1. Introduction
National parks in the United States are visited by people from all over the world. Visits to National Park Service facilities reached 318 million in 2018. Natural features such as mountains, canyons, waterfalls, geothermal basins, lakes, and rivers are the core destinations for most parks. Therefore, these locations are viewed outdoors, usually during daylight hours, and often during the warmest months of the year. Since these are similar to the times of peak lightning occurrence, there is a potential for people to become victims of lightning. Over the last 27 years, there are known to have been 12 fatalities and 39 injuries in Rocky Mountain National Park and 5 deaths and 26 injuries in Grand Canyon National Park, as well as less frequent events in other parks.
Climatologies of cloud-to-ground (CG) lightning flashes over the contiguous United States (CONUS) have been published by time of day (Holle 2014) and time of year (Holle et al. 2016). While regional lightning climatologies have been published within the United States as summarized in Holle et al. (2016), the local lightning scenario within each national park is not readily determinable from these resources. For this reason, the present study focuses on the large concentration of people outdoors at national parks during daylight summer hours in the United States.
During recent years, population-weighted lightning death rates have tended to be greatest in 1) the southeastern states where there is the largest concentration of lightning and 2) in western states (Holle 2016). The maximum in the lightning fatality rate from Arizona northward to Montana has been attributed to low flash incidence leading to a perception of low risk, along with less intense rain at the time of lightning (Hodanish and Zajac 2002; Hodanish et al. 2004; Hodanish 2005, 2012; Hodanish et al. 2015; Holle et al. 1993). There is a tendency for human injury from lightning to occur during daytime in summer, but such data for national parks are not currently available.
This study provides the first comprehensive summary of the incidence of CG lightning and a partial overview of lightning-related casualties in national parks in the CONUS. Casualties in this study refer to combined fatalities and injuries. The study proceeds as follows. First, an overview of the CG flash and stroke frequency, as well as mean areal density will be presented for the 46 parks in the contiguous United States with areas exceeding 100 km2. This will be followed by a more detailed examination of the time and location of CGs within the six most-visited parks: Great Smoky Mountains, Grand Canyon, Rocky Mountain, Zion, Yosemite, and Yellowstone National Parks. The paper continues with a summary of known lightning casualties within these six and several other national parks, primarily Grand Canyon and Rocky Mountain, and relate them to lightning occurrence. The study concludes with calculation for each park of lighting fatality risk, computed as the product of areal visitor and flash densities on an annual basis.
2. Data and methods
a. NLDN data
CG flash and stroke data are from the National Lightning Detection Network (NLDN) from 1999 through 2018. The NLDN reports both CG flashes and CG strokes, as well as a portion of cloud pulses within both CG flashes and cloud flashes that are not examined here. A CG flash has one or more return strokes; the average is about four CG strokes per CG flash; most ground contact separations are within a few kilometers (Rakov 2016). Between 35% and 51% of CG flashes have multiple ground stroke locations (Rakov 2016, chapter 4; Valine and Krider 2002). The estimated detection efficiency (DE) of CG flashes over the CONUS was 90%–95% from 2003 through 2012 (Cummins et al. 2006; Cummins and Murphy 2009) and most recently is 95% or higher following a networkwide upgrade in 2013 (Murphy and Nag 2015). The DE of individual CG strokes from 2009 to 2018 is estimated as 76% for the NLDN during the period of study (Mallick et al. 2014; Nag et al. 2015). For each CG flash and stroke, the latitude, longitude, time, signal strength, and polarity were collected within the national park boundaries. NLDN reports with positive peak currents <15 kA have been excluded because of their tendency to be cloud pulses (Cummins and Murphy 2009). Additional recent studies supporting these results of NLDN performance are in Koshak et al. (2015) and Medici et al. (2017). Definitions and context for these lightning performance measures are provided in Nag et al. (2015). NLDN flash data were also shown in the studies of diurnal (Holle 2014) and seasonal (Holle et al. 2016) variations over the CONUS.
Three-dimensional animated perspectives are also provided for each of the six most-visited parks in the online supplemental material. CG stroke data were accumulated in 30-arc-s grids (approximately 800 m), draped over smoothed terrain, and then illuminated from above to provide a clearer perception of terrain gradient direction and complexity. The terrain information was derived from the Digital Terrain Elevation Data (DTED) developed by the National Imagery and Mapping Agency (NIMA). For DTED, the elevation is given in meters above mean sea level (MSL). Its accuracy is ±50 m in the horizontal plane and ±30 m in the vertical direction. The horizontal latitude–longitude resolution of the dataset is 30 arc s, matching the lightning density resolution.
b. Fatalities and injuries
At the present time, there is no unified database of lightning casualties in national parks. For this study, the best summary of lightning casualty data is for Rocky Mountain National Park, where data were provided by local staff from park archives in mid-1992. Additional Rocky Mountain cases were gathered from studies of individual events by Hodanish (2005, 2012) and Hodanish et al. (2015). Grazulis (1997) includes some national park lightning events, and Ghiglieri and Myers (2001) include several Grand Canyon incidents within the canyon.
Additional lightning casualty events were collected with variable success by online search engines in the last two decades for all other parks. In these reports, the location is likely to be accurate with respect to the park location, the year and month are correct, the number of deaths and injuries are usually accurate, age and gender are often reported and likely to be correct when they are stated, and the location and activity within the parks are often provided. While these media reports are not likely to be complete for all parks in the United States, they provide a useful indication of whether the lightning occurrence climatologies match the victims’ scenarios. The monthly National Weather Service publication Storm Data is available for online searches since 1992 and was reviewed for the six major parks in this study; however, this resource has not been scanned manually from its inception in 1959 through 1991.
3. Lightning density within the 46 CONUS national parks
There are 46 national parks within the CONUS with areas larger than 100 km2. The two smallest parks, Gateway Arch and Hot Springs, are not included since the small areas do not provide stable lightning frequency estimates. Table 1 alphabetically lists the area, 2018 attendance, annualized number of CG flashes and strokes, and CG flash and stroke densities reported by the NLDN. In terms of CG flash density, Everglades National Park has the largest value of 6.10 CG flashes per kilometer squared, while the smallest is in California’s Pinnacles National Park where an average of two CG flashes per year were detected within the 20-yr period for the entire park. Inspection of Table 1 shows that the stroke density ranking is similar to CG flashes but is not quite the same because of differing ratios of strokes per flash. (Areas are from the National Park Service at https://www.nps.gov/. Visitor numbers are available for 2018 at https://irma.nps.gov/DataStore/DownloadFile/620857.)
National parks and their corresponding state(s) and areas; 2018 attendance rank and number of visitors; average CG flashes per year, CG flash density (km−2 yr−1), CG flash density rank; CG strokes per year and CG stroke density (km−2 yr−1) for the 46 national parks in the CONUS with areas >100 km2 from 1999 through 2018.
The park locations in Table 1 have been overlaid in Fig. 1 onto the NLDN flash density map from 1999 to 2018. The ranking of flash densities within the national parks matches the mapped NLDN pattern. The top 10 CG flash densities within national parks are in Florida (1, 4), South Carolina (2), Kentucky (3), southern Arizona (5), New Mexico (6), west Texas (7), South Dakota (8), Ohio (9), and southwest Colorado (10). Most notable is that many of the national parks are in Intermountain West and West Coast states where there are low to extremely low CG flash densities.
CG flash density on a 2-km grid from the NLDN for 1999–2018 overlaid with the ranking (color circles) by CG flash density of the 46 CONUS national parks larger than 100 km2 from Table 1. The scale for the national map is in the lower right.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
4. Lightning within the six most-visited national parks
The lightning distributions and topography within each park exhibit important small-scale variations such that it is useful to examine the most-visited parks in more detail, in order of their 2018 attendance ranking. All six of these popular parks have notable elevation changes within them.
a. Great Smoky Mountains
This is the most-visited national park, where 11 421 200 people arrived in 2018. Figure 2 shows the format of the data to be presented for six parks. The elevation map for the east–west-oriented Great Smoky Mountains (Fig. 2a) illustrates how the park is divided by a ridge that separates Tennessee to the northwest and North Carolina to the southeast. Elevation within the park ranges from 267 to 2025 m. CG flash density averages 2.37 km−2 yr−1 within the park boundary (11th of the 46 parks in this study) and CG stroke density averages 5.32 km−2 yr−1 (Table 1). Figure 2b indicates that many more CG flashes occur on the north and west sides than to the south and east. This distribution is part of a larger regional pattern of smaller lightning densities over all of the Appalachian Mountains that is apparent in the national NLDN map in Fig. 1. A point plot of each CG stroke from 2009 to 2018 in Fig. 2c overlaid on a topographic representation also indicates the maximum on the north and west sides; a 10-yr period is used so that individual strokes can be seen separately. Major park and natural features are shown for reference in Fig. 2d. The three-dimensional Animation S1 in the online supplemental material also shows a dominance of lightning over the higher peaks in the park itself. The complementary expanded regional view in Animation S1 depicts larger CG flash densities at both lower and higher elevations in nearly all directions away from the park. The pink dot and bar in Animation S1 indicate Cades Cove. The only known Great Smoky Mountains casualty incident was when two people were injured near vehicles at a campground above 1525 m in the east-central portion of the park during a Saturday afternoon in June.
(a) Great Smoky Mountains National Park elevation map, (b) 1999–2018 CG flash density on a 2-km grid, (c) 2009–18 point plot of CG strokes overlaid on terrain, and (d) map of significant features and locations within the park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
b. Grand Canyon
A total of 6 380 495 people came to this second most-visited national park in 2018. The primary location of interest is the long, narrow, and very deeply incised canyon containing the Colorado River that flows downstream from east to west across much of northern Arizona. Elevation within the park ranges from 732 to 2794 m (Fig. 3a). CG flash density averages 1.57 km−2 yr−1 within the park boundary (19th of all parks) and CG stroke density averages 3.91 km−2 yr−1 (Table 1). CG flashes are most frequent over the higher elevations to the south and north of the canyon; considerably less lightning is apparent within the canyon itself (Fig. 3b). The point plot of CG strokes in Fig. 3c also shows the very large incidence on the higher elevations, and sharp reductions inside deep canyons that appear as part of a narrow rootlike structure in this image. Figure 3d shows the location of the river and other features. Animation S2 in the online supplemental material also illustrates how lightning tends to occur along the rims, especially in the eastern portion of the park along the South Rim. There is a strong impact of terrain on lightning attachment, even within the canyon. The animation also illustrates the concentration of lightning along the rim’s edge near the visitor center (pink dot and bar), the in-canyon minimum, and the tendency for greatest lightning activity to correlate well with highest elevations as shown by the elevation map in Fig. 3a.
As in Fig. 2, but for Grand Canyon National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
Information on 5 lightning deaths and 26 injuries in 13 events since 1987 has been collected from newspapers, online media reports, Grazulis (1997), and Ghiglieri and Myers (2001) for within the canyon (Fig. 4). Although this is not a complete summary of lightning-related events since Grand Canyon National Park was established in 1919, general information is provided by this collection of reports. From 2009 to 2018, 14 people were killed by lightning in all of Arizona so that the Grand Canyon total of five deaths from 1987 to 2018 contributes about one death per six years. Arizona is ranked number 33 in the United States in CG flashes during this decade, so it is not an especially high lightning incidence state. However, lightning incidence in Arizona is much greater in some areas (Fig. 1), and the Grand Canyon is one such region. All casualty events took place from May through October; July and August accounted for 54% of the incidents. All events with time identified took place between 1200 and 2115 LST. The same number of males as females was affected, ages 40 to 49 were the most frequent decadal age group, and 77% of the reported origins of people were from outside Arizona. Events were spread widely throughout the week. In terms of activity, viewing the canyon from the rim was most common (6), while photographing (2), hiking (2), and guiding river rafters (1) were the others with a known activity. In terms of location, the South Rim was most frequent (8), below the rim in the canyon was next (3), and one event each was on the North Rim and the Colorado River. Viewing the canyon from the rims is the most common activity, especially on the South Rim where a large concentration of people views this natural feature. However, the South Rim is also where the large CG flash and stroke densities are located (Figs. 3 and 4). The coincidence of large visitor concentrations and large flash densities on a park-by-park basis is explored in section 5b.
Locations of known lightning casualty events within Grand Canyon National Park overlain on the point plot of CG strokes and terrain from Fig. 3c. Triangles include the number of events at each location.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
c. Rocky Mountain
Rocky Mountain was the third most-visited national park in 2018 and had an attendance of 4 590 493 people. This park in Colorado is located at high altitudes over and near the Continental Divide; elevation within the park ranges from 2396 to 4346 m (Fig. 5a). CG flashes average 1.59 km−2 yr−1 within the park boundary (18th of all parks) and CG stroke density averages 4.10 km−2 yr−1 (Table 1). Flashes are more frequent on the east side than over the highest terrain and to the west (Fig. 5b), although there are local maxima in density along the larger east-facing slopes. This pattern is part of the larger-scale west-to-east increase across Colorado in Fig. 1. The point plot of CG strokes in Fig. 5c also depicts more lightning to the east of the highest elevations. The often-visited mountains Longs Peak and Twin Sisters are identified in Fig. 5d. The regional Animation S3 in the online supplemental material expands on how lightning tends to occur over the highest-elevation peaks in the southeast portion of the park, and also shows additional larger and stronger flash density maxima over the high terrain to the south and minima within the deeper canyons. The pink dot and bar in the animation are at Estes Park as in Fig. 5d.
As in Fig. 2, but for Rocky Mountain National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
Information on all lightning casualties from 1915 to 1992 was provided from park archives by staff at Rocky Mountain National Park in mid-1992; an update from the park has not been located. There were seven single-fatality events between 1922 and 1992, and six injuries during five events that were between 1982 and 1988. An additional 14 events since mid-1992 have been collected from newspaper and online media reports, nevertheless, this list may not be complete. Locations of 25 events within the park are plotted in Fig. 6; one location was not known. Several events have received detailed analyses of lightning occurrence relative to the casualties within Rocky Mountain by Hodanish (2012) and Hodanish et al. (2015). A total of 12 fatalities and 39 injuries were identified in these 26 incidents since 1915. For all of Colorado from 2009 to 2018, 10 people were killed by lightning so that the Rocky Mountain total of six deaths from 1992 to 2018 contributes about one death per four or five years to the state total. Colorado is ranked rather low at 32 in the United States in CG flashes during this decade. All events took place from May through October; July and August accounted for 83% of them. One event took place at 1120 LST, and the rest were between 1200 and 1900. Males accounted for 61% of the deaths and injuries, ages 30 to 39 were the most frequent decadal age group, and 62% of the people were from outside Colorado. There were slightly more weekend events when compared with other days of the week. Figure 6 shows the locations reported for 25 of the events; the most frequent are in the vicinity of Trail Ridge Road (9), Longs Peak (5), Hallett Peak (3), and Twin Sisters Peak (2). Climbing (5) and hiking (4) accounted for all but two of the reported activities. Eight of the 13 people with reported origins were from outside Colorado.
As in Fig. 4, but for Rocky Mountain National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
d. Zion
In 2018, Zion was the fourth most-visited national park and had an attendance of 4 320 033. The most popular feature in this relatively small Utah park is a narrow canyon that opens to the south; elevation within the park ranges from 1116 to 2660 m (Fig. 7a). More CG flashes occur over the higher terrain to the north and east, and less to the south and west (Fig. 7b). They average 1.19 km−2 yr−1 within the park boundary (22nd of all parks) and CG stroke density averages 2.84 km−2 yr−1 (Table 1). The CG stroke point plot (Fig. 7c) shows how lightning clusters somewhat over the higher terrain. The largest concentration of visitors is near Zion Canyon that is reached from the main visitor center on the south border of the park (Fig. 7d). No record of a casualty has been found in the available data for Zion.
As in Fig. 2, but for Zion National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
The regional Animation S4 in the online supplemental material emphasizes a CG flash minimum in the narrow canyon on the south side and frequent lightning over mountains, especially to the west of the park. Note the large lightning frequency over a terrain feature just inside the southern entrance to the park where visitors often congregate. The dark red region near a peak to the northeast of the park is the result of lightning strokes to tall radio towers, as described in Kingfield et al. (2017) and the references therein. The pink dot and bar in the animation are at Castle Dome as in Fig. 7d.
e. Yellowstone
The fifth most-visited national park, Yellowstone, had an attendance of 4 115 000 in 2018. Geothermal features, canyons, and wildlife are spread across this large high-altitude park in Wyoming, Idaho, and Montana. The highest elevations are on the eastern border; altitudes within the park range from 1610 to 3462 m (Fig. 8a). The relatively small overall incidence of CG flashes increases to the northeast and tends to be at its minimum in the south-central regions of the park (Fig. 8b). They average only 0.64 km−2 yr−1 within the park boundary (29th of all parks), and CG stroke density averages 1.59 km−2 yr−1 (Table 1). The stroke point plot (Fig. 8c) shows uniform but mostly infrequent lightning occurrence except to the east over high terrain along the park’s border. Visitors often gather at Old Faithful geyser in the center of the park but are dispersed across the area due to variety of sites to visit (Fig. 8d). The regional Animation S5 in the online supplemental material shows somewhat more lightning over higher peaks in all directions around the park except to the west; the pink dot and bar in the animation are at Old Faithful.
As in Fig. 2, but for Yellowstone National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
Three casualty events for Yellowstone include two at Old Faithful when 11 were injured in one event and nine at another; both cases were during the afternoon in June. An additional event occurred when five campers were injured near Yellowstone Lake in the early evening in August.
f. Yosemite
Yosemite was the sixth most-visited national park in 2018 and had an attendance of 4 009 436. Numerous lakes, rivers, and canyons prevail at an elevation within this California park ranging from 642 to 3997 m; high mountains form the eastern border (Fig. 9a). CG flashes are very infrequent, and average only 0.37 km−2 yr−1 within the park boundary (33rd of all parks), while CG stroke density averages 0.76 km−2 yr−1 (Table 1). The density and point plots (Figs. 9b,c) show how most of the lightning is toward the east side, and minimal activity is to the west. People tend to concentrate in the vicinity of the visitor center and Half Dome (Fig. 9d) with low lightning incidence other than slight increases in lightning occurrence over higher elevations. The regional Animation S6 in the online supplemental material illustrates larger flash densities over some mountain peaks a short distance outside the park; the pink dot and bar in the animation are at the Yosemite Valley visitor center. One known Yosemite casualty event involved four injuries while climbing Cathedral Peak on a June afternoon; the peak is near Tuolumne Meadows located in Fig. 9d.
As in Fig. 2, but for Yosemite National Park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
g. Monthly variations
The variation of CG flashes through the year for these six parks in Fig. 10 shows a dominance during late spring, summer, and early autumn months, with maxima in July or August. Great Smoky Mountains has the broadest distribution from March through September, and Rocky Mountain and Yellowstone are also widely spread from May into October. In contrast, the western parks of Grand Canyon, Zion, and Yosemite have little lightning before the start of July when the southwest monsoon (Adams and Comrie 1997; Holle and Murphy 2015) starts abruptly in most years, then it diminishes before ending in September (Holle et al. 2016). The maximum in the warmer months is consistent with the annual cycle of heating of the land, resulting in low-level moisture carried in updrafts that reach altitudes colder than the freezing level to initiate lightning (MacGorman et al. 2007). Among the six most-visited parks, the most pronounced annual cycle is the July single-month peak in Yosemite that comprises 46% of the year’s CG flashes. The Grand Canyon (85%) and Zion (84%) have most of the lightning activity concentrated during the three months of July through September.
Monthly variations of CG flashes within the six most-visited national parks from 1999 to 2018.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
h. Diurnal variations
Hourly variations of cloud-to-ground lightning flashes in Fig. 11 show a primary maximum in the afternoon to early evening. Great Smoky Mountains, Grand Canyon, and Zion have some CG flashes in the morning before 1000 LST while the rest are nearly lightning-free until 1000. All of the parks except Yosemite have lightning lingering after 1800 LST, as in Holle (2014; his Fig. 2). The afternoon maximum is consistent with heated land during the day producing updrafts that reach the altitudes where lightning is formed over mountainous terrain (Whiteman 2000; MacGorman et al. 2007). All of the parks have nearly two-thirds, or more, of their CGs between late morning and early evening. Most pronounced is the 8-h daytime period in Yosemite that accounts for 90% of the day’s CG flashes, and a 9-h period in Rocky Mountain that has 94% of the day’s lightning.
Hourly variations of CG flashes within the six most-visited national parks from 1999 to 2018.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
i. Local terrain effects
The very localized effects of mountainous terrain on lightning occurrence are evident in Animations S1–S6 in the online supplemental material. Cummins (2012) examined local variations in lightning relative to rugged terrain at two locations in the CONUS and emphasized that mountain slopes oriented toward the direction from which warm moist air arrives will enhance lightning occurrence. Such a preferred orientation for increased thunderstorm formation was shown for northeastern Colorado by Toth and Johnson (1985) and eastern Colorado by López and Holle (1986). Vogt and Hodanish (2014, 2016) and Hodanish et al. (2019) describe NLDN-derived 1 km × 1 km lightning grids for Colorado and several examples of these local variations across the state. To date, however, in-depth investigations of localized topographic effects have not been made for national parks other than the six in this study.
j. Grand Teton and Everglades: Special cases
Grand Teton National Park ranks number eight in visitors in 2018 and had five casualty events, so it is examined separately in Fig. 12. A total of two people were killed and 25 injured in four climbing events on Grand Teton Mountain and one on Table Mountain. These peaks are in locations with larger lightning frequencies than at many other lower elevations within the park. All but one occurred during July, and all but one was on a weekday. Two were prior to 1200 LST, and three were in the afternoon; one of the rescues involved the efforts of 83 people.
Locations of known lightning casualty events within Grand Teton National Park overlain on point plot of CG strokes and terrain. Triangles include number of events at each location.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
Everglades National Park has the largest flash and stroke densities of all of the 46 parks in Table 1. To illustrate this situation, Fig. 13 plots every 20th CG stroke using the same plotting size as in previous figures (a plot of every stroke made the map solid red). The most frequent lightning is over the interior of the park and somewhat less over the surrounding water areas; nevertheless, the density is large everywhere. Although Everglades has the 28th most visitors and a large amount of lightning, no record of a casualty has been found in the available data.
Everglades National Park map with 2009–18 point plot of every 20th CG stroke and two visitor centers within the park.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
5. Discussion
a. Summary of casualties
In addition to casualty reports mentioned earlier, newspaper stories and online media reports identified events in Bryce Canyon (3), Carlsbad Caverns (1), Gateway Arch (1), and Glacier (1) National Parks. Note that Gateway Arch is one of the very small parks not included in this study. In addition, the Bryce website mentions four deaths and six injuries in the last 23 years. In summary, information was available for 22 lightning deaths and 128 injuries in 54 known incidents in national parks. The three meteorological summer months of June, July, and August accounted for 87% (Fig. 14a) of the events and 1200 to 1800 LST accounted for 70% of the events (Fig. 14b). Of particular note is the sharp reduction in casualties after 1800 LST (Fig. 14b) despite the continuation of lightning into the evening in most parks (Fig. 11). This discrepancy could be attributed to people deciding to finish their outdoor daylight activities earlier than the lightning that decreases more gradually after sunset.
(a) Monthly and (b) hourly variations of known lightning casualty events within all national parks.
Citation: Weather, Climate, and Society 13, 3; 10.1175/WCAS-D-19-0155.1
Combining all known park casualties, the following results apply:
Gender: 63% were males, similar to many other locations (Cooper and Holle 2018). The prevalence of more risk taking by males in nonagricultural situations matches other studies of recreation (Badoux et al. 2016; Cooper and Holle 2018; Holle 2005; Jensenius 2016). In the national parks, though, the 63% male ratio may be somewhat lower because of the nature of families and groups visiting these parks on leisure time.
Age: The ages from 20 to 49 accounted for 66% of all casualties, and most commonly from 30 to 39. The age is spread more widely into the 30 and 40s, which is somewhat older than the U.S. average for recreation (Holle 2005).
Location: Mountains and rock formations (19) were the most frequent, followed by Grand Canyon rims (12), in the vicinity of Trail Ridge Road in Rocky Mountain (8), hiking trails (4), below the rims in Grand Canyon (3), and Old Faithful Geyser in Yellowstone (2). Note that casualty locations often do not coincide with the largest flash densities. Instead, most important is the concentration of people at specific locations in the local environment, often at isolated high peaks, canyon rim overlooks, and high-elevation roads.
Activity: Climbing mountains (12 incidents) was the most common, followed by hiking (9), and viewing scenery from an overlook (8).
Day of week: There was no tendency for more weekend events when compared with other days of the week.
Origin of visitors: Most people were from outside the state (78%) where the parks are located. Of particular note is that most people were visiting the parks from elsewhere, sometimes from outside the United States, such that people may not have been especially aware of the local lightning risk or decided to move forward with their long-planned visit despite the possibility of lightning.
Lightning-related deaths and injuries within National Parks concentrate in summer afternoons. The events are not more prevalent on weekends, indicating that people traveled some distance away from home. Exposure to lightning on mountain peaks, high-elevation roads, and isolated canyon rim overlooks comprised a majority of the casualty events. However, many of these locations are not where lightning occurs most often. Instead, these are where large numbers of visitors are engaged in lightning-vulnerable activities. Being outdoors away from the safety of a large substantial building or fully enclosed metal-topped vehicle completely compromises lightning safety (Cooper and Holle 2018). In addition, being outdoors at the time of year and time of day when lightning most often occurs is risky. Moreover, being in the open at high elevations or along sharply defined canyon rims is an especially critical factor when lightning is in the vicinity or about to occur.
b. Lightning fatality risk
Since the amount of lightning is not the sole determinant of casualties, another approach is explored here. Table 2 shows the lightning fatality risk that results from multiplying visitor density and the park-average CG flash density, as used in a study of U.S. lightning fatalities by Roeder et al. (2015). The units are visitor flashes per kilometer to the fourth power per year. Lightning fatality risk combines the number of people exposed to the hazard and the amount of lightning. When related to fatalities, Roeder et al. (2015) found that lightning fatality risk showed skill in identifying regions with the most reported deaths in the United States. Table 2 lists the parks by CG flash density rank with the following extreme values:
Lightning fatality risk is > 10 000 at Cuyahoga Valley (41 902), Bryce Canyon (31 414), Great Smoky Mountains (12 805), Wind Cave (12 744), and Mammoth Cave (10 958). Most of these are small parks with large visitor densities. Note that Bryce Canyon within this group had three casualty events, as mentioned in section 5b.
Lightning fatality risk is <100 in North Cascades (2), Isle Royale (6), Channel Islands (7), Olympic (25), Redwood (26), Death Valley (36), and Pinnacles (41). These parks have very low flash densities or small visitor counts, or both.
National parks ordered by CG flash density rank; 2018 number of visitors, park area and visitor density (km−2); CG flash density (km−2) and its rank; lightning fatality risk (visitor flashes km−4 yr−1), and number of casualty incidents (U = unknown) for the 46 national parks in the CONUS with areas >100 km2 from 1999 through 2018.
This study started with the six most-visited parks. While Great Smoky Mountains has a large lightning fatality risk, none of the other five most-visited parks rank near the top in risk. Yet there are numerous casualty events in Grand Canyon and Rocky Mountain. It can be concluded that the value of this parameter is diminished when examined only on a parkwide basis. Instead, the risk is locally greater where large numbers of people are congregated despite not being a location with an extremely large lightning density. This local risk applies on Grand Canyon rims and mountain peaks in Rocky Mountain and Grand Teton that are always of concern. These areas need to be emphasized although the entire park may not have a large lightning fatality risk. From a practical point of view, lightning safety advice can be minimized at parks with very low values of parkwide lightning fatality risk.
6. Summary and conclusions
A summary of CG flashes has been presented for the 46 CONUS national parks with areas >100 km2. The parks were ranked according to CG flashes per square kilometer per year, and the ranking was plotted on an NLDN map from 1999 to 2018. Most of the national parks are in western states where lightning density is lower than in other regions of the CONUS.
Focus was made on the six most-visited parks in 2018. Maps of CG flash density within the parks varied due to both local topographic and larger-scale regional patterns. Most CG flashes occur between May and October; the parks west of the Continental Divide often have most of the lightning activity after the arrival of the southwest monsoon in early July. During the course of the day, most lightning was concentrated from 1200 to 1800 LST, although some lingers after sunset.
Known lightning deaths and injuries within the parks usually occurred in the summer months during the afternoon when lightning occurs most often. The locations of these incidents were typically in high-risk local environments such as on mountains and at overlooks along roads and canyon rims, although they often are not locations with locally high incidences of CG flash density. Casualties were somewhat more often male than female, and ages were spread widely. Most lightning casualties were from outside the state where the park is located.
Lightning fatality risk was calculated for each park based on areal visitor and CG flash densities. Very small parkwide lightning fatality risk values indicate parks where lightning awareness efforts can be minimized. Other parks with moderate risk may contain areas of high visitor concentration that need special attention with regard to warnings of vulnerability. Some parks have a very high risk due to large visitor or flash densities or both. However, many of the casualty events were not located in parks with overall high lightning fatality risk. As a result, safety advice should focus on sites where lightning-vulnerable activities are engaged by a large number of visitors such as on frequently visited canyon rims, mountains, and high-altitude roads, regardless of low to moderate parkwide values of lightning fatality risk.
Acknowledgments
The authors are very grateful to Kelly Gassert of Vaisala, Inc., in Tucson, Arizona, for revising many figures and developing new figures for the revision. The comments of three reviewers improved the paper substantially, and they are thanked for identifying how to make the study more complete.
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