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11 activity--including thunderstorms and lightning--peaks in the mid to late afternoon, with a secondary nighttime maxi- mum across the Midwest. Lightning Detection Technologies Lightning flashes and strokes can be detected in many dif- ferent ways. Most notably the discharge of thousands of amperes of current in a fraction of a second generates temperatures estimated to be as hot as 30,000 °C, hotter than the surface of the sun, with a brilliant flash of light and an acoustic shock wave we call thunder. At the same time, the surging electrical currents release a wide spectrum of electro- magnetic radiation and modify the strength of the local atmospheric electrical field. Flash and Bang The flash and bang of nearby lightning strikes are hard to ignore, even without special instrumentation. Distant light- ning flashes can often be seen by an alert observer, particu- larly at night. For applications involving safety, however, these techniques are not reliable and are only appropriate in the Figure 4. A well-known picture of a lightning flash absence of more quantitative technologies. made with a special lightning camera with film While these "technology free" approaches can only esti- that moves rapidly during the exposure. Stepped mate the position of a lightning flash in a general way, the leaders are frozen, while the multiple return strokes difference in time between the observation of a flash and the show up as separate strokes that follow exactly the arrival of the sound of thunder is a useful and practical way same path (4). to estimate the distance (but not necessarily the direction) of the lightning. The lightning flash is seen virtually instanta- shows the overall annual average flash density distributions. neously, while sound travels at about 750 mph, or approxi- The main features are the concentration of the lightning flashes mately 1 mi every 5 sec. The interval, in seconds, between the over land and a general gradient from low flash densities in flash and the bang multiplied by 5 thus gives a useful estimate the Pacific Northwest to very high flash densities over Florida. of the distance of the strike, in miles. The annual pattern, however, reflects both the geographical Unfortunately, many small airports do not have any light- and seasonal frequencies of thunderstorms, with the south- ning detection capabilities, and rules-of-thumb, such as the ern states having a much longer annual lightning season. "flash-bang-multiply-by-5" estimate of the proximity of the The lower panel shows the monthly average flash density lightning, provide the only information. for the month of August, displayed in terms of the expected annual lightning flash densities that would result if the August Acoustic detectors. While the sound of thunder is usu- flash rates were continued for a full year. The August plot ally easy to recognize, it is difficult to use in any quantitative shows that during the late summer, thunderstorms are wide- sense. Networks of acoustic detectors have been tested to try to spread throughout all areas of CONUS, with the exception of locate lightning strikes, but with limited success, and acoustic the extreme northwestern and northeastern states. Lightning detection systems have never been used operationally. can be a hazard every place in the lower 48 states; although not well represented in the satellite climatology, lightning storms Optical detectors. The instantaneous flash of light asso- are also a major hazard in Alaska during the long hours of ciated with lightning can be difficult to see in the daytime and, summer sun, and they trigger a great number of forest fires until lately, has not often been used for quantitative applica- every year. tions. By using sensitive detectors and narrow bandwidth fil- In addition to the geographic and seasonal variations in the ters, however, optical lightning detection systems have been frequency of lightning hazards, there is also an important developed that can be used in the daytime and which have daily diurnal variation. Over continental areas, convective been incorporated into ground-based sensors in conjunction

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12 INTER-CLOUD STRIKE DISCHARGE WITHIN CLOUD BETWEEN (CLOUD-TO-CLOUD) NEGATIVE BASE AND POSITIVE TOP (INTRA-CLOUD) DISCHARGE BETWEEN NEGATIVE AND POSITIVE CHARGE CENTERS TYPICAL CLOUD-TO-GROUND LIGHTNING BETWEEN GROUND AND NEGATIVE CHARGE CENTERS Figure 5. Multiple clouds with complex charge distributions. This figure illustrates the typical cloud-to-ground lightning flashes, as well as discharges between different portions of a single cloud and discharges between adjacent clouds (8). CG Flash Cloud Flash VLF 1 to 10 kHZ LF 100 kHz MF 1 Mhz VHF 10 Mhz Scale 0.5 second Figure 6. CG and IC flash emissions in various frequency ranges. VHF emissions are generally limited to line of sight propagation (200­300 km, or 125­185 mi.), while LF emissions propagate by ground waves that can follow the curvature of the earth and can be detected to ranges of 300­600 km, or 185­375 miles. VLF emissions can be reflected off the ionosphere and can be detected for thousands of kilometers, but in variably decreasing efficiencies (4).

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13 Annual NASA Satellite Climatology, Flashes per km2 per year 70 50 40 30 20 15 10 8 6 4 2 1 .8 .6 .4 .2 .1 Monthly NASA Satellite Climatology, Flashes per km2 per year Figure 8. Electric field mill (from Boltek). 70 50 40 30 although there are other instruments, including some that are 20 proprietary, that can also be used to monitor the electric field. 15 10 Nearby lightning discharges will produce sudden changes 8 in the strength of the local electrical field, and these distinctive 6 4 changes can be used to detect lightning--although without 2 any direct way of measuring the distance or range to the light- 1 .8 ning flash. Nearby charge centers, such as a cloud developing .6 .4 directly overhead, can dominate the local electric field and August .2 may limit the detection of distant lightning strikes. Perhaps .1 more important than detecting lightning, electric field mills Figure 7. NASA satellite climatologies of "total" can also monitor the buildup in the local electrostatic field, lightning (CG plus IC) flashes in terms of the which normally precedes a lightning strike. Most currently average number of flashes per square kilometer available lightning detection systems that employ field mills per year, compiled over a 10-yr period. The upper use them to alert users to the electric field buildup and to panel shows the annual averages, while the warn them of a potential lightning event. This application is bottom panel shows the monthly average flash unique in focusing on anticipating the lightning "threat" density for the month of August (data provided rather than on detecting lightning strikes after they occur. The by the Global Hydrology and Climate Center, technology, however, has a somewhat uncertain range and de- NASA Marshall Space Flight Center). tection efficiency, along with a potential for false alarms. Field mills are sensitive instruments that require periodic monitoring and cleaning; their readings can be influenced by with magnetic and electrostatic pulse analysis to reduce false blowing dust and by local air pollution. The strong electric fields alarms (for example, the Vaisala TSS-928 local-area lightning that signal a potential lightning event, however, are normally detection sensor). More important, optical detection systems easy to detect with fields mills that are properly maintained. have also been adapted for satellite-based lightning detection Lightning alert and lightning prediction systems making systems (satellite-based systems will be discussed in a separate use of electric field mills are available commercially and are a section). key component of the lightning hazard and launch evaluation systems employed at NASA's Kennedy Space Flight Center. Atmospheric Electric Field Measurements Electromagnetic Emissions Electric field measurements have a long and important his- from Lightning Strokes tory of use by scientists interested in atmospheric electricity and lightning. The most common instrument to measure Most lightning detection systems currently available make the atmospheric electric field is the field mill (see Figure 8), use of the electromagnetic emissions, predominately RF,

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14 associated with the electrical discharge (see Figure 6). Light- receivers, most lightning strokes can be detected by three to ning strokes produce RF static (mostly in the MF band) and four different sensors. are familiar to listeners of AM radios. CG strokes generate Sensor networks can also locate the position of a lightning strong signals in the LF band, which can be detected at ranges strike by making use of the high-accuracy time references of many hundreds of kilometers. IC strokes, on the other provided by global positioning system (GPS) satellites to hand, predominately generate VHF, line-of-sight emissions. determine the difference in time between two or more de- Lightning detectors based on RF electromagnetic emissions tectors' observations of the same lightning stoke. Using range from relatively simple, low-cost, handheld devices to sophisticated algorithms, the differences in the "time of ar- sophisticated sensors and groups of sensors organized into rival" of the signal can be used to identify the location and detection networks. Low-end systems, however, are of uncer- time of the lightning strike. Depending on the position of the tain sensitivity and are subject to false detections. They are lightning strike and the position and spacing of the detectors, most commonly marketed for hikers, sports activities, and time of arrival solutions can require as many as three or more outdoor gatherings. The most basic systems do not try to detectors to record the signal from the same lightning stroke. identify the direction of the lightning, but may try to produce Using sensitive receivers designed to minimize false detec- a rough estimate of the lightning distance by measuring the tions, lightning detection networks have been shown to be amplitude of the signal. capable of detecting cloud-to-ground lightning strokes with This technology can be enhanced by using more sophisti- a detection efficiency of over 90% and position accuracy of cated receivers that can monitor the signal at multiple fre- significantly better than 1 km (0.625 mi). Two such networks, quencies and analyze the time evolution and properties of run by commercial companies, currently provide lightning the signal to minimize false alarms. Analysis of the incoming information for CONUS. signal can also be used to distinguish between CG flashes and Ground-based lightning detection networks are primarily discharges from an IC stroke. designed to detect CG lightning and can provide information With the addition of orthogonally crossed loop antennas about each individual stroke within a lightning flash. With or other radio direction finding technologies (the SAFIR recent improvements to these same detectors they can now de- lightning detection systems developed in France, for example, tect a significant percentage of the nearby IC lightning strokes, use VHF interferometric dipole antennas for direction find- but at a variable and as yet not well characterized detection ing), it is also possible to determine the direction from the efficiency that depends on the properties of the stroke and the detector to the source of the lightning signal. Used individu- distance from the network sensors. Since the IC lightning ally, high-end receivers of this sort are employed to identify strokes are frequently horizontal and extend for great dis- the direction of nearby lightning strikes and, with a simple tances, it is harder to assign a single position to each stroke. CG signal amplitude algorithm, to also estimate the range. Such flashes also extend over long distances inside the cloud, while sensors are often included in automatic weather stations de- the ground strike positions are normally well defined. Since signed to produce fully automatic METAR reports (aviation there are significantly more cloud lightning strikes than ground routine weather reports) summarizing the current weather at strikes, and since within-cloud lightning is normally observed an airport. For this application, the lightning detection sys- preceding the first ground strokes, cloud lightning detection tem is used as an indicator of the nearby presence of a thun- systems that are optimized for VHF emissions have a great po- derstorm and gives an approximate indication of the storm's tential for enhancing our current detection capabilities. These position and distance relative to the airport. systems will, however, require a significantly higher density of stations to provide uniform, high-detection-efficiency cover- age for future applications. At present, there are a number of Lightning Detection by Networks regional "total lightning" detection systems that are being used of Electromagnetic Sensors for research and for the testing of future application products. Networks of sophisticated electromagnetic sensors can provide very accurate position information for CG lightning Lightning Detection from Space strokes. The most immediately obvious approach is through triangulation of the direction information obtained by two Space-borne sensors can also be used to detect lighting. or more sensors. Since the strong LF and VLF signals from While some satellite-based sensors can detect the electrical ground lightning tend to follow the surface of the earth and emissions from the lightning flash, the most promising space- are detectable at ranges of many hundreds of kilometers, it is borne approach is based on optical detection of the lightning possible to construct a network to cover a very large area with strikes. a reasonable number of detectors--something on the order Optical detectors, normally filtered to look at a strong of slightly over 100 sensors for CONUS. With this density of oxygen emission band in the near infrared (IR) and analyzed