The map is based on data collected from 1998–2013 by the Lightning Imaging Sensor (LIS) on NASA’s Tropical Rainfall Measuring Mission satellite, and from 1995–2000 by the Optical Transient Detector (OTD) on the OrbView-1/Microlab satellite. Flashes above 38 degrees North were observed by OTD only, as the satellite flew to higher latitudes.
NASA scientists say that the higher frequency of lighting over land makes sense because solid earth absorbs sunlight and heats up faster than water; this means there is stronger convection and greater atmospheric instability, leading to the formation of thunder and lightning producing storms. This conclusion is similar to a NASA publication released in 2001 which showed that Florida, for example, is one place where the rate of strikes is unusually high because the Sunshine State experiences two sea breezes: one from the east coast and one from the west coast. The “push” between these two breezes forces ground air upward and triggers thunderstorms.
According to NASA’s Daniel Cecil, a member of the Global Hydrology and Climate Center’s lightning team, the data also have revealed some interesting regional trends. Scientists have observed a large number of flashes during the month of May in the Brahmaputra Valley of far eastern India. The heating and weather patterns are unstable and changeable at that time—just before the onset of the monsoon, which brings plenty of rain but much less lightning. In contrast, locations in Central Africa and Northwestern South America have large amounts of lightning throughout the entire year.
As the map shows, the highest amounts of lightning flashes occur in the far eastern Democratic Republic of Congo, and Lake Maracaibo in northwestern Venezuela.
Cecil noted that more years of data has not necessarily brought notable big-picture differences when compared to the earlier maps. “The longer record allows us to more confidently identify some of these finer details,” he said. “We can examine seasonality, and variability through the day and year-to-year.”