Lightning activity varies with the seasons and with the time of day. This figure shows the hour of the day when lightning is most probable at each location in and around the U.S., using the color coding shown in the color bar at the bottom.

Red colors, for example, indicate maximum lightning activity at noon, whereas yellow colors indicate maximum activity near 4 PM. Darker, grayer colors (the colors at the base of the color bar) indicate that the time of the maximum at that location is difficult to determine, because lightning activity is erratic. High-value colors (along the top of the color bar) indicate fairly regular changes in lightning activity with the time of day. Note that the association of lightning with the time of day in the Great-Plains area is fairly weak (dark colors). This is believed to be because storms that occur in this region are often remnants of storms that were initiated earlier in the afternoon near the Rocky Mountains and have taken many hours to propagate eastward to this area. The statistics shown are based only on summer months (June-August) for 1998-2006. Lightning detection weakens with distance from the land-based detectors, and this is partly why lightning activity appears to diminish off the coasts. It is well known, however, that there is less lightning over the oceans than over land (see story). Areas with little lightning (less than 0.001 strikes per 15 min per (8 km)² have been masked.

The rapid change in the maximum hour of lightning activity near the coasts is due in part to the land/sea contrast in temperature. In the summertime, the land heats up more than the water during the day, and winds blow inland from the sea, helping storms to develop over the land. At night, the land cools more rapidly than the ocean, and the winds reverse, helping storms to develop at night over the nearby ocean. Mountain ridges tend to catch the morning sun, and storms tend to build up over them earlier than over the surrounding land.

The cloud-to-ground lightning data were produced by Vaisala's National Lightning Detection Network (NLDN) and were provided by the NASA Lightning Imaging Sensor (LIS) instrument team and the LIS data center via the Global Hydrology Resource Center (GHRC) located at the Global Hydrology and Climate Center (GHCC), Huntsville, Alabama through a license agreement with Global Atmospherics, Inc (GAI). (The data available from the GHRC are restricted to LIS science team collaborators and to NASA EOS and TRMM investigators.)

(submitted by Thomas Bell)