength is observed. This variation is ning. Despite the observations and models of these fire characterized by a maximum that occurs at about 1900 balls, the exact mechanism(s) for naturally occurring ball UTC for all points on the earth and is now believed to be lightning is unknown. produced by thunderstorms that, for geographic regions, are more numerous for the world as a whole at that uni- Beaded lightning--(Also called chain lightning, pearl light- versal time than at any other. It is now believed that thun- ning) A particular aspect of a normal lightning flash occa- derstorms, by replenishing the negative charge to the sionally seen when the observer happens to view end-on a earth's surface, provide the supply current to maintain the number of segments of the irregular channel (zigzag lightning) fair-weather electric field in spite of the continued flow of and hence receives an impression of higher luminosity at a the airearth current that tends to neutralize that field. The series of locations along the channel. range of the electric field in fair weather varies consider- ably with geographical area, from one part of the globe to Blue jets--Weakly luminous upward propagating discharges, another. If, however, there are no local sources of pollu- blue in color, emanating from the tops of thunderstorms. tion, the surface electric field has its maximum amplitude around 1900 UTC. Following their emergence from the top of the thunder- cloud, they typically propagate upward in narrow cones of Atmospherics--(Also called atmospheric interference, about 15 full width at vertical speeds of roughly 100 km s-1 strays, sferics) The radio frequency electromagnetic radiation (Mach 300), fanning out and disappearing at heights of

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65 about 4050 km. Their intensities are on the order of 800 kR Direction finder--An instrument consisting of two orthog- near the base, decreasing to about 10 kR near the upper onal magnetic loop antennas and associated electronics for terminus. These correspond to an estimated optical energy the purpose of detecting the azimuth to a cloud-to-ground of about 4 kJ, a total energy of about 30 MJ, and an energy lightning stroke. density on the order of a few millijoules per cubic meter. Blue jets are not aligned with the local magnetic field. Electrical breakdown--The sudden decrease of resistivity of a substance when the applied electric field strength rises Cloud flash--(Also called intracloud flash, cloud-to-cloud above a certain threshold value (the substance's dielectric flash) A lightning discharge occurring between a positively strength). charged region and a negatively charged region, both of which may lie in the same cloud. For air at normal pressures and temperatures, experi- ment has shown that the breakdown process occurs at The most frequent type of cloud discharge is one between a field strength of about 3 106 V m-1. This value de- a main positively charged region and a main negatively creases approximately linearly with pressure, and is de- charged region. Cloud flashes tend to outnumber cloud- pendent upon humidity and traces of foreign gases. In to-ground flashes. In general, the channel of a cloud flash the region of high field strength just ahead of an actively will be wholly surrounded by cloud. Hence, the channel's growing leader in a lightning stroke, breakdown occurs luminosity typically produces a diffuse glow when seen in the form of a rapidly moving wave of sudden ioniza- from outside the cloud and this widespread glow is called tion (electron avalanche). The dielectric strength in a sheet lightning. cloud of water drops is less than that in cloud-free humid air. Cloud-to-ground flash--A lightning flash occurring between a charge center in the cloud and the ground. Electric field mill--see field mill. On an annual basis, negative charge is lowered to the Elve--Transient laterally extensive illumination of the airglow ground in about 95% of the flashes. The remaining flashes layer, at about 90 km, over thunderstorms, and associated lower positive charge to the ground. This type of lightning with the electromagnetic pulse from the return stroke of a flash, which can be contrasted with an intracloud flash or lightning flash to ground. cloud flash, consists of one or more return strokes. The first stroke begins with a stepped leader followed by an in- Field mill--An instrument that obtains a continuous tense return stroke that is the principal source of luminos- measurement of the sign and magnitude of the local elec- ity and charge transfer. Subsequent strokes begin with a tric potential gradient by alternately shielding and expos- dart leader followed by another return stroke. Most of the ing a conductor that is grounded through a resistance to strokes use the same channel to ground. The time interval develop an alternating potential that is proportional to the between strokes is typically 40 s. field. Forked lightning--The common form of cloud-to-ground Dart leader--(Also called continuous leader) The leader which, discharge always visually present to a greater or lesser degree after the first stroke, typically initiates each succeeding stroke that exhibits downward-directed branches from the main of a multiple-stroke flash lightning. (The first stroke is initi- lightning channel. ated by a stepped leader.) In general, of the many branches of the stepped leader, The dart leader derives its name from its appearance on only one is connected to the ground, defining the primary, photographs taken with streak cameras. The dart leader's bright return stroke path; the other incomplete channels brightest luminosity is at its tip which is tens of meters decay after the ascent of the first return stroke. Compare in length, propagating downward at about 107 m s-1. In streak lightning, zigzag lightning. contrast to stepped leaders, dart leaders do not typically exhibit branching because the previously established Ground flash--Same as cloud-to-ground flash or cloud-to- channel's low gas density and residual ionization provide ground discharge. a more favorable path for this leader than do any alter- native ones. Ground-to-cloud discharge--A lightning discharge in which Chalmers, J. A., 1957: Atmospheric Electricity, p. 239. the original leader process starts upward from some object

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66 on the ground; the opposite of the more common cloud-to- changes in cloud discharges. It is reasonable to expect that ground discharge. cloud discharge K changes are similar to the in-cloud por- tion of ground discharges. Ground-to-cloud discharges most frequently emanate from very tall structures that, being at the same potential Leader--(Or leader streamer) The electric discharge that as the earth, can exhibit the strong field intensities near initiates each return stroke in a cloud-to-ground lightning their upper extremities necessary to initiate leaders. discharge. Heat lightning--Nontechnically, the luminosity observed from It is a channel of high ionization that propagates through ordinary lightning too far away for its thunder to be heard. the air by virtue of the electric breakdown at its front pro- duced by the charge it lowers. The stepped leader initiates Since such observations have often been made with clear the first stroke in a cloud-to-ground flash and establishes skies overhead, and since hot summer evenings particu- the channel for most subsequent strokes of a lightning dis- larly favor this type of observation, there has arisen a pop- charge. The dart leader initiates most subsequent strokes. ular misconception that the presence of diffuse flashes in Dart-stepped leaders begin as dart leaders and end as the apparent absence of thunderclouds implies that light- stepped leaders. The initiating processes in cloud dis- ning is somehow occurring in the atmosphere merely as a charges are sometimes also called leaders but their prop- result of excessive heat. erties are not well measured. Intracloud flash--A lightning discharge occurring between Lightning--Lightning is a transient, high-current electric a positive charge center and a negative charge center, both of discharge with path lengths measured in kilometers. which lie in the same cloud; starts most frequently in the re- gion of the strong electric field between the upper positive The most common source of lightning is the electric and lower negative space charge regions. charge separated in ordinary thunderstorm clouds (cumulonimbus). Well over half of all lightning discharges In summer thunderstorms, intracloud flashes precede the occur within the thunderstorm cloud and are called intra- occurrence of cloud-to-ground flashes; they also outnum- cloud discharges. The usual cloud-to-ground lightning ber cloud-to-ground flashes. Intracloud lightning devel- (sometimes called streak lightning or forked lightning) has ops bidirectionally like a two-ended tree: one end of the been studied more extensively than other lightning forms tree is a branching negative leader, the other is a branch- because of its practical interest (i.e., as a cause of injury and ing positive leader. Later in the flash, fast negative leaders death, disturbances in power and communication sys- similar to dart leaders (also called K changes) appear in the tems, and ignition of forest fires) and because lightning positive end region and propagate toward the flash origin. channels below cloud level are more easily photo- In weather observing, this type of discharge is often mis- graphed and studied with optical instruments. Cloud-to- taken for a cloud-to-cloud flash, but the latter term should cloud and cloud-to-air discharges are less common than be restricted to true intercloud discharges, which are far intracloud or cloud-to-ground lightning. All discharges less common than intracloud discharges. Cloud discharges other than cloud-to-ground are often lumped together tend to outnumber cloud-to-ground discharges in semiarid and called cloud discharges. Lightning is a self-propagating regions where the bases of thunderclouds may be several and electrodeless atmospheric discharge that, through kilometers above the earth's surface. In general, the chan- the induction process, transfers the electrical energy of an nel of a cloud flash will be wholly surrounded by cloud. electrified cloud into electrical charges and current in its Hence the channel's luminosity typically produces a dif- ionized and thus conducting channel. Positive and nega- fuse glow when seen from outside the cloud, and this wide- tive leaders are essential components of the lightning. spread glow is called sheet lightning. Only when a leader reaches the ground does the ground potential wave (return stroke) affect the lightning process. K changes--The K process is generally viewed as a recoil Natural lightning starts as a bidirectional leader, although streamer or small return stroke that occurs when a propagat- at different stages of the process unidirectional leader de- ing discharge within the cloud encounters a pocket of charge velopment can occur. Artificially triggered lightning starts opposite to its own. on a tall structure or from a rocket with a trailing wire. Most of the lightning energy goes into heat, with smaller In this view, the J process represents a slowly propagating amounts transformed into sonic energy (thunder), radia- discharge that initiates the K process. This is the case for K tion, and light. Lightning, in its various forms, is known by

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67 many common names, such as streak lightning, forked clouds (intercloud or cloud-to-cloud discharge), or between lightning, sheet lightning, and heat lightning, and by the a cloud charge and the air (air discharge). less common air discharge; also, the rare and mysterious ball lightning and rocket lightning. An important effect of It is a very large-scale form of the common spark discharge. worldwide lightning activity is the net transfer of negative A single lightning discharge is called a lightning flash. charge from the atmosphere to the earth. This fact is of great important in one problem of atmospheric electricity, Lightning flash--The total observed lightning discharge, the question of the source of the supply current. Existing generally having a duration of less than 1 s. evidence suggests that lightning discharges occurring spo- radically at all times in various parts of the earth, perhaps A single flash is usually composed of many distinct lumi- 100 per second, may be the principal source of negative nous events that often occur in such rapid succession that charge that maintains the earthionosphere potential dif- the human eye cannot resolve them. ference of several hundred thousand volts in spite of the steady transfer of charge produced by the airearth cur- Lightning mapping system--A network of lightning detec- rent. However, there also is evidence that point discharge tion equipment for locating the electromagnetic sources of a currents may contribute to this more significantly than lightning flash. lightning. See also cloud-to-ground flash, intracloud flash, lightning discharge. The flash, both intracloud and cloud-to-ground, is mapped in three-dimensional space using equipment with a time Chalmers, J. A., 1957: Atmospheric Electricity, 235255. resolution of less than 1 s. Since cloud-to-cloud and Schonland, B. F. J., 1950: The Flight of Thunderbolts, 152 pp. cloud-to-air are rare lightning phenomena, mapping them Hagenguth, J. H., 1951: Compendium of Meteorology, has little or no importance. 136143. Lightning stroke--In a cloud-to-ground discharge, a leader Lightning channel--The irregular path through the air along plus its subsequent return stroke. which a lightning discharge occurs. In a typical case, a cloud-to-ground discharge is made up The lightning channel is established at the start of a dis- of three or four successive lightning strokes, most follow- charge by the growth of a leader, which seeks out a path of ing the same lightning channel. least resistance between a charge source and the ground or between two charge centers of opposite sign in the thun- Negative cloud-to-ground lightning--A lightning flash or dercloud or between a cloud charge center and the sur- stroke between a cloud and the ground that lowers negative rounding air or between charge centers in adjacent clouds. charge to the ground. Lightning detection network--An integrated array of light- Negative ground flash--Same as negative cloud-to-ground ning direction finders that provide information for trigono- lightning. metric location of cloud-to-ground lightning discharges. Peak current--Usually refers to the maximum current in a Timing and direction information from individual re- lightning return stroke. ceivers are combined to provide evolving maps of light- ning occurrences across vast regions that sometimes reach Pearl lightning--Same as beaded lightning. beyond the range of storm surveillance radars. See sferics receiver. Point discharge current--The electrical current accompa- nying any specified source of point discharge. Lightning direction finder--See sferics receiver. In the electrical budget of the earthatmosphere system, Lightning discharge--The series of electrical processes tak- point discharge currents are of considerable significance as ing place within 1 s by which charge is transferred along a dis- a major component of the supply current. Estimates made charge channel between electric charge centers of opposite by Schonland (1928) of the point discharge current from sign within a thundercloud (intracloud flash), between a trees in arid southwest Africa suggest that this process ac- cloud charge center and the earth's surface (cloud-to-ground counts for about 20 times as much delivery of negative flash or ground-to-cloud discharge), between two different charge to the earth during typical thunderstorms as do

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68 lightning discharges. Although the great height of thun- several coulombs on the preceding negative leader channel, dercloud bases in arid regions, such as that referred to in thus charging earth negatively. In positive cloud-to- Schonland's study, tends to favor point discharge over ground flashes, the return stroke deposits the negative lightning charge transfer, point discharge still seems more charge of several tens of coulombs on the preceding posi- significant than lightning even in England, where Wormell tive leader channel, thus increasing positive charge on the (1953) found for Cambridge a ratio of about 5:1 in favor ground. In negative cloud-to-ground flashes, multiple re- of point discharge over lightning charge transfer. turn strokes are common. Positive cloud-to-ground flashes, in contrast, typically have only one return stroke. The return Chalmers, J. A., 1957: Atmospheric Electricity, 156175. streamer of cloud-to-ground discharges is so intense be- Wormell, T. W., 1953: Atmospheric electricity: some cause of the high electrical conductivity of the ground, and recent trends and problems. Quart. J. Roy. Meteor. Soc., hence this type of streamer is not to be found in air dis- 79, 350. charges, cloud discharges, or cloud-to-cloud discharges. Schonland, B. F. J., 1928: The polarity of thunderclouds. Proc. Roy. Soc. A, 118, 233251. Hagenguth, J. H., 1951: Compendium of Meteorology, 137141. Positive cloud-to-ground lightning--A lightning flash or stroke between a cloud and the ground that lowers positive Ribbon lightning--Ordinary cloud-to-ground lightning that charge from the cloud to the ground. appears to be spread horizontally into a ribbon of parallel luminous streaks when a very strong wind is blowing at right Positive discharge--A positive discharge lowers positive angles to the observer's line of sight. charge to the ground via a lightning flash. Successive strokes of the lightning flash are then displaced The flash may be initiated in the cloud or from the ground. by small angular amounts and may appear to the eye or camera as distinct paths. The same effect is readily created Positive ground flash--Same as positive cloud-to-ground artificially by rapid transverse movement of a camera dur- lightning. ing film exposure. Return stroke--The intense luminosity that propagates Rocket-triggered lightning--A form of artificial lightning upward from earth to cloud base in the last phase of each discharge initiated with a rocket trailing wire that may or may lightning stroke of a cloud-to-ground discharge. not be connected to the ground. In a typical flash, the first return stroke ascends as soon The first phase of the discharge is a unidirectional leader as the descending stepped leader makes electrical contact starting from the tip of the wire. When the low end of the with the earth, often aided by short ascending ground wire is not connected to ground, bidirectional leader de- streamers. The second and all subsequent return strokes velopment occurs from both ends of the wire, similar to differ only in that they are initiated by a dart leader and not lightning initiation from aircraft. In the case of negative a stepped leader. It is the return stroke that produces space charge overhead (usual summer thunderstorm con- almost all of the luminosity and charge transfer in most dition), a triggered lightning may only be a positive leader cloud-to-ground strokes. Its great speed of ascent (about or may become a sequence of dart leaderreturn stroke 1 108 m s-1) is made possible by residual ionization of processes following the initial positive leader. The latter is the lightning channel remaining from passage of the analogous to the subsequent return stroke process in a immediately preceding leader, and this speed is enhanced negative cloud-to-ground flash with the initial positive by the convergent nature of the electric field in which leader being analogous to the first return stroke. In the case channel electrons are drawn down toward the ascending of positive space charge overhead (usual winter storm con- tip in the region of the streamer's electron avalanche. Cur- dition), the triggered lightning is a single negative leader. rent peaks as high as 3 105 A have been reported, and values of 3 104 A are fairly typical. The entire process of Sferics fix--The determination of the bearing to the lightning the return stroke is completed in a few tens of microsec- source usually based on the measurement of the horizontal onds, and even most of this is spent in a long decay period magnetic field with orthogonal coils or loop antennas. following an early rapid rise to full current value in only a few microseconds. Both the current and propagation Sferics observation--The detection of electromagnetic speed decrease with height. In negative cloud-to-ground radiation from lightning generally in the frequency range flashes the return stroke deposits the positive charge of 1030 kHz.

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69 The physical measurement can include the electric field, Spider lightning--Lightning with extraordinary lateral extent the magnetic field, or both. Sferics are generally attributed near a cloud base where its dendritic structure is clearly visible. to the high current phases of source, that is, return strokes and K changes. This lightning type is prevalent beneath the stratiform anvil of mesoscale convective systems and is often associ- Sferics receiver--(Also called lightning direction finder.) An ated with positive ground flashes. This discharge form is instrument that measures, electronically, the direction of also referred to as sheet lightning. arrival, intensity, and rate of occurrence of atmospherics; a type of radio direction finder, it is most commonly used to Sprite--Weak luminous emissions that appear directly above detect and locate cloud-to-ground lightning discharges from an active thunderstorm and are coincident with cloud-to- distant thunderstorms. ground or intracloud lightning flashes. In its simplest form the instrument consists of two or- Their spatial structures range from small single or multi- thogonally crossed antennas that measure the electromag- ple vertically elongated spots, to spots with faint extrusions netic field changes produced by a lightning discharge and above and below, to bright groupings that extend from the determine the direction from which the changes arrived. cloud tops to altitudes up to about 95 km. Sprites are pre- Negative and positive polarity cloud-to-ground discharges dominantly red. The brightest region lies in the altitude can be distinguished. Cloud-to-cloud discharges can be dis- range 6575 km, above which there is often a faint red tinguished based on characteristics of the received signal, glow or wispy structure that extends to about 90 km. Below the bright red region, blue tendril-like filamentary and the geometry of nearby discharge channels may be structures often extend downward to as low as 40 km. determined. See also lightning detection network. High-speed photometer measurements show that the duration of sprites is only a few milliseconds. Current Sferics source--That portion of a lightning discharge that evidence strongly suggests that sprites preferentially occur radiates strongly in the frequency interval 1030 kHz. in decaying portions of thunderstorms and are correlated with large positive cloud-to-ground flashes. The optical The physical source is generally identified with the return intensity of sprite clusters, estimated by comparison with stroke in flashes to ground and the K change in the case of tabulated stellar intensities, is comparable to a moderately intracloud flashes. bright auroral arc. The optical energy is roughly 1050 kJ per event, with a corresponding optical power of Sheet lightning--(Also called luminous cloud.) A diffuse, but 525 MW. Assuming that optical energy constitutes 10-3 sometimes fairly bright, illumination of those parts of a thun- of the total for the event, the energy and power are on the dercloud that surround the path of a lightning flash, particu- order of 10100 MJ and 550 GW, respectively. Early larly a cloud discharge or cloud-to-cloud discharge. research reports for these events referred to them by a variety of names, including upward lightning, upward dis- Thus, sheet lightning is no unique form of lightning but charges, cloud-to-stratosphere discharges, and cloud-to- only one manifestation of ordinary lightning types in the ionosphere discharges. Now they are simply referred to presence of obscuring clouds. Compare heat lightning. as sprites, a whimsical term that evokes a sense of their fleeting nature, while at the same time remaining non- Spark discharge--That type of gaseous electrical discharge judgmental about physical processes that have yet to be in which the charge transfer occurs transiently along a rela- determined. Compare blue jets. tively constricted path of high ion density, resulting in high luminosity. Stepped leader--The initial leader of a lightning discharge; an intermittently advancing column of high ionization and It is of short duration and to be contrasted with the non- charge that establishes the channel for a first return stroke. luminous point discharge, the corona discharge, and the continuous arc discharge. The exact meaning to be at- The peculiar characteristic of this type of leader is its step- tached to the term "spark discharge" varies somewhat wise growth at intervals of about 50100 s. The velocity in the literature. It is frequently applied to just the tran- of growth during the brief intervals of advance, each only sient phase of the establishment of any arc discharge. about 1 s in duration, is quite high (about 5 107 m s-1), A lightning discharge can be considered a large-scale spark but the long stationary phases reduce its effective speed to discharge. only about 5 105 m s-1.

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70 Streak lightning--Ordinary lightning, of a cloud-to-ground 10,000 C, with the result that a violent quasi-cylindrical discharge, that appears to be entirely concentrated in a single, pressure wave is sent out, followed by a succession of rar- relatively straight lightning channel. efactions and compressions induced by the inherent elas- ticity of the air. These compressions are heard as thunder. Compare forked lightning, zigzag lightning Most of the sonic energy results from the return streamers of each individual lightning stroke, but an initial tearing Streamer--A sinuous channel of very high ion density that sound is produced by the stepped leader; and the sharp propagates itself though a gas by continual establishment of click or crack heard at very close range, just prior to the an electron avalanche just ahead of its advancing tip. main crash of thunder, is caused by the ground streamer ascending to meet the stepped leader of the first stroke. In lightning discharges, the stepped leader, dart leader, and Thunder is seldom heard at points farther than 15 miles return stroke all constitute special types of streamers. from the lightning discharge, with 25 miles an approxi- mate upper limit, and 10 miles a fairly typical value of the Stroke--See lightning stroke. range of audibility. At such distances, thunder has the characteristic rumbling sound of very low pitch. The pitch Stroke density--The areal density of lightning discharges over is low when heard at large distances only because of the a given region during some specified period of time, as num- strong attenuation of the high-frequency components of ber per square mile or per square kilometer. the original sound. The rumbling results chiefly from the varying arrival times of the sound waves emitted by the Supply current--The electrical current in the atmosphere portions of the sinuous lightning channel that are located that is required to balance the observed airearth current of at varying distances from the observer, and secondarily fair-weather regions by transporting positive charge upward from echoing and from the multiplicity of the strokes of a or negative charge downward. composite flash. Accounting for the supply current has been for many years Thunderstorm cell--The convective cell of a cumulonimbus a key problem of the field of atmospheric electricity and cloud having lightning and thunder. has received much attention. A quasi-steady current of about 1800 A for the earth as a whole is estimated to be re- Thunderstorm--(Sometimes called electrical storm.) In quired to balance the airearth current. Wilson (1920) general, a local storm, invariably produced by a cumulonim- suggested that the thunderstorms present in widely scat- bus cloud and always accompanied by lightning and thunder, tered regions of the earth at any one time might be re- usually with strong gusts of wind, heavy rain, and sometimes sponsible for the supply current. Although this suggestion with hail. has not been fully confirmed, there is growing conviction that this is correct. When one considers an average over It is usually of short duration, seldom over two hours many storms, thunderstorm lightning transports negative for any one storm. A thunderstorm is a consequence of charge downward to earth, as does point discharge in the atmospheric instability and constitutes, loosely, an over- regions below thunderstorms. Also, positive ions flow up- turning of air layers in order to achieve a more stable density ward above active thunderstorms. See airearth current, stratification. A strong convective updraft is a distin- point discharge current. guishing feature of this storm in its early phases. A strong Gish, O. H., 1951: Compendium of Meteorology, 113118. downdraft in a column of precipitation marks its dissi- Wilson, C. T. R., 1920: Investigations on lightning dis- pating stages. Thunderstorms often build to altitudes of charges and on the electric field of thunderstorms. Phil. 40,00050,000 ft in midlatitudes and to even greater heights Trans. A, 221, 73115. in the Tropics; only the great stability of the lower strato- sphere limits their upward growth. A unique quality of Thunder--The sound emitted by rapidly expanding gases thunderstorms is their striking electrical activity. The study along the channel of a lightning discharge. of thunderstorm electricity includes not only lightning phe- nomena per se but all of the complexities of thunderstorm Some three-fourths of the electrical energy of a lightning charge separation and all charge distribution within the discharge is expended, via ionmolecule collisions, in realm of thunderstorm influence. In U.S. weather observing heating the atmospheric gases in and immediately around procedure, a thunderstorm is reported whenever thunder is the luminous channel. In a few tens of microseconds, heard at the station; it is reported on regularly scheduled ob- the channel rises to a local temperature of the order of servations if thunder is heard within 15 minutes preceding

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71 the observation. Thunderstorms are reported as light, estimate the distance to the lightning flash. On the aver- medium, or heavy according to 1) the nature of the light- age, a time arrival difference of five seconds indicates ning and thunder; 2) the type and intensity of the precipi- that a lightning flash occurred one mile away from the tation, if any; 3) the speed and gustiness of the wind; 4) the observer, since the speed of sound in air is approximately appearance of the clouds; and 5) the effect upon surface 1000 ft s-1. temperature. From the viewpoint of the synoptic meteorol- ogist, thunderstorms may be classified by the nature of the Whistler--A type of VLF electromagnetic signal generated overall weather situation, such as airmass thunderstorm, by some lightning discharges. frontal thunderstorm, and squall-line thunderstorm. Byers, H. R., and R. R. Braham Jr., 1949: The Thunder- Whistlers propagate along geomagnetic field lines and can storm, U.S. Government Printing Office, 287 pp. travel back and forth several times between the Northern Byers, H. R., 1951: Compendium of Meteorology, p. 681. and Southern Hemispheres. So named from the sound they produce in radio receivers. Time-of-arrival technique--The time-of-arrival technique refers to locating the source of an emitted signal from a precise Zigzag lightning--Ordinary lightning of a cloud-to-ground recording of the time that a signal is observed. discharge that appears to have a single, but very irregular, lightning channel. For example, the time interval between an observed light- ning flash and the arrival of the thunder can be used to Compare streak lightning, forked lightning.