Lightning Occurrence in the Six Most-Visited U.S. National Parks

Large numbers of visitors come to the National Parks in the United States from all over the world. The attendance ranges from over 11 million in the Great Smoky Mountains, to more than 6 million in Grand Canyon, and over four million per year in Yosemite, Rocky Mountain, and Zion National Parks. For much of the time, visitors come to view natural features outdoors in a wide range of activities. The juxtaposition of lightning and large numbers of people in the open during the lightning season necessarily results in an enhanced risk of lightning exposure. This study will provide a general overview of the time of year, time of day, and location of cloud-to-ground lightning strokes in each of these five Parks based on data from the National Lightning Detection Network. Note that other National Parks may have greater lightning frequencies; this summary focuses on the five most-visited locations.

The average annual number of NLDN-detected cloud-to-ground strokes ranges from 938 within the boundaries of Yosemite to 15,854 within Grand Canyon National Park; large interannual variability is apparent. By time of year, the three western parks have a concentrated lightning season, dominated by the Southwest Monsoon. Grand Canyon has 91% of its cloud-to-ground lightning between 01 July and 15 September; Zion and Yosemite have similar concentrated summer periods. Rocky Mountain and Great Smoky National Parks have more disperse lightning occurrence from May through September. By time of day, these Parks have most of their lightning from as early as 1000 LST through 1900 LST, although some strokes linger into the evening. In terms of location, each park has a different but distinctive pattern of lightning that depends on local and regional topography. As a result, it is possible to specify that at the Grand Canyon, for example, lightning exposure is greatest from noon to 1900 LST from 01 July to 15 September, especially on the rims and higher-altitude mesas. Such information can assist in advising visitors to avoid prolonged lightning exposure during typical times and in specific locations. Visitors at these times are encouraged to reach the safety of large substantial buildings or fully enclosed vehicles at any sign of lightning. An informal data collection identifies several lightning-related deaths and tens of injuries at Grand Canyon and Rocky Mountain National Parks in the last 20 years, and several deaths and injuries in each of the other parks.