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A momentary cloud created by artist Berndnaut Smilde floats in St. Helen’s Church on the Island of Lundy in England’s Bristol Channel. On average, at any given time, two-thirds of our planet is cov...

The existence of mesoscale lightning discharges on the order of 100 km in length has been known since the radar-based findings of Ligda in the mid-1950s. However, it took the discovery of sprites in 1989 to direct significant attention to horizontally extensive “megaflashes” within mesoscale convective systems (MCSs). More recently, 3D Lightning Mapping Arrays (LMAs) have documented sprite-initiating lightning discharges traversing several hundred kilometers. One such event in a 2007 Oklahoma MCS having an LMA-derived length of 321 km, has been certified by the WMO as the longest officially documented lightning flash. The new Geostationary Lightning Mapper (GLM) sensor on GOES-16/17 now provides an additional tool suited to investigating mesoscale lightning. On 22 October 2017, a quasi-linear convective system moved through the central United States. At 0513 UTC, the GLM indicated a lightning discharge originated in northern Texas, propagated north-northeast across Oklahoma, fortuitously traversed the Oklahoma LMA (OKLMA), and finally terminated in southeastern Kansas. This event is explored using the OKLMA, the National Lightning Detection Network (NLDN), and the GLM. The NLDN reported 17 positive cloud-to-ground flashes (+CGs), 23 negative CGs (−CGs), and 37 intracloud flashes (ICs) associated with this massive discharge, including two +CGs capable of inducing sprites, with others triggering upward lightning from tall towers. Combining all available data confirms the megaflash, which illuminated 67,845 km2, was at least 500 km long, greatly exceeding the current official record flash length. Yet even these values are being superseded as GLM data are further explored, revealing that such vast discharges may not be all that uncommon.

“Mount Irazu has not erupted since 1994,” our guide assured us as we began the long drive up to the summit of Costa Rica's highest volcanic peak, one of the many in this geologically active country...

Is fog the Rodney Dangerfield of the elements? It gets no respect from weather aficionados. When fog is in the forecast, do scores of “fog chasers” pile into their vehicles to travel hundreds of mi...

September, 2020. Hong Kong residents, like those in most locales, were locked down by COVID-19. Respite from home isolation for some came in the form of hikes into remote places such as Needle Hill...

How did the tiger elve get its stripes? It's simple, really: a massive and impulsive CG emits an intense burst of EMP, detectable worldwide at ELF radio frequencies, resulting in a large glowing el...

CHRISTOPHER SINGER/CLOUD APPRECIATION SOCIETYThere is a birthday coming up. On March 23, 2018, World Meteorological Day, asperitas will be one year old. On that date in 2017, the World Meteorologic...

In upcoming issues, we will undoubtedly show many examples of ACSL—altocumulus standing lenticularis—the often spectacular lens-shaped cloud sculptures that mark the contorted flows of strong winds...

Two sprite-producing thunderstorms were observed on 8 and 25 June 2012 in northeastern Colorado by a combination of low-light cameras, a lightning mapping array, polarimetric and Doppler radars, the National Lightning Detection Network, and charge moment change measurements. The 8 June event evolved from a tornadic hailstorm to a larger multicellular system that produced 21 observed positive sprites in 2 h. The majority of sprites occurred during a lull in convective strength, as measured by total flash rate, flash energy, and radar echo volume. Mean flash area spiked multiple times during this period; however, total flash rates still exceeded 60 min(sup 1), and portions of the storm featured a complex anomalous charge structure, with midlevel positive charge near 20degC. The storm produced predominantly positive cloud-to-ground lightning. All sprite-parent flashes occurred on the northeastern flank of the storm, where strong westerly upper level flow was consistent with advection of charged precipitation away from convection, providing a pathway for stratiform lightning. The 25 June event was another multicellular hailstorm with an anomalous charge structure that produced 26 positive sprites in less than 1 h. The sprites again occurred during a convective lull, with relatively weaker reflectivity and lower total flash rate but relatively larger mean flash area. However, all sprite parents occurred in or near convection and tapped charge layers in adjacent anvil cloud. The results demonstrate the sprite production by convective ground strokes in anomalously charged storms and also indicate that sprite production and convective vigor are inversely related in mature storms.

On the morning of 5 June 2013, two cameras of the SpriteCam network concurrently captured a red sprite with diffuse halo over a mesoscale convective system (MCS) passing the panhandle area of Oklahoma. This sprite was produced by a negative cloud-to-ground (CG) stroke with peak current of −103 kA in a manner different from previous observations in several aspects. First of all, the causative stroke of sprite is located by the National Lightning Detection Network (NLDN) in the trailing stratiform of MCS, instead of the deep convection typically for negative sprites. Second, the sprite-producing stroke was likely the first stroke of a multistroke negative CG flash (with ≥6 CG strokes) whose evolution was mainly confined in the lower part of thunderstorm; although the parent flash of sprite might contain relatively long in-cloud evolution prior to the first stroke, there is no evidence that the negative leader had propagated into the upper positive region of thundercloud as typically observed for the sprite-producing/class negative CG strokes. Third, as shown by the simulation with a two-dimensional full-wave electrodynamic model, although the impulse charge moment change (−190 C km) produced by the main stroke was not sufficient to induce conventional breakdown in the mesosphere, a second impulse charge transfer occurred with ~2 ms delay to cause a substantial charge transfer (−290 C km) so that the overall charge moment change (−480 C km) exceeded the threshold for sprite production; this is a scenario different from the typical case discussed by Li et al. (2012). As for the source of the second current pulse that played a critical role to produce the sprite, it could be an M component whose charge source was at least 9 km horizontally displaced from the main stroke or a negative CG stroke (with weak peak current for the return stroke) that was not detected by the NLDN.

Recent video observations have shown that elves—the visible manifestation of the lightning electromagnetic pulse (EMP) interacting with the lower ionosphere—can be modulated by neutral density fluctuations at lower ionosphere altitudes due to gravity waves. The elve emissions thus illuminate the gravity wave structure. Here we use a finite difference time domain (FDTD) model of the lightning EMP to simulate elve emissions in the presence of neutral density fluctuations representative of gravity waves. We show that observable elve striations can be generated by gravity waves with a neutral density perturbation of as low as 5% at elve altitudes near 85 km. Higher perturbations lead to more pronounced striations. The depth of the optical signature perturbation is found to be similar in magnitude to the neutral density perturbation at elve altitudes, although the relationship depends on viewing geometry, gravity wave geometry, and the elve-causative lightning peak current moment.

On 31 May 2013, a line of severe tornadic thunderstorms (the El Reno event) developed during the local afternoon in central Oklahoma, USA. Within range of the Oklahoma Lightning Mapping Array, the evolution of the event can be separated into three distinct periods: an Early period (before 02:00 UTC on 1 June) when the storm consisted of discrete supercells, a Middle period (02:00–05:00 UTC) when the convection began merging into a linear feature and stratiform precipitation developed, and a Late period (after 05:00 UTC) featuring a mature mesoscale convective system (MCS). Each of these periods demonstrated distinct patterns in the large (>100 C km) charge moment change (CMC) lightning that was produced. During the Early period, large-CMC positive cloud-to-ground (+CG) lightning was produced in the convective cores of supercells. These flashes were small in area (typically 500 km2, >300 C km) in the developing stratiform, similar to typical sprite-parent lightning in MCSs. During the Late period, convective large CMC +CGs ceased and instead large-CMC negative CGs were produced in and near the MCS convection. These flashes neutralized charge both in convection as well as in adjacent stratiform and anvil precipitation. The results suggest that the CMC metric has potential applications for studying tropospheric weather.

We report on a markedly striated elve (Emissions of Light and Very Low Frequency Perturbations due to Electromagnetic Pulse Source) (a “tiger elve”) observed using an intensified high-speed Phantom camera system at the Yucca Ridge Field Station near Fort Collins, Colorado, on the night of 12 June 2013. This elve was induced by a 204 kA positive cloud-to-ground lightning flash within a mesoscale convective system in western South Dakota. A halo and a sprite followed the elve. The banded structure in the elve was aligned with convectively generated gravity waves (CGGWs) independently observed by a collocated color near-infrared camera. Assuming the height of the OH layer and elve both to be 85 km, photogrammetry allowed projection of the elve and the CGGWs onto the same geographic map. The tiger elve stripes approximately overlay on the troughs (dark bands) of CGGWs. This is consistent with model predictions that the ionization rate in the D region ionosphere is inversely proportional to the air density, which is modulated by the CGGWs.

The use of both total charge moment change (CMC) and impulse charge moment change (iCMC) magnitudes to assess the potential of a cloud-to-ground (CG) lightning stroke to induce a mesospheric sprite has been well described in the literature, particularly on a case study basis. In this climatological study, large iCMC discharges for thresholds of >100 and >300 C km in both positive and negative polarities are analyzed on a seasonal basis. Also presented are local solar time diurnal distributions in eight different regions covering the lower 48 states as well as the adjacent Atlantic Ocean, including the Gulf Stream. The seasonal maps show the predisposition of large positive iCMCs to dominate across the northern Great Plains, with large negative iCMCs favored in the southeastern United States year-round. During summer, the highest frequency of large positive iCMCs across the upper Midwest aligns closely with the preferred tracks of nocturnal mesoscale convective systems (MCSs). As iCMC values increase above 300 C km, the maximum shifts eastward of the 100 C km maximum in the central plains. Diurnal distributions in the eight regions support these conclusions, with a nocturnal peak in large iCMC discharges in the northern Great Plains and Great Lakes, an early to midafternoon peak in the Intermountain West and the southeastern United States, and a morning peak in large iCMC discharge activity over the Atlantic Ocean. Large negative iCMCs peak earlier in time than large positive iCMCs, which may be attributed to the growth of large stratiform charge reservoirs following initial convective development.

A major central U.S. winter cyclone on 1–2 February 2011 produced a band of high winds, up to 75 cm of snow, and numerous reports of thundersnow from Oklahoma into Ontario over a 26 h period. The National Lightning Detection Network (NLDN) recorded 282 flashes comprised of 1153 events which were >96% negative polarity. Hopes of imaging winter sprites associated with energetic positive cloud-to-ground events that sometimes accompany such winter storms did not materialize. However, the lack of lightning over the Great Lakes waters, plus media reports of numerous thundersnow events in downtown Chicago, prompted a detailed analysis of the NLDN data. This revealed that >93% of all lightning in the snow band was likely or possibly associated with self-initiated upward lightning (SIUL) events from a variety of tall, and some not so tall, structures. In addition to 43 events from two Chicago skyscrapers, many shorter structures were involved, including wind turbines (13.1% of the total) and transmission line towers (6.7%). Wind speeds for all events exceeded the 8 m s−1 minimum threshold associated with SIULs in Japanese winter lightning storms. Radar reflectivities at the event locations had a mean of 28 dBZ and were almost always

Received 28 December 2012; revised 18 June 2013; accepted 24 June 2013; published 26 July 2013. [1] This study examined the meteorological characteristics of precipitation systems that produced 38 “sprite-class” negative cloud-to-ground (CG) strokes (i.e., peak currents in excess of 100kA and charge moment changes in excess of 800Ckm) as well as those that produced three confirmed negative sprites on 23 different days during 2009–2011. Within 15km of the negative sprite-parent/class stroke, the median characteristics for these systems were to produce negative CGs as 69.2% of all CGs, and for the 30dBZ radar reflectivity contour to reach on average 14.2km above mean sea level (MSL), during a 25min period encompassing the occurrence of the stroke. The median contiguous area of 30dBZ composite radar echo (i.e., maximum value in the vertical column) for these systems was 6.73×10 3 km 2 . All but three of the discharges occurred in intense multicellular convection, with 30dBZ exceeding 10km MSL in altitude, while the others occurred in the stratiform regions of mesoscale convective systems. All but six of the systems produced greater than 50% negative CG lightning, though flash rates tended to be low near the stroke (1–2min 1 on average). The results suggest that negative sprite-parent/class lightning typically occurs in precipitation systems of similar size and intensity as those that produce positive sprites, but not necessarily the same systems, and the negative lightning normally strikes ground in the convection rather than the stratiform precipitation. However, upper-level positive charge in the convection may play an important role in sprite-class/parent lightning of either polarity.

[1] The temporal and spatial development of sprite-producing lightning flashes is examined with coordinated observations over an asymmetric mesoscale convective system (MCS) on 29 June 2011 near the Oklahoma Lightning Mapping Array (LMA). Sprites produced by a total of 26 lightning flashes were observed simultaneously on video from Bennett, Colorado and Hawley, Texas, enabling a triangulation of sprites in comparison with temporal development of parent lightning (in particular, negatively charged stepped leaders) in three-dimensional space. In general, prompt sprites produced within 20 ms after the causative stroke are less horizontally displaced (typically 30 km). However, both prompt and delayed sprites are usually centered within 30 km of the geometric center of relevant LMA sources (with affinity to negative stepped leaders) during the prior 100 ms interval. Multiple sprites appearing as dancing/jumping events associated with a single lightning flash could be produced either by distinct strokes of the flash, by a single stroke through a series of current surges superposed on an intense continuing current, or by both. Our observations imply that sprites elongated in one direction are sometimes linked to in-cloud leader structure with the same elongation, and sprites that were more symmetric were produced above the progression of multiple negative leaders. This suggests that the large-scale structure of sprites could be affected by the in-cloud geometry of positive charge removal. Based on an expanded dataset of 39 sprite-parent flashes by including more sprites recorded by one single camera over the same MCS, the altitude (above mean sea level, MSL) of positively charged cloud region tapped by sprite-producing strokes declined gradually from ~10 km MSL (−35°C) to around 6 km MSL (−10°C) as the MCS evolved through the mature stage. On average, the positive charge removal by causative strokes of sprites observed on 29 June is centered at 3.6 km above the freezing level or at 7.9 km above ground level.

[1] We report and analyze 3 years of lightning impulse charge moment change (iCMC) measurements obtained from an automated, real time lightning charge moment change network (CMCN). The CMCN combines U.S. National Lightning Detection Network (NLDN) lightning event geolocations with extremely low frequency (≲1 kHz) data from two stations to provide iCMC measurements across the entire United States. Almost 14 million lightning events were measured in the 3 year period. We present the statistical distributions of iCMC versus polarity and NLDN-measured peak current, including corrections for the detection efficiency of the CMCN versus peak current. We find a broad distribution of iCMC for a given peak current, implying that these parameters are at best only weakly correlated. Curiously, the mean iCMC does not monotonically increase with peak current, and in fact, drops for positive CG strokes above +150 kA. For all positive strokes, there is a boundary near 20 C km that separates seemingly distinct populations of high and low iCMC strokes. We also explore the geographic distribution of high iCMC lightning strokes. High iCMC positive strokes occur predominantly in the northern midwest portion of the U.S., with a secondary peak over the gulf stream region just off the U.S. east coast. High iCMC negative strokes are also clustered in the midwest, although somewhat south of most of the high iCMC positive strokes. This is a region far from the locations of maximum occurrence of high peak current negative strokes. Based on assumed iCMC thresholds for sprite production, we estimate that approximately 35,000 positive polarity and 350 negative polarity sprites occur per year over the U.S. land and near-coastal areas. Among other applications, this network is useful for the nowcasting of sprite-producing storms and storm regions.

[1] An analysis of thunderstorm environment, structure, and evolution associated with six gigantic jets (five negative polarity, one positive) was conducted. Three of these gigantic jets were observed within detection range of very high frequency lightning mapping networks. All six were within range of operational radars and two-dimensional lightning network coverage: five within the National Lightning Detection Network and one within the Global Lightning Detection (GLD360) network. Most of the storms producing the jets formed in moist tropical or tropical-like environments (precipitable water ranged from 37 to 62 kg m−2, and 0–6 km shear from 3.5 to 24.8 m s−1), featuring high convective available potential energy (1200–3500 J kg−1) and low lifted indices (−2.8 to −6.4). The storms had maximum radar reflectivity factors of 54 to 62 dBZ, and 10 dBZ echo contours reached 14–17 km. Storms covered by three-dimensional lightning mappers were near peak altitude of lightning activity (modes of the vertical distributions of radio sources were at altitudes colder than −50°C) and vertical reflectivity intensity, with overshooting echo tops around the times of their jets. Two of the other three jet-producing storms produced their jet around the time of a convective surge as indicated by radar data and likely featured overshooting tops. The observations suggest a link between convective surges, overshooting tops, and the occurrence of gigantic jets, similar to prior modeling studies.

Ripples as seen in airglow imagers are small wavy structures with short horizontal wavelengths (

A few years ago, we identified a deep convective transport mechanism, of water vapor through the tropopause, namely, storm top gravity wave breaking, such that tropospheric water substance can be injected into the lower stratosphere via this pathway. The main evidence presented previously was taken from the lower resolution AVHRR images of the storm anvil top cirrus plumes obtained by polar orbiting satellites. Recent observations have provided further supporting evidence for this important cross-tropopause transport mechanism. There are now many higher resolution satellite images, mainly from MODIS instrument, that show more definitely the existence of these plumes, many of which would probably be unseen by lower resolution images. Furthermore, a thunderstorm movie taken in Denver (USA) area during STEPS2000 field campaign and another thunderstorm movie taken by a building top webcam in Zurich also demonstrate that the jumping cirrus phenomenon, first identified by T. Fujita in 1980s, may be quite common in active thunderstorm cells, quite contrary to previous belief that it is rare. We have used a cloud model to demonstrate that the jumping cirrus is exactly the gravity wave breaking phenomenon that transports water vapor through the tropopause. These additional evidences provide increasing support that deep convection contributes substantially to the troposphere-to-stratosphere transport of water substance. This corroborates well with recent studies of the stratospheric HDO/H 2 O ratio which is much highly than it would be if the transport is via slow ascent. The only explanation that can be used to interpret this observation at present is that water substance is transported through the tropopause via rapid vertical motion, i.e., deep convection.

Gigantic jets emerge from the top of thunderstorms and extend all the way to the ionosphere at altitudes of 90 km. Simultaneous video images and magnetic field measurements of a gigantic jet demonstrate an electric charge transfer between the thunderstorm and the ionosphere that is comparable to that observed in cloud-to-ground lightning.

The heights of horizontal structures in the OH airglow images were observed with two all-sky imagers at Platteville (40.2N, 104.7W) and Yucca Ridge Field Station (40.7N, 104.9W) in Colorado. The sodium temperature-wind lidar of CSU at Fort Collins (40.6N, 105.1W) observed temperature and wind velocities around 80–105-km altitude, providing time-height variations of atmospheric stability. Simultaneous observations with the imagers and the lidar were successful on two nights: November 18 and 19, 2003. The estimated height from OH airglow images were lower (80–86 km) than those observed in Japan, with significant time variation. Comparison with the temperature and wind observed by the lidar showed that the estimated height is likely to coincide with the height of small N 2 (close to 0) values. Ripples (

During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

Historically, the process of transient luminous event (TLE) detection has required an alert human observer on a low light level television (LLTV) monitor, either in real time or playback. The 1999 Sprites Balloon Campaign payloads had all-sky upward looking photometers not sensitive to events below the balloons. The photometer data was examined at the time of National Lightning Detection Network (NLDN) lightening strokes to find TLEs that were missed visually. 3602 events were analyzed in 4.1 h of storm time. Threshold current moments of ∼50 kA km for the positive cloud to ground (+CG) TLEs and ∼−5 kA km for negative cloud to ground (−CG) TLEs are found.

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (Mq) yielded averages of ;800 and ;950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (1CGs). Analyses of the LMA’s very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that 1CGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7‐11.5 km AGL) to much lower altitudes (2‐5 km AGL). The average height of charge removal (Zq) from 15 sprite-parent 1CGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by spriteparent 1CGs were on the order of 200 C. The regional 08C isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS spriteproduction models previously proposed by two of the authors (Lyons and Williams).

A variety of storm top electrical discharges have been observed using several types of low-light imagers, film, and the human eye. Recently, a video recorded an unprecedented, bright blue upward discharge from a tropical thunderstorm top near Puerto Rico. The event reached the base of the ionosphere. The horizontal dimensions of cloud top discharges can range from 100 m to several kilometers. Upward extents vary from 100 m to 70 km. Shapes include “points” of light, upwardly flaring trumpets, and narrow, vertical, lightning-like channels, some topped with expanding blue, flame-like features. Visual appearances range from brilliant white lightning-like channels to a grainy, almost particulate appearing jets of dim blue light, and sometimes as a blue flame within which a brilliant white channel appears. The classical blue jet is at the lower limit of human night vision whereas some upward discharges have been clearly seen during daylight. Cloud top “pixies” last no longer than 16.7 ms, whereas upward lightn...

Waveform monitoring of ELF radio signals in the frequency range of 1– 400 Hz have been carried out on a routine basis at Syowa station (69.0°S, 39.6°E in geographic coordinates), Antarctica since February, 2000. The main purpose of these observations is to monitor global lightning activity and to locate lightning-induced sprites and elves. The ELF observation system consisting of two search coil sensors (geomagnetic north–south (H) and east–west (D) sensors) was installed at a remote unmanned observatory in West Ongul Island located 5 km southwest from Syowa station. As a back up system, the same system was installed near Syowa station in East Ongul Island. Signals from these sensors were digitally sampled at 1000 Hz with a GPS time code. On July 4, 2000 during the STEPS (Severe Thunderstorm Electrification and Precipitation Studies) 2000 campaign carried out over the Great Plains in the US, 57 sprite events were observed from Yucca Ridge Field Station (40.7°N, 104.9°W), Colorado, and 53 out of these sprite events had one-to-one correspondence to transient Schumann resonances (SR) detected at Syowa station. The waveforms of these SR are characterized by sharp initial pulses and following damped oscillations. The great circles representing the propagation paths are determined from the Lissojous plots of the H and D magnetic field data of the transient SR. It has been demonstrated that the minimum distance between the great circles and the locations of causative cloud-to-ground (CG) discharges is ∼240 km on average. It is thus concluded that the method to determine the propagation paths from Lissajous plots is extremely accurate when we use the Syowa ELF waveform data. Consequently, it would be possible to globally triangulate sprite-inducing CG locations by setting up at least one more observation site with the same system.

Flight 3 of the Sprites99 balloon campaign flew from 00:39:32 to 11:12:00 UTC on 21 August 1999, launched from Ottumwa, Iowa. Three axis electric and magnetic field and ground-based low light level TV observations from three sites were made of more than 68 transient luminous events (TLEs) in the middle atmosphere above thunderstorms in South Dakota and Kansas. At least four TLEs were recorded by two or more stations, including sprites at 0955:36, 0541:58 and 0546:10 UTC and a sprite halo at 0746:35. An event triggered on-board memory, sampling 10 quantities at a rate of 50 kHz per channel for 160 ms per event, recorded bursts for some of the sprites and elves. At the range of the balloon from the lightning and TLE (∼400 km), the vertical electric field perturbation is roughly simulataneous with the light emission. The presence of a radial component of some magnetic pulses suggests that signal propagation was not entirely in the TM mode.

Recent time-resolved multi-color photometric data obtained on one class of lightning-related transient upper-atmospheric electromagnetic events called sprites have confirmed an impulsive ionization emission during the sprite initiation. Data have also been obtained on some sprites which do not exhibit observable tendrils and which exhibit ionization emission that, if present, is below our detection limit. This suggests that some sprite events exhibit strong ionization while others do not. These results indicate that conditions causing sprite optical emissions are highly variable.

Using identical observed meteorology for lateral boundary conditions, the Regional Atmospheric Modeling System was integrated for July-August 1973 for south Florida. Three experiments were performed-one using the observed 1973 landscape, another the 1993 landscape, and the third the 1900 landscape, when the region was close to its natural state. Over the 2-month period, there was a 9% decrease in rainfall averaged over south Florida with the 1973 landscape and an 11% decrease with the 1993 landscape, as compared with the model results when the 1900 landscape is used. The limited available observations of trends in summer rainfall over this region are consistent with these trends.

A clear association between large peak current cloud-to-ground lightning flashes of positive polarity and sprites and elves in the stratosphere and mesosphere has been previously demonstrated. This paper reports on the first climatology of large peak current cloud-to-ground (LPCCG) lightning flashes compiled from the U.S. National Lightning Detection Network. Analysis of almost 60 million CG flashes from 14 summer months (1991‐95) reveals distinct geographic differences in the distribution of positive and negative polarity LPCCGs, arbitrarily defined as flashes with peak currents $75 kA. Large peak current positive CGs (LPC1CGs) are concentrated in the High Plains and upper Midwest, the region in which a large majority of optical sprite and elves observations have been obtained. By contrast, large peak current negative CGs (LPC2CGs) preferentially occur over the coastal waters of the Gulf of Mexico and the southeastern United States. A total of 1.46 million LPCCGs were found, of which only 13.7% were 1CGs. Almost 70% of the LPC1CGs, however, occurred in the central United States (308‐508N, 888‐1108W). The percentage of all LPCCGs that were positive approached 30% in the central United States compared to 4.5% for the remainder of the country. A 1CG is 3.1 times more likely to exceed 75 kA than is a 2CG flash on a national basis. Yet in terms of absolute numbers for all ranges of peak current $75 kA, negative CGs are clearly dominant. For peak currents $75 and 200 kA, negative CGs outnumbered positive CGs by ratios of 6.4 and 4.1, respectively. In the central United States, however, during evening hours the number of LPC1CGs almost reaches parity with LPC2CGs. Average stroke multiplicity also exhibited regional differences. Over a half million negative CGs and over 1000 positive CGs were found with multiplicity $10.

This paper reports observations of a distinctive form of sprites associated with positive CG flashes carrying currents of 23 or less to about 100 kA in mesoscale thunderstorms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, and show virtually no variation in brightness along their length. Three dimensional triangulation of what we define as a ‘columniform’ sprite (c-sprite) event on the evening of 19 June 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some show faint diffuse ‘hair’ or tendrils extending above and below the column. The sprite columns are nearly vertical, in video imagery. On some evenings, c-sprites are the dominant form of sprite activity above thunderstorms but, on other nights with many sprites, they may not be observed at all. Comparison of c-sprite forms vs National Lightning Detection Network (NLDN) positive cloud-to-ground current, shows a progression from simple thin vertical forms to brighter and more complicated forms. Theoretical explanations which predict the form and vertical structure of the classical sprites do not at present account for these different forms.

We present preliminary evidence of a high-altitude, electrical discharge process that can produce radar echoes at 24.38 MHz. This conclusion is drawn from pulsed-radar observations of near time-coincident occurrences of impulsive electromagnetic radiation with radar echoes that originated at altitudes well above those of mesoscale convective systems.

We have obtained blue (350–475 nm) video images and simultaneous high-time resolution narrow-band blue (415–435 nm) photometry records of four sprite events. The brightest blue images show a sustained tendril geometry and a nearly constant intensity of emission over the entire vertical extent of the sprite (from 35–90 km altitude). Photometer light curves display an exponential decay with a 0.3 ms time constant, a FWHM on the order of 0.1–1.0 ms, and are probably dominated by 427.8 nm (N+2 1N) emission. The data support the observations of Armstrong et al. (1998) and support the contention that significant ionization occurs during sprite generation.

We have obtained blue photometric measurements of the N 2 second positive 399.8 nm and the N + 2 first negative 427.8 nm emission from sprites, elves and lightning, along with supporting video images. The pulse width and intensity results for sprites are consistent with those of Suszcynsky et al. (1998) . The red emission from sprites has been independently and unambiguously identified by Hampton et al. (1996) and Mende et al. (1995) as the nitrogen first positive band. The source has been attributed to electron impact excitation from low energy electrons (≈1 eV) in the sprite. The short pulse width of the 427.8 nm and 399.8 nm photometer time traces obtained in this investigation are probably not from the same source that produces the red emission. The results reported here indicate an initial energetic ionizing event sufficient to ionize and excite nitrogen followed by secondary electron processes which give rise to the dominant red emission. The photometer results for elves are consistent with the EMP mechanism suggested by Inan et al. (1996) . The photometer traces obtained for lightning indicate emissions consistent with a ‘continuing current’ as the charge redistributes within the thunderstorm cloud. We find that the ratio of the intensity of the 399.8 nm N 2 (2P) emission to that of 427.8 nm N + 2 (1N) emission can be used to discriminate among sprites, elves and lightning.

Horizontal magnetic field variations in the frequency range of the Earth-ionosphere cavity resonances are observed at Silberborn, Germany, simultaneous with 19 sprite-associated lightning flashes in the midwestern United States, on July 15, 1995. The measured horizontal magnetic intensities are linearly related to the horizontal magnetic intensities of slow tails of radio atmospherics which were simultaneously recorded at Palmer Station, Antarctica. Enhancement of the Earth-ionosphere cavity resonances is verified by spectral analysis, and the measured arrival azimuths are in agreement with the expected orientation of the Poynting vector along the great-circle path. The estimated accuracy of the arrival azimuth determination is on the order of ±5°.

Transient luminous events (sprites, blue jets, elves) above large mesoscale convective systems (MCSs) over the U.S. High Plains have been routinely monitored from the Yucca Ridge Field Station near Fort Collins, Colorado using ground-based low-light video systems. We analyzed 36 sprites above the Nebraska MCS of August 6, 1994. The results lend further support to the hypothesis that sprites are almost uniquely associated with positive cloud-to-ground (+CG) lightning flashes. Sprite-associated +CGs also averaged substantially larger peak currents than the remaining +CG population (81 kA versus 30 kA in this storm system). There is some evidence that sprite-associated +CGs also have higher stroke multiplicity. This study yields no evidence of sprites associated with negative CG events. In the central United States an additional requirement appears to be that the parent MCS has a contiguous radar reflectivity area exceeding 20-25,000 km 2 . The majority of the sprites occur above the large stratiform precipitation region and not the high-reflectivity convective core of the MCS. Triangulation of a limited number of paired images (from September 7, 1994) suggests that the sprite is generally centered within 50 km of the parent +CG. Assuming the +CG provides the range, single-image photogrammetric analyses provide estimates of the maximum vertical extent of the sprites. For this storm the sprite tops averaged 77 km with a maximum of 88 km. The bases averaged 50 km but with a few sprite tendrils extending as low as 31 km.

Radio atmospherics launched by sprite-producing positive cloud-to-ground lightning flashes and observed at Palmer Station, Antarctica, exhibit large ELF slow tails following the initial VLF portion, indicating the presence of continuing currents in the source lightning flashes. One-to-one correlation of sferics with NLDN lightning data in both time and arrival azimuth, measured with an accuracy of ±1° at ∼12,000 km range, allows unambiguous identification of lightning flashes originating in the storm of interest. Slow-tail measurements at Palmer can potentially be used to measure continuing currents in lightning flashes over nearly half of the Earth's surface.

Observations of optical phenomena at. high alti- tude a, bove thunderstorms using a multichannel high-speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Field Station (40040 ' N, 104o.56 ' W), Colorado as part of the SPRITES'95 campaign from 15 June to August 6, 1995. These newneasurements indicate that diffuse optical flashes with a duration of < I ms and a hori- zontal scale of-.- 100-300 km occur at 75-105 km altitude in the lower ionosphere just after the onset of cloud-to-ground lightning discharges, but preceding the onset of sprites. Here we designate these events as 'alves" to distinguish them from 'i'ed sprites" . This finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges as suggested by several authors.

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler-induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.

Scattering by the conductive columns of red sprites of VLF waves ("VLF sprites") traveling in the Earth- ionosphere waveguide is characterized by wide angle scattering (to 180o). This enabled the first VLF detection of sprite conductivity and is now used routinely for sprite detection and location by measurement of the phase and amplitude at both of the frequencies transmitted by NAA, NSS, NLK and NPM. In a blind test using only one VLF receiver and the first three of these transmitters, all of the VLF events fitting the criteria for VLF sprites were found to correspond to optical sprites and vir- tually all of the sprites observed optically corresponded to VLF events. Using only a single receiver in the high noise envi- ronment of local thunderstorms enabled the range to the sprite to be found to within 100 km and the direction to within 90 o. Use of a large number (-10) of suitably spaced antennas greatly increases the location accuracy and provides some information on the lateral structure of sprite conductivity.

Red sprites occur high above the stratosphere, just under the ionosphere. Although the first reported observation was over 100 years ago, and the first theory was 40 years ago, only over the last year or so has the subject spread into the popular science magazines, and into the secular media. Most of the studies of the sprite structure have been optical, using the light they emit for a few tens of milliseconds for imaging (low-light video and photography) and spectroscopy. Here, we concentrate on the scattering by sprites of man-made VLF radio waves. This scattering shows that the columnar elements of sprites have a substantial electrical conductivity.