The occurrence of lightning also has significant impacts on utility and many other lightning-vulnerable industrial operations. Personal safety situations include recreation, schools, defense, and aviation. Lightning activity is highly variable on all scales including the diurnal, and this variability should be taken into account for individual safety and for planning outdoor lightning-vulnerable activities. In some Midwest and plains locations, lightning is most frequent after midnight.Ĭloud-to-ground lightning has a major impact on a wide variety of activities and industries. These storms pose a threat to late-afternoon and evening recreation.
An additional result of the study is the midday beginning of lightning over the higher terrain of the western states, then the maximum activity moves steadily eastward. These areas are where recurring lightning-vulnerable recreation and workplace activities should expect the threat at these times, rather than view them as an anomaly. However, lightning activity starts before noon over western mountains and onshore along the Atlantic and Gulf of Mexico coasts. The afternoon from 1200 to 1800 local mean time provides two-thirds of the day’s lightning. Composite maps of the 2-h periods with the most lightning in each grid square were also prepared. NLDN flashes were assigned to 2-h time periods in 5° longitude bands. A unique feature of this study is that maps were prepared to coincide with local time, not time zones. For this study, NLDN cloud-to-ground flash data were compiled in 20 km by 20 km grid squares from 2005 to 2012 for the lower 48 states. Cloud-to-ground lightning has strong and variable diurnal changes across the United States that should be taken into account for outdoor lightning-vulnerable activities, particularly those involving human safety.
However, no single publication includes maps of cloud-to-ground flash density across the domain and adjacent areas during the entire diurnal cycle. National maps of cloud-to-ground lightning flash density (in flashes per square kilometer per year) for one or more years have been produced since the National Lightning Detection Network (NLDN) was first deployed across the contiguous United States in 1989.
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