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63 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). A P P E N D I X B Glossary of Lightning Terms
Airâearth currentâThe transfer of electric charge from the positively charged atmosphere to the negatively charged earth. This current is made up of the air-earth conduction cur- rent, a point-discharge current, a precipitation current, a convection current, and miscellaneous smaller contri- butions. Of these, the airâearth conduction current is by far the largest. This is not just true locally, but through- out the world where there are no thunderstorms occur- ring, which is estimated to be 80%â90% percent of the earth. The existence of this quasi-steady current in fair weather and the observed maintenance of the earthâs 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. âGish, O. H., 1951: Compendium of Meteorology, p. 113. Atmospheric electric fieldâA 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 earthâs surface typically is about 100 volts (V) mâ1 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 V mâ1 at an altitude of about 10 km. Near thunderstorms, and under clouds of vertical development, the surface electric field (the electric field measured at the surface of the earth) varies widely in magnitude and direction, usually reversing its direction immediately beneath active thunderstorms. In areas of minimal local disturbance, a characteristic diurnal varia- tion of electric field strength is observed. This variation is characterized by a maximum that occurs at about 1900 UTC for all points on the earth and is now believed to be produced by thunderstorms that, for geographic regions, are more numerous for the world as a whole at that uni- versal time than at any other. It is now believed that thun- derstorms, by replenishing the negative charge to the earthâs surface, provide the supply current to maintain the fair-weather electric field in spite of the continued flow of the airâearth current that tends to neutralize that field. The range of the electric field in fair weather varies consider- ably with geographical area, from one part of the globe to another. If, however, there are no local sources of pollu- tion, the surface electric field has its maximum amplitude around 1900 UTC. Atmosphericsâ(Also called atmospheric interference, strays, sferics) 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 frequencies, such as those used to broadcast AM radio signals. Since any acceleration of electric charge leads to emission of electro- magnetic radiation, and since the several processes in- volved in propagation of lightning lead to very large charge accelerations, the lightning channel acts like a huge trans- mitter, sending out radiation with frequencies of the order of 10 kHz. Atmospherics may occasionally be detected at distances in excess of 3500 km (2000 mi) from their source. Advantage has been taken of this characteristic by using radio direction-finding equipment to plot cloud-to-ground lightning locations, and to locate active thunderstorm areas in remote regions and in-between weather reporting stations. Ball lightningâ(Also called globe lightning) A rare and ran- domly occurring bright ball of light observed floating or mov- ing through the atmosphere close to the ground. Observations have widely varying identifying characteris- tics for ball lightning, but the most common description is that of a sphere having a radius of 15â50 cm, orange or reddish in color, and lasting for only a few seconds before disappearing, sometimes with a loud noise. Most often ball lightning is seen in the vicinity of thunderstorms or a re- cent lightning strike, which may suggest that ball lightning is electrical in composition or origin. Considered contro- versial due to the lack of unambiguous physical evidence for its existence, ball lightning is becoming more accepted due to recent laboratory recreations resembling ball light- ning. Despite the observations and models of these fire balls, the exact mechanism(s) for naturally occurring ball lightning is unknown. Beaded lightningâ(Also called chain lightning, pearl light- ning) A particular aspect of a normal lightning flash occa- sionally seen when the observer happens to view end-on a number of segments of the irregular channel (zigzag lightning) and hence receives an impression of higher luminosity at a series of locations along the channel. Blue jetsâWeakly luminous upward propagating discharges, blue in color, emanating from the tops of thunderstorms. Following their emergence from the top of the thunder- cloud, they typically propagate upward in narrow cones of about 15° full width at vertical speeds of roughly 100 km sâ1 (Mach 300), fanning out and disappearing at heights of 64
about 40â50 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 millijoules per cubic meter. Blue jets are not aligned with the local magnetic field. Cloud flashâ(Also called 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 positively charged region and a main negatively 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 channelâs luminosity typically produces a diffuse glow when seen from outside the cloud and this widespread glow is called sheet lightning. Cloud-to-ground flashâA lightning flash occurring between a charge center in the cloud and the ground. On an annual basis, negative charge is lowered to the ground in about 95% of the flashes. The remaining flashes lower positive charge to the ground. This type of lightning flash, which can be contrasted with an intracloud flash or cloud flash, consists of one or more return strokes. The first stroke begins with a stepped leader followed by an in- tense return stroke that is the principal source of luminos- ity 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 μs. Dart leaderâ(Also called continuous leader) The leader which, after the first stroke, typically initiates each succeeding stroke of a multiple-stroke flash lightning. (The first stroke is initi- ated by a stepped leader.) The dart leader derives its name from its appearance on photographs taken with streak cameras. The dart leaderâs brightest luminosity is at its tip which is tens of meters in length, propagating downward at about 107 m sâ1. In contrast to stepped leaders, dart leaders do not typically exhibit branching because the previously established channelâs low gas density and residual ionization provide a more favorable path for this leader than do any alter- native ones. âChalmers, J. A., 1957: Atmospheric Electricity, p. 239. Direction finderâAn instrument consisting of two orthog- onal magnetic loop antennas and associated electronics for the purpose of detecting the azimuth to a cloud-to-ground lightning stroke. Electrical breakdownâThe sudden decrease of resistivity of a substance when the applied electric field strength rises above a certain threshold value (the substanceâs dielectric strength). For air at normal pressures and temperatures, experi- ment has shown that the breakdown process occurs at a field strength of about 3 à 106 V mâ1. This value de- creases approximately linearly with pressure, and is de- pendent 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 ioniza- tion (electron avalanche). The dielectric strength in a cloud of water drops is less than that in cloud-free humid air. Electric field millâsee field mill. ElveâTransient laterally extensive illumination of the airglow layer, at about 90 km, over thunderstorms, and associated with the electromagnetic pulse from the return stroke of a lightning flash to ground. Field millâAn instrument that obtains a continuous measurement of the sign and magnitude of the local elec- tric potential gradient by alternately shielding and expos- ing a conductor that is grounded through a resistance to develop an alternating potential that is proportional to the field. Forked lightningâThe common form of cloud-to-ground discharge always visually present to a greater or lesser degree that exhibits downward-directed branches from the main lightning channel. In general, of the many branches of the stepped leader, only one is connected to the ground, defining the primary, bright return stroke path; the other incomplete channels decay after the ascent of the first return stroke. Compare streak lightning, zigzag lightning. Ground flashâSame as cloud-to-ground flash or cloud-to- ground discharge. Ground-to-cloud dischargeâA lightning discharge in which the original leader process starts upward from some object 65
on the ground; the opposite of the more common cloud-to- ground discharge. Ground-to-cloud discharges most frequently emanate from very tall structures that, being at the same potential as the earth, can exhibit the strong field intensities near their upper extremities necessary to initiate leaders. Heat lightningâNontechnically, 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 particu- larly favor this type of observation, there has arisen a pop- ular misconception that the presence of diffuse flashes in the apparent absence of thunderclouds implies that light- ning is somehow occurring in the atmosphere merely as a result of excessive heat. Intracloud flashâA 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 re- gion of the strong 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 outnum- ber cloud-to-ground flashes. Intracloud lightning devel- ops bidirectionally like a two-ended tree: one end of the tree is a branching negative leader, the other is a branch- ing 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 mis- taken 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 earthâs surface. In general, the chan- nel of a cloud flash will be wholly surrounded by cloud. Hence the channelâs luminosity typically produces a dif- fuse glow when seen from outside the cloud, and this wide- spread glow is called sheet lightning. K changesâThe K process is generally viewed as a recoil streamer or small return stroke that occurs when a propagat- ing 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 por- tion of ground discharges. Leaderâ(Or leader streamer) The electric discharge that initiates each return stroke in a cloud-to-ground lightning discharge. It is a channel of high ionization that propagates through the air by virtue of the electric breakdown at its front pro- duced 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 dis- charge. The dart leader initiates most subsequent strokes. Dart-stepped leaders begin as dart leaders and end as stepped leaders. The initiating processes in cloud dis- charges are sometimes also called leaders but their prop- erties are not well measured. LightningâLightning is a transient, high-current electric discharge with path lengths measured in kilometers. The most common source 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 intra- cloud discharges. The usual cloud-to-ground lightning (sometimes called streak lightning or forked lightning) has been studied more extensively than other lightning forms because of its practical interest (i.e., as a cause of injury and death, disturbances in power and communication sys- tems, and ignition of forest fires) and because lightning channels below cloud level are more easily photo- graphed and studied with optical instruments. Cloud-to- cloud and cloud-to-air discharges are less common than intracloud or cloud-to-ground lightning. All discharges other than cloud-to-ground are often lumped together and called cloud discharges. Lightning is a self-propagating and electrodeless atmospheric discharge that, through the induction process, transfers the electrical energy of an electrified cloud into electrical charges and current in its ionized and thus conducting channel. Positive and nega- tive leaders are essential components of the lightning. Only when a leader reaches the ground does the ground potential wave (return stroke) affect the lightning process. Natural lightning starts as a bidirectional leader, although at different stages of the process unidirectional leader de- velopment 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), radia- tion, and light. Lightning, in its various forms, is known by 66
many common names, such as streak lightning, forked lightning, sheet lightning, and heat lightning, and by the less common air discharge; also, the rare and mysterious ball lightning and rocket lightning. An important effect of worldwide 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 spo- radically at all times in various parts of the earth, perhaps 100 per second, may be the principal source of negative charge that maintains the earthâionosphere potential dif- ference of several hundred thousand volts in spite of the steady transfer of charge produced by the airâearth cur- rent. However, there also is evidence that point discharge currents may contribute to this more significantly than lightning. See also cloud-to-ground flash, intracloud flash, lightning discharge. âChalmers, J. A., 1957: Atmospheric Electricity, 235â255. âSchonland, B. F. J., 1950: The Flight of Thunderbolts, 152 pp. âHagenguth, J. H., 1951: Compendium of Meteorology, 136â143. Lightning channelâThe irregular path through the air along which a lightning discharge occurs. The lightning channel is established at the start of a dis- charge 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 thun- dercloud or between a cloud charge center and the sur- rounding air or between charge centers in adjacent clouds. Lightning detection networkâAn integrated array of light- ning direction finders that provide information for trigono- metric location of cloud-to-ground lightning discharges. Timing and direction information from individual re- ceivers are combined to provide evolving maps of light- ning occurrences across vast regions that sometimes reach beyond the range of storm surveillance radars. See sferics receiver. Lightning direction finderâSee sferics receiver. Lightning dischargeâThe series of electrical processes tak- ing place within 1 s by which charge is transferred along a dis- charge channel between electric charge centers of opposite sign within a thundercloud (intracloud flash), between a cloud charge center and the earthâs surface (cloud-to-ground flash or ground-to-cloud discharge), between 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 flashâThe total observed lightning discharge, generally having a duration of less than 1 s. A single flash is usually composed of many distinct lumi- nous events that often occur in such rapid succession that the human eye cannot resolve them. Lightning mapping systemâA network of lightning detec- tion 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 1 μs. Since cloud-to-cloud and cloud-to-air are rare lightning phenomena, mapping them has little or no importance. Lightning strokeâIn 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 follow- ing the same lightning channel. Negative cloud-to-ground lightningâA lightning flash or stroke between a cloud and the ground that lowers negative charge to the ground. Negative ground flashâSame as negative cloud-to-ground lightning. Peak currentâUsually refers to the maximum current in a lightning return stroke. Pearl lightningâSame as beaded lightning. Point discharge currentâThe electrical current accompa- nying any specified source of point discharge. In the electrical budget of the earthâatmosphere system, point discharge currents are of considerable significance as a major component of the supply current. Estimates made by Schonland (1928) of the point discharge current from trees in arid southwest Africa suggest that this process ac- counts for about 20 times as much delivery of negative charge to the earth during typical thunderstorms as do 67
lightning discharges. Although the great height of thun- dercloud bases in arid regions, such as that referred to in Schonlandâs study, tends to favor point discharge over lightning charge transfer, point discharge still seems more significant than lightning even in England, where Wormell (1953) found for Cambridge a ratio of about 5:1 in favor of point discharge over lightning charge transfer. âChalmers, J. A., 1957: Atmospheric Electricity, 156â175. âWormell, T. W., 1953: Atmospheric electricity: some recent trends and problems. Quart. J. Roy. Meteor. Soc., 79, 3â50. âSchonland, B. F. J., 1928: The polarity of thunderclouds. Proc. Roy. Soc. A, 118, 233â251. Positive cloud-to-ground lightningâA lightning flash or stroke between a cloud and the ground that lowers positive charge from the cloud to the ground. Positive dischargeâA positive discharge lowers positive charge to the ground via a lightning flash. The flash may be initiated in the cloud or from the ground. Positive ground flashâSame as positive cloud-to-ground lightning. Return strokeâThe intense luminosity that 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 that produces almost all of the luminosity and charge transfer in most cloud-to-ground strokes. Its great speed of ascent (about 1 Ã 108 m sâ1) 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 electric field in which channel electrons are drawn down toward the ascending tip in the region of the streamerâs electron avalanche. Cur- rent peaks as high as 3 Ã 105 A have been reported, and values of 3 Ã 104 A are fairly typical. The entire process of the return stroke is completed in a few tens of microsec- onds, 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 posi- tive leader channel, thus increasing positive charge on the ground. In negative cloud-to-ground flashes, multiple re- turn 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 be- cause of the high electrical conductivity of the ground, and hence this type of streamer is not to be found in air dis- charges, cloud discharges, or cloud-to-cloud discharges. âHagenguth, J. H., 1951: Compendium of Meteorology, 137â141. Ribbon lightningâOrdinary 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 observerâs 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 dur- ing film exposure. Rocket-triggered lightningâA form of artificial lightning discharge initiated with a rocket trailing wire that 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, bidirectional leader de- velopment occurs from both ends of the wire, similar to lightning initiation from aircraft. In the case of negative space charge overhead (usual summer thunderstorm con- dition), a triggered lightning may only be a positive leader or may become a sequence of dart leaderâreturn 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 the case of positive space charge overhead (usual winter storm con- dition), the triggered lightning is a single negative leader. Sferics fixâThe determination of the bearing to the lightning source usually based on the measurement of the horizontal magnetic field with orthogonal coils or loop antennas. Sferics observationâThe detection of electromagnetic radiation from lightning generally in the frequency range 10â30 kHz. 68
The physical measurement can include the electric field, the magnetic field, or both. Sferics are generally attributed to the high current phases of source, that is, return strokes and K changes. Sferics receiverâ(Also called lightning direction finder.) An instrument that measures, electronically, the direction of arrival, intensity, and rate of occurrence of atmospherics; a type of radio direction finder, it is most commonly used to detect and locate cloud-to-ground lightning discharges from distant thunderstorms. In its simplest form the instrument consists of two or- thogonally crossed antennas that measure the electromag- netic field changes produced by a lightning discharge and determine the direction from which the changes arrived. Negative and positive polarity cloud-to-ground discharges can be distinguished. Cloud-to-cloud discharges can be dis- tinguished based on characteristics of the received signal, and the geometry of nearby discharge channels may be determined. See also lightning detection network. Sferics sourceâThat portion of a lightning discharge that radiates strongly in the frequency interval 10â30 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. Sheet lightningâ(Also called luminous cloud.) A diffuse, but sometimes fairly bright, illumination of those parts of a thun- dercloud that surround the path of a lightning flash, particu- larly 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. Compare heat lightning. Spark dischargeâThat type of gaseous electrical discharge in which the charge transfer occurs transiently along a rela- tively 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, the corona discharge, and the continuous arc discharge. The exact meaning to be at- tached to the term âspark dischargeâ varies somewhat in the literature. It is frequently applied to just the tran- sient phase of the establishment of any arc discharge. A lightning discharge can be considered a large-scale spark discharge. Spider lightningâLightning with extraordinary lateral extent near a 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 associ- ated with positive ground flashes. This discharge form is also referred to as sheet lightning. SpriteâWeak 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 multi- ple vertically elongated spots, to spots with faint extrusions above and below, to bright groupings that extend from the cloud tops to altitudes up to about 95 km. Sprites are pre- dominantly red. The brightest region lies in the altitude range 65â75 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 as 40 km. High-speed photometer measurements show that the duration of sprites is only a few milliseconds. 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 10â50 kJ per event, with a corresponding optical power of 5â25 MW. Assuming that optical energy constitutes 10â3 of the total for the event, the energy and power are on the order of 10â100 MJ and 5â50 GW, respectively. Early research reports for these events referred to them by a variety of names, including upward lightning, upward dis- charges, 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 non- judgmental about physical processes that have yet to be determined. Compare blue jets. Stepped leaderâThe initial leader of a lightning discharge; an intermittently advancing column of high ionization and charge that 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 50â100 μs. The velocity of growth during the brief intervals of advance, each only about 1 μs in duration, is quite high (about 5 à 107 m sâ1), but the long stationary phases reduce its effective speed to only about 5 à 105 m sâ1. 69
Streak lightningâOrdinary lightning, of a cloud-to-ground discharge, that appears to be entirely concentrated in a single, relatively straight lightning channel. Compare forked lightning, zigzag lightning StreamerâA sinuous channel of very high ion density that propagates itself though a gas by continual establishment of an electron avalanche just ahead of its advancing tip. In lightning discharges, the stepped leader, dart leader, and return stroke all constitute special types of streamers. StrokeâSee lightning stroke. Stroke densityâThe areal density of lightning discharges over a given region during some specified period of time, as num- ber per square mile or per square kilometer. Supply currentâThe electrical current in the atmosphere that is required to balance the observed airâearth current of fair-weather regions by transporting positive charge upward or negative charge downward. 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 A for the earth as a whole is estimated to be re- quired to balance the airâearth current. Wilson (1920) suggested that the thunderstorms present in widely scat- tered regions of the earth at any one time might be re- sponsible 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 up- ward above active thunderstorms. See airâearth current, point discharge current. âGish, O. H., 1951: Compendium of Meteorology, 113â118. âWilson, C. T. R., 1920: Investigations on lightning dis- charges and on the electric field of thunderstorms. Phil. Trans. A, 221, 73â115. ThunderâThe sound emitted by rapidly expanding gases along the channel of a lightning discharge. Some three-fourths of the electrical energy of a lightning discharge is expended, via ionâmolecule collisions, in heating the atmospheric gases in and immediately around the luminous channel. In a few tens of microseconds, the channel rises to a local temperature of the order of 10,000 °C, with the result that a violent quasi-cylindrical pressure wave is sent out, followed by a succession of rar- efactions and compressions induced by the inherent elas- ticity of the air. These compressions are heard as thunder. Most of the sonic energy results from the return streamers of each individual lightning stroke, but an initial tearing sound is produced by the stepped leader; and the sharp click or crack heard at very close range, just prior to the main crash of thunder, is caused by the ground streamer ascending to meet the stepped leader of the first stroke. Thunder is seldom heard at points farther than 15 miles from the lightning discharge, with 25 miles an approxi- mate upper limit, and 10 miles a fairly typical value of the range of audibility. At such distances, thunder has the characteristic rumbling sound of very low pitch. The pitch is low when heard at large distances only because of the strong attenuation of the high-frequency components of the original sound. The rumbling results chiefly from the varying arrival times of the sound waves emitted by the portions of the sinuous lightning channel that are located at varying distances from the observer, and secondarily from echoing and from the multiplicity of the strokes of a composite flash. Thunderstorm cellâThe convective cell of a cumulonimbus cloud having lightning and thunder. Thunderstormâ(Sometimes called electrical storm.) In general, a local storm, invariably produced by a cumulonim- bus cloud and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. It is usually of short duration, seldom over two hours for any one storm. A thunderstorm is a consequence of atmospheric instability and constitutes, loosely, an over- turning of air layers in order to achieve a more stable density stratification. A strong convective updraft is a distin- guishing feature of this storm in its early phases. A strong downdraft in a column of precipitation marks its dissi- pating stages. Thunderstorms often build to altitudes of 40,000â50,000 ft in midlatitudes and to even greater heights in the Tropics; only the great stability of the lower strato- sphere limits their upward growth. A unique quality of thunderstorms is their striking electrical activity. The study of thunderstorm electricity includes not only lightning phe- nomena per se but all of the complexities of thunderstorm charge separation and all charge distribution within the realm of thunderstorm influence. In U.S. weather observing procedure, a thunderstorm is reported whenever thunder is heard at the station; it is reported on regularly scheduled ob- servations if thunder is heard within 15 minutes preceding 70
the observation. Thunderstorms are reported as light, medium, or heavy according to 1) the nature of the light- ning and thunder; 2) the type and intensity of the precipi- tation, if any; 3) the speed and gustiness of the wind; 4) the appearance of the clouds; and 5) the effect upon surface temperature. From the viewpoint of the synoptic meteorol- ogist, thunderstorms may be classified by the nature of the overall weather situation, such as airmass thunderstorm, frontal thunderstorm, and squall-line thunderstorm. âByers, H. R., and R. R. Braham Jr., 1949: The Thunder- storm, U.S. Government Printing Office, 287 pp. âByers, H. R., 1951: Compendium of Meteorology, p. 681. Time-of-arrival techniqueâThe 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 light- ning flash and the arrival of the thunder can be used to estimate the distance to the lightning flash. On the aver- age, a time arrival difference of five seconds indicates that a lightning flash occurred one mile away from the observer, since the speed of sound in air is approximately 1000 ft sâ1. WhistlerâA type of VLF electromagnetic signal generated by some lightning discharges. Whistlers propagate along geomagnetic field lines and can travel back and forth several times between the Northern and Southern Hemispheres. So named from the sound they produce in radio receivers. Zigzag lightningâOrdinary lightning of a cloud-to-ground discharge that appears to have a single, but very irregular, lightning channel. Compare streak lightning, forked lightning. 71