Copyright © 2021 Albuquerque Journal
As the summer skies start to roil with thunderstorms, New Mexicans get a front-and-center seat to some spectacular lightning displays. We may even count the time between spotting the flash and hearing the roar, a means to calculate the distance of the lightning that ensures we’re a safe distance from the strike.
In most cases, if the flash of light and clap of thunder are more than 30 seconds apart, it usually indicates enough distance from the storm to avoid being struck. But this applies only to normal thunderstorms that are small and send bolts vertically from the cloud directly to the ground. For one in ten lightning flashes, globally, this is not the case.
In the early 1980s, an astronaut looked down upon the Earth from the Space Shuttle and saw a massive flash of lightning that seemed to grow horizontally through the cloud cover, spreading in a long web – long enough, obviously, to be seen by the naked eye from space. These long horizontal flashes have been observed periodically since the 1970s and given such descriptive names as “spider lightning” or “anvil crawlers,” but our understanding of them has been incomplete.
Recently, however, we’ve made great strides in documenting and analyzing this spectacular type of lightning. We now know that the largest of these flashes – termed “megaflashes” – can exceed 62 miles in length and occur throughout the southern Great Plains in North America and the La Plata basin in South America. Previously, we could detect these large flashes only if they happened to occur in specific regions monitored by a special type of sensor called the Lightning Mapping Array, invented at New Mexico Tech. But now, thanks to a set of new geostationary satellites developed by the National Oceanic and Atmospheric Administration, we can identify megaflashes wherever and whenever they occur over most of the western hemisphere.
Last year, the World Meteorological Organization confirmed two new record-setting megaflashes. The first was for the longest bolt, which stretched horizontally from the coast of Brazil 440 miles into Argentina, roughly equivalent to traveling from Albuquerque to Denver. The second record was for duration of flash, another bolt detected in Argentina that lasted for 16.7 seconds, or about the time of a short television commercial.
These records more than doubled anything previously documented. And from analyzing these and other megaflashes, we’ve been able to learn more about our global climate and the nature of lightning worldwide. It has also led to important safety realizations that should change how people respond to these electrical storms.
Specifically, we’ve learned that these unique flashes are produced by mesoscale storms, weather systems that can cover entire regions of a continent. The La Plata Basin in South America and North America’s Midwest prairies are ideal megaflash zones in the western hemisphere due to a favorable set of environmental conditions – including interactions between the atmosphere and terrain, and transport of moist air from the Gulf of Mexico or Amazon rainforest – that allow storm events to grow into massive systems that span the horizons.
What we’ve found is that raindrops and ice crystals in these massive weather systems collide as they are lofted upwards, and they become electrified as electrons are transferred from one to the other during the collision. Once the charged precipitation particles reach the cloud-top, they can no longer continue their ascent and are blown horizontally into the clouds surrounding the storm core. Charge builds up in these clouds as the electrified precipitation particles accumulate. Then a spark occurs. A bolt flashes horizontally, spreading outward to fill as much of these electrified clouds as it can access, consuming the built-up charge along its meandering path.
These megaflashes curve for hundreds of miles through the stratified clouds like a defined river, sending off thousands of small electrified tributaries. From the ground, a megaflash might appear as a crackling glow high above, muted by cloud cover. A series of cloud-to-ground strikes may also be visible, so far apart they seem to be completely separate bolts of lightning. We now know these disparate bolts are all, in fact, connected within the clouds.
On average, a 180-mile megaflash will create 20 cloud-to-ground bolts. This increases exponentially with the length of the megaflash. We’ve also learned that, unlike typical lightning, these bolts will often be positively charged, which tend to deposit more energy for a longer duration into whatever they hit.
In a practical sense, this research means that, in areas prone to megaflashes, we may need to improve engineering codes to account for more intense lightning phenomena. It also means people shouldn’t always depend on the 30-30 rule, which says that if the time difference between a visible flash and the audible crack is less than 30 seconds, people should seek shelter in their home for 30 minutes.
The 30-30 rule might be suitable in most scenarios, but perhaps not during storms that produce megaflashes, when one bolt can generate multiple strikes that range hundreds of miles apart.
While we’re finally gaining deep insights into the cause of megaflashes, much more work remains to be done. But, for now, we can venture to warn that if you live in an active megaflash zone and an organized storm system is in the area, it’s safest to heed another popular rule of lightning safety: if thunder roars, go indoors.
Michael Peterson is a remote-sensing scientist at Los Alamos National Laboratory.