Diurnal Variations of NLDN Cloud-to-ground Lightning in the United States

National maps of cloud-to-ground lightning flash density (flashes/km2/year) for one or more years have been produced since Vaisala's National Lightning Detection Network (NLDN) was first deployed across the contiguous 48 U.S. states in 1989. However, detailed diurnal national lightning maps have not been prepared.

For this study, NLDN flash data were compiled in 20 by 20 km squares from 2006 to 2010 for the lower 48 states. A unique feature of this study is that maps were prepared to coincide with local overhead time, not time zones. NLDN flashes were assigned to two-hour time periods in five-degree longitude strips. This approach proved to be adequate in identifying patterns of hourly local lightning. A map of the two-hour periods with the most lightning in each grid square will also be prepared.

The time-of-day maps show that the afternoon hours from noon to 6 pm provide around two-thirds of the day's lightning. However, lightning activity starts before noon over the western mountains and onshore along the Atlantic and Gulf coasts, which are locations that contribute to difficulties with lightning avoidance during outdoor activities. An additional specific result of the study is that lightning begins over the higher terrain of the western states, then moves very steadily eastward onto the plains and Midwest states during the night and morning hours, local time.