Superbolts of lightning are rare; they only make up about one percent of all lightning strikes. But they are incredibly strong; compared to the average lightning strike, which carries about 300 million volts of electricity, superbolts can be as much as 1,000 times stronger and cause serious damage to infrastructure and vessels like ships. Previous work has also shown that this is a real phenomenon, and is not because of instrument or interpretation errors. Now scientists have learned more about the conditions that right for superbolt strikes.
Reporting in the Journal of Geophysical Research: Atmospheres, scientists determined that when a storm is closer to the ground, and its electrical charging zone is lower, suprbolts are far more likely. This situation creates superbolt hotspots around some oceans and tall high mountains. Previous work has shown that the Northeast Atlantic Ocean, Mediterranean Sea, and Peru and Bolivia's Altiplano are areas that experience usually high rates of superbolt strikes; these areas also have short distances between charging zones and surfaces.
Although superbolts are only an extremely small percentage of all lightning, they are "a magnificent phenomenon," said lead study author Avichay Efraim, a physicist at the Hebrew University of Jerusalem. The researchers were interested in why they tend to form in some places over others.
Previous studies have investigated how shipping emissions, desert dust, sea spray or salinity could influence superbolt strength. Those studies have focused on limited geographical areas, however, and have not adequately explained the global distribution of superbolts.
Storm clouds can be 12 to 18 kilometers (7.5 to 11 miles) in height, and contain a wide range of temperatures. Clouds have to sit over air temperatures of 0 degrees Celsius (32 degrees Fahrenheit) for lightning to form. In the upper parts of storm clouds and above the freezing line, a charging zone emerges where electrification takes place.
The researchers used lightning data about time, location, and energy content to find information about the storms where they occurred, like charging zone height, temperature ranges, aerosol content, and land and water surface height. This work indicated that aerosols actually do not affect the strength of superbolts very much, contrary to previous studies. It also revealed that shorter distances between charging zones and the surface of either land or water generated lightning that was significantly more energized. The study determined that higher-energy bolts tended to form in storms that were closer to the surface because electrical resistance is reduced over shorter distances, leading to higher currents and stronger bolts.
The regions with highest rates of superbolts also have the lowest distances between charging zones and surfaces.
"The correlation we saw was very clear and significant, and it was very thrilling to see that it occurs in the three regions. This is a major breakthrough for us," said Efraim.
This research can hep scientists predict how superbolts might be impacted by climate change. Higher temperatures may weaken lightning, but increased levels of moisture in the atmosphere could counteract that, Efraim suggested. More research will be needed to know more about superbolt formation as well, such as how the magnetic field or solar cycle are involved.
Sources: American Geophysical Union, Journal of Geophysical Research: Atmospheres