Lightning-Warning Systems for Use by Airports (2008)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

45 Current Systems From a safety point of view, the existing lightning warning systems for airports seem to be doing a very good job. Because airports and airlines are safety conscious and closely monitor the weather, lightning injuries to ramp and other outdoor workers have been infrequent and fatalities rare. At the same time, however, there appears to be no systematic attempt to collect or maintain lightning-related ramp injury records for the cases that do occur, and the information that is available is mostly in the form of anecdotal stories or in the corporate memory of long-term employees. On the federal side, no statistics on aviation-related lightning injuries are collected by the Occupational Safety and Health Administration, the National Transportation Safety Board, or the FAA. With increasing pressure for on-time operations and effi- ciency, ramp closures resulting from nearby lightning can have a serious impact on local airport operations and reduce the efficiency of the national air transportation system. Although lightning frequently halts ramp operations at many airports, it is difficult to analyze the true scope and magnitude of the problem because neither airlines nor airports routinely record the frequency or duration of ramp closures. The serious impact of lightning on ramp safety and operational efficiency, and the potential impact on the national air transportation system, need to be reflected in better efforts to collect and maintain records. On the industry side, lightning safety studies emphasize techniques designed to improve lightning detection capabil- ities and predictions. Researchers examine “gaps” in warning detection systems and try to eliminate potential “failures to warn” of imminent lightning strikes. At the same time, how- ever, it is important to examine proposed system improve- ments for their potential to increase the number or duration of ramp closures without any increase in worker safety, and to focus on identifying the earliest time at which the “all clear” can be announced and ramp operations resumed. What is needed is a balanced system that preserves or improves ramp safety, while better defining the hazard area and duration for maximum efficiency. A system that is too conservative and generates prolonged shutdowns will even- tually be ignored or disregarded. Lightning warning systems in the United States are gener- ally based on lightning observations from the national light- ning detection networks, primarily the NLDN. These systems are a national resource and provide high-accuracy location of cloud-to-ground lightning strokes from Seattle to Miami and from Maine to San Diego. With real-time access to lightning data, an airport monitoring system can track the develop- ment of lightning storms and their movement toward the airport. Warnings and ramp closures are then generated on the basis of the distance of flashes from the airport and the time since the most recent nearby stroke. This is an efficient system since it is based on directly detecting and monitoring the cloud-to-ground lightning strokes that pose the hazard. Appropriate Systems for Airports of All Sizes Ramp safety is essential for all airports. All commercial air- ports with scheduled operations in lightning-prone areas should have lightning detection and warning systems to alert managers and ramp personnel of approaching hazards. While our study has concentrated on the higher-end lightning warning systems designed for large airports, there are also less sophisticated and less expensive lightning warning systems appropriate for smaller airports. Smaller airports may be well served by Internet-based lightning monitoring systems that provide real-time access to lightning information from the NLDN, but without the ded- icated high-speed communication lines, sophisticated display workstations, or automatic sirens and alarms typically used at larger airports. Smaller airports with fewer operations have more flexibility and can more easily absorb delays caused by C H A P T E R 4 Conclusions

ramp closures than can large airports. This means that when lightning is near, they may be able to shut down earlier and wait a bit longer to declare an “all clear” than major airports with higher traffic volumes and tighter schedules. Safety issues become more critical and require closer, more expen- sive, monitoring of the situation when airport users are trying to push the envelope and keep operations going as long as possible without interruption. Warnings based on NLDN monitoring the approach of active thunderstorms can identify perhaps 90% of the light- ning events that affect an airport, with the remainder coming from new storms that develop in the immediate vicinity of the airport (28). To respond to this developing storms hazard, high-end lightning detection systems typically augment the NLDN observations with locally installed EFMs, which can detect the buildup of the local electric field that normally pre- cedes lightning. EFMs do, however, add significantly to the cost of a warning system. An EFM can cost as much as $16,000, and they would need to be installed at several locations around the airport to provide a useful indication of the developing potential for lightning strikes. Each EFM would require its own set of com- munication cables and regular maintenance to ensure reliable performance. While EFMs are routinely used at lightning- sensitive locations, such as the Kennedy Space Center in Florida and weapons testing sites, because they can provide early warning of developing storms, they are subject to false alarms since not all developing storms actually produce light- ning. In most cases, the buildup of the electric field should be considered a necessary, but not sufficient, criterion for lightning activity (29). From an airport operational perspective, the most impor- tant improvements that could be made in current lightning detection and warning systems would be to develop more precise and better defined warnings that still give operators time to effectively clear the ramp and suspend operations, and then get back to work as quickly as possible with less downtime, but without compromising safety. There are a number of promising ways to refine and im- prove lightning detection and warning systems for airports by making better use of all the currently available weather observations, through the development of smarter software and analysis algorithms, and by incorporating new technolo- gies. These options are highlighted in the following sections. Smart Algorithms and Software The performance of any lightning warning system is criti- cally dependent on the specific warning criteria that are used to stop work and clear the ramp, as well as the guidelines that are subsequently used to decide when to resume work. These criteria affect both safety and efficiency. Conservative criteria may enhance safety, but at the cost of excessive downtime. On the other hand, standards designed to minimize disruptions may put airport workers at risk. System providers will nor- mally recommend an initial set of warning criteria, but allow users to set their own criteria for alerts and warnings based on their collective experience with typical weather patterns at their airport. As a practical matter, this means that the spe- cific warning criteria used at different airports can vary greatly. One approach to improving this situation and helping individual airports and airlines refine their warning criteria would be to make use of intelligent, self-monitoring warning systems. A lightning detection and warning system with this sort of capability would be able to monitor its own perform- ance and evaluate the adequacy of the specific warning criteria being used. Any unanticipated lightning strikes in the imme- diate vicinity of the airport, or strikes that follow the declara- tion of an alert too closely for the ramp to be cleared, would be evaluated to see if reasonable changes to the warning criteria would have provided a better warning. Such a sys- tem could also keep track of excessive warnings or lengthy ramp closures and evaluate to what extent safety would have been compromised with slightly more relaxed criteria. The system would be, in effect, self-training and would provide an objective approach for making gradual adjustments to the specific warning criteria used at an airport in response to the actual lightning events it experiences over time. This approach could also be used to refine warning criteria to reflect the local storm climatology, and permit seasonable adjustments to optimize performance. For example, consider an airport with a lightning warning system that recommends that outdoor operations be stopped whenever a lightning strike is detected within 6 mi of the airport and declares an “all clear” when there have been no additional lightning strikes within this distance for 15 min. As a routine matter, the lightning system could be designed to keep track of the number of recommended alerts and alarms, the duration of the work stoppages, the number of lightning strikes over the immediate airport area (or other designated “area of concern”), and related statistics. Lightning strikes in the area of concern without adequate prior warning would be of particular importance and would be identified and recorded. In parallel with the statistics for the operational set of warning criteria, system software could also generate com- parable statistics for other possible combinations of warning criteria. For example, there could be separate statistics gener- ated for all distance thresholds from 3 mi to 10 mi, and for “all clear” times from 5 min to 30 min. These statistics would be collected and reviewed, perhaps once a year, identifying possible changes to the warning criteria that could improve airport efficiency, while preserving safety. Any changes of this sort would need to be done gradually and incrementally, but 46

would eventually move the airport to an optimum balance of safety and efficiency. With this sort of capability, the system operator could also be provided with periodic summaries of system performance and daily reporting of all lightning events. With an additional option for manual entry of actual ramp closure times and durations, the system could provide a permanent record of lightning activity and ramp closures. These same capabilities, perhaps including a 24-hr tempo- rary archive of nearby lightning strikes, would also be useful for airport and airline accident/incident investigations, hard land- ings, or lightning-related injuries and damage. While much of this information could be recovered or reconstructed from the national NLDN permanent data archive, the full set of local in- formation should also be available at airports that operate lightning detection and warning systems. This capability would be particularly valuable in providing airport authorities, air- lines, and other tenants with rapid access to recent lightning in- formation and local statistics in response to emergencies. Integrating Technologies for Improved Performance Perhaps the most obvious way to improve the performance of existing lightning warning systems is to incorporate addi- tional weather information into the warning algorithms. Meteorological radars have traditionally been the observing sys- tem of choice for monitoring thunderstorms. The current na- tional U.S. network of high-quality Doppler radars (NexRad) is a uniquely valuable resource for tracking the development and movement of lightning-producing storms and should be able to be used in conjunction with standard NLDN observations to produce a new set of comprehensive warning products. Radar Echo Properties and Tracking Radar studies of storm structure have led to radar-based predictions of the likelihood of lightning (30, 31). Although such second-order products are of little direct use when NLDN observations are available, they indicate that radar echo patterns and properties may be useful in helping to identify specific meteorological situations that may be particularly problematic or require additional safeguards. Studies of this sort are currently underway and may lead to improved light- ning warning products (32). From an airport operational per- spective, the most important potential contribution of radar data may be to provide a better estimate of the end of the lightning hazard as storms move away from the airport area. Radar observations can also be used to track the movement of storm cells. Thunderstorm cells generally have great spa- tial and time continuity and are relatively easy to track by radar. While it is also possible to identify and track areas of lightning activity, lightning “cells” are composed of individ- ual, discrete lightning strokes and are more difficult to define and harder to track. While radar and lightning cells are clearly linked, it is important to remember that the radar echo bound- aries do not always coincide with the limits for lightning strikes, as exemplified by the “bolt from the blue” phenom- ena discussed in Chapter 1. Nevertheless, a blended product identifying the boundaries of the most active lightning- producing areas through a combination of radar echoes and direct lightning observations should provide a good estimate of the expected movement of active lightning areas. A better delineation of the boundaries of the active lightning strike areas should in turn allow a better estimate of the onset and termination of the lightning threat. Knowledge of the advection direction of the lightning- producing cells can also provide additional direct benefits. One of the most interesting results of our study generating synthetic airport closure statistics based on archived NLDN observations (discussed in Chapter 3) was the relatively high number of closures that resulted from a single lightning strike, or from an extremely short burst of lightning activity extending less than 1 min. While this phenomena needs additional study, it is likely the result of lightning-producing storms drifting past the airport, just barely within the dis- tance criteria used for shutting down ramp operations. These storms would presumably be producing lightning as they approach and move on past the airport area, but are only within the warning range for a short time. In this case, it may be possible to significantly reduce the total number of ramp closures by adjusting the warning area boundaries based on storm motion vectors. Adjustments of this sort are essentially equivalent to modifying the current distance-based lightning proximity warning criteria to also consider the time before an approaching storm is likely to reach the airport. Warning boundaries in the direction of storms moving rapidly directly towards the airport, for example, may need to be extended to provide adequate time to shut down oper- ations before the storm reaches the airport. Shrinking the dimensions of the warning area in the directions perpendic- ular to the motion of the lightning cells, on the other hand, would reduce the number of storms that just brush along the side of the normal warning area and then move on without becoming a real hazard. Once fast-moving storms have passed the airport, knowledge of the storm speed and direction of movement may also permit an earlier declaration of an “all clear” without compromising safety. Total Lightning Systems The NLDN has been designed to provide high-quality, high- collection-efficiency observations of CG lightning strikes. While these ground strikes are the specific hazard that 47

endangers airport workers, they only represent a small frac- tion of the total lightning in a storm. The majority of the lightning discharges stay within the cloud or strike adjacent clouds and are generally described as IC strikes. Measurement systems that can detect and locate both CG and IC lightning are termed total lightning systems. CG lightning strikes are predominantly vertically orientated and can be associated with a single geographical position, essentially their impact point. IC lightning, on the other hand, often extends in complicated patterns over long hori- zontal distances. The most sophisticated total lightning detection systems can track the full path of an IC stroke and, by combining the tracks of several successive strokes, can pro- duce two-dimensional coverage plots. Because there are many more IC lightning strokes than CG strokes, and since their positions can be mapped in a two-dimensional grid, they provide a valuable description of the overall extent of active lightning in a cloud system. Total lightning patterns can be monitored and tracked with more precision than can be done with CG strokes alone, and since IC strokes are gener- ally observed several minutes before the first CG strokes they may be able to be used to identify potential hazards in storms that are developing overhead before the first CG stroke is observed. Total lightning systems require special VHF sensors to track the IC strokes and are currently only available over a few regional areas where they are being tested. Because the IC lightning patterns identify areas that have already developed active charge separation processes and are actively producing lightning strikes, they represent a uniquely valuable enhance- ment to operational lightning warning systems. Integrated systems based on total lightning detection networks may be able to provide significantly improved lightning warnings, in terms of a better delineated hazard area and a reduction in total downtime for airport operations. While it is not yet clear to what extent total lightning systems will become available, or who will install, operate, and fund their operation, they may eventually provide significant im- provements for lightning detection and warning systems, as well as enhancing short-term weather forecasts for the entire terminal area. Predicting Lightning Hazards Mesoscale “nowcasting” systems are quite effective at iden- tifying the growth and motion of developing convective sys- tems and are used by the FAA for both terminal and en route air traffic management. These forecasting systems can also be used to identify developing storms that are likely to produce lightning. Airline operations are time-sensitive and have a very low tolerance for false alarms. Most lightning prediction products should therefore only be used to generate “advisory” products that call attention to the potential for storm development. Such an advisory would serve as a “heads-up” and not in itself call for a “stand down.” Predictive systems may be valuable for operational planning, but are not likely to replace or elim- inate the need for lightning-specific detection and warning systems. Making Use of Existing Data Integration Systems Integrating multiple data sets into a decision support system can be a difficult and expensive process. Data access and latency are particularly critical issues. One way to mini- mize these efforts and costs is to make use of existing data integration systems instead of developing new systems that process much the same information. Potentially valuable additions to airport lightning detec- tion and warning systems include meteorological radar data, cell identification and tracking algorithms, and observations from regional total lighting detection systems. Radar data and associated cell identification and tracking algorithms are fundamental to both the FAA-sponsored ITWS developed by Raytheon (33) and Vaisala’s WSDDM system that was developed at NCAR (34). Because both systems already include access to real-time NLDN lightning reports, it should be relatively straightforward to transfer spe- cific lightning warning algorithms to these existing opera- tional systems for easy access to their extended data sets and processing algorithms. The expanded weather systems, how- ever, would need to support additional communication links, lightning user displays, and integration of electric field mill data, as well as be able to trigger the needed alarms and noti- fication systems. An ITWS-based integration would also extend the government’s use of lightning data beyond the limits of the current U.S. contract with Vaisala and directly compete with Vaisala’s commercial lightning warning prod- ucts. The terms and conditions of the NLDN contract, how- ever, could be renegotiated when the contract comes up for renewal in 2010. On the other hand, customized products or output fields could be generated by ITWS, WSDDM, or other data inte- gration platforms for export to existing lightning warning systems, with the final integration being done there. In both cases, the technical challenges for these types of integration should not be too difficult, but the issues of data rights and the generation of customized products for use by other, separately funded systems could become a major impediment. Integrating aviation-related weather decision support systems into unified systems, however, should be the most efficient and cost-effective way to ensure a higher level of operational safety. 48

Additional Issues In reviewing the current state of airport lightning detection and warning systems, it was immediately evident that there are no common system standards, no certification or testing procedures for lightning detectors, and no general agreement as to who should provide these warning services at U.S. airports. While we are not in a position to make recommendations in these areas, they are important issues that limit our options for enhanced systems. Who Should Provide Lightning Warning Services? At present, there is no general agreement as to who should provide lightning warning services for airports. This is a dif- ficult issue that hinges on the relative roles of the government and private industry and that is complicated by potential lia- bility issues and the very significant cost of system installation and maintenance. In some cases, the largest or dominant airline at an airport will purchase a system or contract for lightning warning serv- ices, with the other carriers following their lead in deciding when to clear the ramp, but occasionally making a contrary decision on their own. In other cases, airports may maintain warning systems for their own use, but not share their infor- mation with tenant airlines and other users. FAA and other government agencies often have access to NLDN lightning data, sometimes on systems operated at the airport, but are prohibited by the terms of the government contract with the NLDN’s commercial operator to make the data available for nongovernment use. The result is an often inefficient delivery of lightning warnings, with hit-or-miss application of safety measures and great potential for duplication of services. Standardization With different organizations providing lightning warning services at different airports, it is not surprising that there is lit- tle or no standardization of procedures or of shut down and restart criteria. Individual operators make their own decisions, sometimes following their own established standards, and other times responding to a supervisor’s individual decision. Standardization is generally the result of regulations and mandatory procedures passed down from above or promul- gated by the agency providing the services. With no agreement as to who should provide these services, it is natural that there is no standardization to how the warnings are determined and what warning criteria are applied. A lack of standardiza- tion, however, permits individual operators to respond to their own needs and is often welcome. Certification While there are a large number of commercial lightning de- tection systems available, it is difficult to evaluate them since there is no certification process to assess their performance. For relatively low cost systems that only signal when light- ning is near, comparison with lightning detection and position information from the NLDN should be adequate to evaluate and document each system’s performance and limitations. Because these products are sold and advertised for the general consumer market, nonprofit organizations such as the Con- sumers Union might be willing to perform such tests. Performing an end-to-end evaluation of higher-end prod- ucts with sophisticated warning algorithms and workstation displays would be more difficult. Unfortunately, these are the systems that are typically used at large airports. There has not been a comprehensive comparison of the relative accuracy and detection efficiency of the two competing national light- ning detection networks. Performing such a test would require an independent detection capability to serve as ground-truth. In practice, validation studies have made use of photographic or video imagery from multiple viewing angles and by rocket triggered lightning strikes. This means that validation testing is a time-consuming, expensive effort, with each study con- centrating on a single geographical area. A number of such validation tests have been published in the refereed literature for the NLDN (17, 18), but not for the USPLN. Government laboratories such as the FAA’s William J. Hughes Technical Center, NOAA Laboratories, and NOAA’s university-based Cooperative Institutes could perform such tests, but these organizations are not general testing labora- tories. They are government-funded organizations that do applied research in response to the needs of their sponsors. From a government perspective, there has not been a reason to provide certification or testing since the government has not purchased these systems to provide lightning detection and warning services. Looking Toward the Future Next Generation Air Transportation System (NextGen) Planning is currently underway to modernize and upgrade the U.S. air transportation systems to meet the needs of the 21st century. The demand for air traffic services is expected to double or triple by 2025. Planning for the Next Generation Air Transportation System (NextGen) is the responsibility of the Joint Planning and Development Office (JPDO), com- posed of representatives from the FAA, NASA, Department of Transportation, Department of Commerce, Department of Defense (DoD), Department of Homeland Security, and the White House Office of Science and Technology Policy. 49

NextGen will require a systemwide transformation, which is expected to be completed in 2025, with initial system enhancements beginning to come on line by 2012. Weather information and weather observations are crucial to NextGen, and the required upgrades will impact all elements of our aviation weather system (35). To achieve NextGen goals, all aspects of the aviation system, including airport and ramp operations, will be tightly inte- grated to provide a shared awareness of all aspects of the system for joint planning and system management (36). Weather information will be fully integrated in the NextGen environment, with observational data and forecast products available from a single authoritative source and distributed through a network-enabled weather information sharing system. At the core of this capability will be a virtual four- dimensional database formed by expert system fusion of var- ious gridded fields, model output, statistical systems, climate information, observations, and human forecaster input (37). These anticipated changes may well become a vehicle for more standardization of weather products, including lightning detection and warning systems. JPDO planning, however, is still in its early stages, and the specific details of the new pro- cedures and policies, and how they will be implemented, will take time to be resolved. The concept of a “single authoritative source” for weather information suggests increased central- ization of weather observations and dissemination of weather information. The “four-dimensional weather information database,” will, however, be a virtual database and not neces- sarily mean a single information provider. The database concept also includes provisions for restricted or classified information for DoD users, as well as ways to include propri- etary commercial products. This will take time to sort out, but NextGen clearly has a potential to change the way ramp operations are managed and the way lightning detection and warning services are provided to all users of the air trans- portation system. GOES-R Geostationary Lightning Mapper Sometime after 2014, the United States will launch the first of its new generation of geostationary meteorological satellites—the GOES-R series of spacecraft. These new satel- lite systems will, for the first time, include a GLM. The GLM is an optical total lightning detector that can detect and locate lightning strokes over most of the visible earth disk with very high efficiency (38). The GLM will provide real-time lightning information to ground users. While the details of the dissemination system are still being developed, it is likely that the GLM will provide information on the location and extent of lightning discharges, including two-dimensional flash density products. From geo- stationary orbit, the instrument is expected to provide gridded data sets with a grid size of about 8 km. This is significantly coarser than the density mappings that can be provided by surface-based total lightning detection systems, but will be provided at no cost to the user and with relatively uniform resolution coverage over CONUS lightning activity areas. Prior to launch, there will be a number of efforts to use currently available regional ground-based total lightning networks to test the potential application of this new satellite-based data source at airports. Summary and Recommendations From the safety perspective, currently available lightning detection and warning systems seem to be meeting airport and aviation industry needs. There are, however, a number of potential options for enhancing and improving the current systems to reduce the number and duration of ramp closures and to improve operational efficiency, including the following: • Refining the warning algorithms and criteria through the use of self-monitoring software. While this approach is not necessarily guaranteed to shorten ramp closures, it would provide an objective standard for selecting warning crite- ria to balance safety and efficiency. • Additional meteorological data sets, primarily meteoro- logical radar data, can be used to better define the spatial and temporal limits of the lightning hazard. Using inte- grated data sets to define the geometrical extent of the lightning cells and then tracking their evolution and move- ment should be particularly valuable. • Most specifically, lightning cell tracking and echo move- ment vectors may also be used to adjust the warning criteria to minimize the number of short-duration ramp closures triggered by storms that are not likely to impact the airport area. • Recent demonstrations and tests of total lightning systems are showing great promise for enhancing and refining lightning warnings. Limited regional total lightning net- works are currently available for experimentation, but rou- tine availability of these systems for operational use is still years away. • Given the economic pressure on the aviation industry to reduce costs, enhancements in lightning detection and warning systems will need to be critically reviewed to de- termine their cost effectiveness. Software enhancements and optimization of warning criteria should be a relatively low cost system enhancement, but would have to be im- plemented by lightning warning system vendors. The costs of integrating radar and other meteorological observations may be able to be minimized by making use of existing data integration platforms, such as ITWS and WSDDM, or by moving the processing to regional or national analysis 50

centers (either governmental or commercial), and then transmitting only the information needed for the local airport display systems and warning decisions to each individual airport system. Another way to realize signifi- cant cost savings would be to develop new technologies and new algorithms to detect and monitor thunderstorms that develop over the airport, and then minimize or elim- inate the use of EFMs as an essential component in airport lighting warning systems. NextGen and, to a lesser extent, lightning observations from the next generation of geostationary weather satellites may eventually provide enhanced capabilities or increased federal support, but they cannot be counted on in the near future. Those developments are too far off to influence near- term operational decisions, but should be monitored for future potential. For current planning purposes, airports and airlines will need to depend on commercial vendors and cur- rent technology. We recommend that industry trade groups such as the American Association of Airport Executives, Airports Council International–North America, and Air Transport Association encourage, on a voluntary basis, the routine collection and reporting of ramp closure statistics and associated lightning- related injuries and material damage. There are a number of important follow-on studies that will be needed to further the advancement of improved lightning warning systems for airports. Of particular importance is the consideration and evaluation of remote sensing observations, most likely meteorological radars and total lightning systems, as replacements for EFMs in operational lightning detection and warning systems. Warning systems based exclusively on routine surface observations, numerical models, and remote sensing may be able to remove any need for lightning-specific detection hardware to be installed or maintained at individ- ual airports. If successful, this transformation should result in lower costs to airports and airlines, while preserving or improving lightning hazard identification. Airport-specific studies should also be directed at evaluating the performance of currently available lightning systems, optimizing the warn- ing criteria for these systems, and quantifying the potential tradeoff between safety and efficiency. Another topic for additional research and evaluation can address ramp lightning facility mitigation strategies. For example, it may be possible to design a facilities mitigation concept, where ramp workers could safely unload baggage during a lightning event. A program could be developed where a set of ramp mitigation ideas would be collected via survey and analyzed. A cost/benefit analysis could then be de- veloped so each airport could calculate the potential utility of introducing various ramp mitigation strategies based on their individual circumstances. To the extent possible, we urge airports and airlines that operate lightning detection and warning systems to collabo- rate with research efforts designed to test or enhance warning products by granting researchers access to monitor the per- formance of their installed operational systems and observe ramp operations. Lightning is but one of many weather factors causing economic loss for the airlines. It would seem appropriate to conduct a follow-on study to analyze all weather factors affecting airline delays, such as, high winds, heavy rains, snow, ice, and fog. This analysis would employ a different economic approach than used for lightning-caused delays and enable a focus on air traffic flow delays, with ramp closings as a secondary impact. 51