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15 to detect short bursts of radiation such as expected from a a standard as short as 10 min since the last lightning strike lightning strike, can be used both day and night. In the 1970s before going back to work (see Chapter 2). In any situation of and 1980s, Bernard Vonnegut designed an early handheld this sort, there is a continual tension between providing an detector of this sort for use by U-2 research aircraft and Space adequate warning to prevent injuries and not stopping work Shuttle astronauts (13, 14). This approach was subsequently unnecessarily. refined and employed in NASA's LIS on the TRMM satellite Advocates of conservative (safety first) warning criteria and in the OTD flown on the Microlab-1 satellite. Similar emphasize that lightning injury statistics show that injuries optical detection systems are currently being developed for are most likely at either the very beginning of the lightning use on the GOES-R series of U.S. operational geostationary event or near its end (15, 16). Their goal is to be able to issue weather satellites. a lightning warning before the first strike reaches the ground The Geostationary Lightning Mapper (GLM) being devel- and then to allow enough time before work is resumed to oped for GOES-R is expected to provide full coverage over ensure that the hazard has passed. the United States, South America, and adjacent oceanic areas. This is a serious problem, since individual lightning strikes From geostationary orbit, the GLM lightning sensor will not are essentially impossible to predict, either as to time or loca- be able to match the accurate positioning of the current tion. There are well-documented examples of "bolts from the ground-based networks, but will provide uniform, high- blue"--lightning strikes that occur when an observer can see efficiency detection of total lightning, including both cloud blue sky above (see Figure 9). On the other hand, current pro- and ground flashes over virtually all of the visible earth disk cedures are generally believed to provide a safe environment as seen from space. This new data set will not replace the for ramp workers, as evidenced by the very low numbers of current ground-based lightning networks, but will provide reported lighting deaths or injuries. extremely valuable "total lighting" information to augment the high-resolution CG flash information currently available. Review of Current Airport Lightning Detection Technologies Warning Criteria This section reviews and evaluates most of the lightning Detecting lightning strokes is a critical initial step in any detection systems and technologies currently in use at airports lightning safety system, but needs to be combined with a set of or marketed for use by airports, airlines, and ramp workers. warning criteria. In general, most dedicated lightning systems As a rather specialized market, it is surprising how many dif- provide two levels of warning: an alert, saying that lightning ferent commercial systems are available. may develop or move into the area in the near future, and an All the systems included in this discussion seem to be rep- alarm, saying that lightning has been detected in the imme- utable and should be able to detect lightning strikes within diate vicinity or is expected to develop at any moment. ranges of concern to airport workers. There are, however, no In systems based exclusively on measurements of the formal standards for lightning detectors, and no agency or atmospheric electric field, unusually high fields will trigger an organization is responsible for routinely testing these instru- alert, with alarms being triggered by electric fields reaching a ments for accuracy, reliability, or durability. While it would be level where imminent discharges could be expected (typically relatively easy to perform comparisons between instruments, 2000 V/m). this would require cooperation from the various system ven- In systems based on detections of CG flashes, the warning dors. Lacking a requirement for a license or a certification criteria are based on the distance to the lightning strokes process, this is not likely to happen. More important, verifi- being detected and the time since the last stroke was detected cation and validation of lightning detection system perfor- within a specified distance from the area of interest. While mance requires an independent system for detecting ground there are no universally recognized standards for issuing alerts truth. Some limited testing of this sort has been done, mostly or alarms for airport ramp operations, the American Meteo- to document the performance of the National Lightning De- rological Society and the National Oceanic and Atmospheric tection Network (NLDN) using triggered lightning strikes or Administration (NOAA) have endorsed the "30-30 rule." triangulation of strike impact positions from simultaneous This rule states that outdoor activities should be limited or photographs taken by multiple cameras. curtailed whenever there has been a lightning strike detected within 6 mi (based on 30 sec between an observed flash and Handheld or Portable Systems the sound of the thunder) within the past 30 min. Based on RF Emissions Lightning warning system vendors often recommend this standard, but allow users to set their own criteria for alerts Handheld systems are the entry-level product for lightning and warnings. In some cases, airport operators report using detection. These systems are relatively low cost (some priced
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16 A Bolt from the Blue! Figure 9. Two illustrations of lightning strikes that develop within a convective storm, but exit the side of the storm and strike the ground relatively far from the visible edge of the storm. These two illustrations are from different storms, but show a strikingly similar pattern. The picture on the left was taken by Al Moller. The illustration on the right, provided by Bill Rison from the New Mexico Institute for Mining and Technology, is a vertical cross-section of a storm, as seen by a research radar, overlaid with a full depiction of a lightning stroke based on a specialized lightning mapping system capable of detecting each step in the lightning stroke. In this case, the lightning strikes the ground about 5 km (3 mi) from the edge of the radar echo. under $100) and only detect the RF static discharges of a Directional Detectors Based nearby lightning strike. While the systems may not be uni- on RF Emissions formly sensitive in all directions, they have no way to detect or indicate the direction of the lightning strike. They do, how- These systems are a step up from the handheld or ever, generally try to give some indication of the relative range portable systems discussed earlier. The systems add a fixed of the strike, based on the amplitude of the RF signal. These antenna to identify the direction to the detected lightning systems often monitor the amplitude of the lightning signals strike. The distance to the lightning strike, however, is still over time and indicate whether the lightning is getting closer estimated from the amplitude of the lightning signal. Prod- or further away, based on the trend in the signal amplitude. ucts in this category can range from fairly basic systems This is not a particularly accurate way to estimate range, using personal computers, which are primarily targeted at making the devices mostly useful as an "objective" detection meteorological hobbyists or commercial users seeking a system that might be carried by individual workers or used at general awareness of nearby lightning activity, to sophisticated a small airport to help them notice or evaluate a potential systems engineered for specific airport applications (for lightning hazard. example, automatic thunderstorm detection for METAR In general, these systems are not appropriate for workers at reports). large airports or for airport operations managers. As single sensor detection systems, these systems are some- Specific products in this category include what limited for applications that require high-accuracy de- tection and tracking of lightning strikes in the vicinity of an · StrikeAlert (www.strikealert.com), operational facility, such as airport ramp operations. These · SkyScan (www.skyscanusa.com/main.html), and systems can be quite useful, but they should not be used for · ThunderBolt (www.spectrumthunderbolt.com). applications for which they were not intended.
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17 This category includes Boltek (www.boltek.com). Note, TOA Systems offers an Advanced Lightning Warning Sys- however, that there is a cooperative lightning detection network tem (ALWS), based on three or more electrical field mills and based on shared observations by Boltek system users--the designed to monitor an area 6 mi or more in range. Their field StrikeStar Lightning Detection Network--but as a coopera- mill systems can also be integrated with lightning reports tive effort it is clearly and properly labeled as "Not for use for from a separate network (such as their own U.S. Precision protection of life or property." The following two thunder- Lightning Network, USPLN). storm sensors designed for automatic weather stations are Field mills can offer important information on the initial also in this category: development of electrical activity in the vicinity of an airport, but are probably best used as a component of a detection · All Weather Inc. Thunderstorm Detector, Model 6500 (www. and warning system that also uses RF lightning detection allweatherinc.com/meteorological/lightning_detection_ technologies. 6500.html), and · Vaisala Local Lightning Sensor TSS928 (www.vaisala. Commercial Lightning Detection Networks com/businessareas/measurementsystems/thunderstorm/ producingsystems/tss9281). At present, the United States is covered by two separate, in- dependent lightning detection networks. These networks are intended to provide real-time lightning data for a wide vari- The single-sensor thunderstorm detection systems in- ety of commercial and government applications. tended for use with an automatic weather station may not be The older of the two networks, NLDN, is now operated and appropriate for high-resolution detection and tracking of maintained by Vaisala. NLDN (www.vaisala.com/weather/ nearby lightning activity. Two or more of these units could, products/lightning/knowledgecenter/aboutnldn) was recently however, be combined into a local lightning detection network upgraded with new sensors that combine both magnetic direc- that could provide a local-area, high-accuracy, real-time tion finding (MDF) and time of arrival (TOA) technologies lightning detection capability. At present, no commercial to increase system reliability, detection efficiency, and location vendors offer this type of system. accuracy. The current system is estimated to have a 90%95% detection efficiency for CG flashes, with a median location Electric Field Mills (or Other Electric Field accuracy of better than 500 m (17, 18). In 2005, Vaisala's Monitoring Systems) NLDN received a 5-yr contract to provide lightning detection data to the National Weather Service and other U.S. govern- By monitoring the buildup of the local electric field strength, ment agencies. electric field mills (or other electric field monitoring systems) USPLN (www.uspln.com/index2.html) is owned and main- can sense the increasing potential for a nearby lightning strike. tained by TOA Systems, Inc., in collaboration with its partner, In this sense, field mills are a rather unique product in that they WSI Corporation. WSI is responsible for sales and marketing offer the promise of being able to "predict" the first lightning of the data, including sales to value-added retailers. This re- strike and offer protection for airport personnel in the case of cently completed network is based on a new generation of a storm that develops lightning directly overhead and does not sensors, exclusively using time-of-arrival technologies. TOA move into range as a fully developed, active thunderstorm. Systems estimates that their national network provides greater There are a number of manufacturers of field mills, mostly than 90% detection efficiency and an accuracy equal to, or sold as individual units and not as an integrated lightning de- better than, 250 m. tection and warning product suitable for airport applications. Several commercial lightning detection systems employ, or have the option to employ, field mills as a component of their Airport Lightning Detection Systems Based systems. on National Lightning Networks Only two vendors offer full commercial systems based Given the availability, accuracy, and impressive effi- exclusively on the monitoring of the atmospheric electric ciency of the national lightning detection networks, it is field: Thor Guard (thorguard.com) and TOA Systems (www. natural that most of the commercially available airport toasystems.com/TOASystems/ALWS.htm). lightning detection systems are based on the network light- Thor Guard provides complete lightning warning systems, ning data sets. complete with horns and lights. Their standard installation While some systems make use of the lightning data by is based on a single sensor, but for larger areas they provide itself, other vendors integrate the lightning data with other, systems with several sensors. They have an extensive customer more general-purpose weather information, including radar list, including some small airports. products. The lightning-specific products are clearly directed
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18 toward applications such as ramp operations, with the intent alarms and electronic notification. The most recent software of providing a focused product that meets the specific user's upgrade supports an unlimited number of circular or polygon needs. The more general integrated displays, on the other hand, warning areas, with the alert and alarm criteria customized by are normally directed toward a broader audience, including the user (see Figure 10). users such as airline managers and dispatchers that need to The current version of the Vaisala lightning warning system monitor both flight and ramp operations. Ideally, an inte- is the TWX300, which was released in 2007 (www.vaisala.com/ grated product should provide separate displays or tools to weather/products/lightning/). Earlier versions of the Vaisala switch focus between different, independently optimized views system were distributed as the Precision Lightning Warning of the available data. Versatile systems, optimized for meteo- System (PLWS), which was released in 1995, and the TWX1200, rologists, are often too complicated for focused applications which was made available in 2004. All of these versions of the such as ramp operations. Vaisala system are currently in use at a variety of airports. The Vaisala thunderstorm warning system is based on real- ARINC is a licensed installer and value-added reseller of time lightning observations provided by Vaisala's NLDN. The Vaisala lightning equipment and can provide customized in- system can optionally be augmented by the addition of up to stallations with external alarms (horns and beacons) and a seven electric field mills. The warning system provides an variety of different options for communication links (www. extensive set of custom displays showing the location of light- arinc.com/products/weather/forewarn/index.html). ARINC's ning strikes and generating specific alert and alarm messages. ForeWarn precision lightning system is based on Vaisala's The warning system can be customized by visual and audible Thunderstorm Warning System software, with user options. Figure 10. A captured image of the main display screen of the Vaisala TWX300 lightning warning system (with annotations added). The age of the displayed lightning strokes is indicated by their color, while the bottom panel summarizes the current alarm and alert status. As the storm approaches the airport, the display can be zoomed in for a closer, higher resolution view (figure courtesy of Vaisala).