Residents of Cordoba, Argentina, and Porto Alegre in southern Brazil, often see the sky ripple with dramatic storms. Some of these illuminate the sky with such stunning electrical force, that the term ‘megaflash’ has arisen – the rare phenomenon of single lightning flashes traveling hundreds of kilometres.
The World Meteorological Organization recently confirmed that on 31 October 2018, a single flash covering a horizontal distance of 709 kilometres across southern Brazil – equivalent to the distance between London and the border of Switzerland – set a new record for the longest reported distance of a single lightning flash. The megaflash more than doubled the previous record of 321 kilometres, which occurred on 20 June 2007 across the US state of Oklahoma.
Records for the greatest duration for a single lightning flash have also been extended: on 4 March 2019, a megaflash occurred for 16.73 seconds over northern Argentina.
Most lightning is formed when warm air is pushed upward during storms. As the water droplets in the warm air meet ice crystals in the cold air above they bump together and move apart, resulting in static electrical charges in the clouds. Over time, the bottom of the cloud gains a higher negative charge, which seeks to link up with the ground’s positive charge. As a flow of negative charges rush toward the Earth and positive charges flow upward, a strong electric current occurs – the bolt.
Megaflashes have some unique characteristics, however: ‘In storms, negative charge is often equalised to the positive charge of the ground, but in megaflashes, the equalisation happens with positive charges at the tops of other clouds,’ explains Randall Cerveny, professor of geographical sciences at Arizona State University and World Meteorological Organization’s rapporteur of weather and climate extremes. ‘Their cloud-to-cloud equalisation allows megaflashes to cover broad distances and to discharge for long durations.’
Some thunderstorms, termed ‘mesoscale convective systems’ (MCSs) provide the optimal conditions for megaflashes: ‘These storms are created by massive surface heating, rather than by frontal uplift, like the storms common in the UK. These hot-air based MCSs form in large open plains, like the plains of Argentina and southern Brazil,’ says Cerveny.
Before these recent record-breakers were detected, lightning was monitored using on-the-ground geolocation. However, advances in space-based lightning mapping have allowed scientists to measure flash extent and duration over broader ranges, allowing them to detect new extremes. ‘We are literally rewriting the definition of “lightning”,’ says Cerveny, who is passionate about the value of this monitoring. ‘Lightning has great importance to our basic understanding of the science of the atmosphere. As meteorologists learn more about how our atmosphere works, our ability to predict weather events improves. Knowledge of extremes helps engineers to design better planes and buildings. The records also draw attention to basic lightning human safety practices.’
While engineers, meteorologists and climatologists will be fascinated by the implications of ‘megaflashes’ for advancing knowledge in atmospheric science, Cerveny relates to the more elemental stirrings that lightning incites. ‘I’m told that announcements of these extremes draws the youngest generation of meteorologists into the field. It draws attention to the excitement of atmospheric science.’
Cerveny thinks that space-based lightning mapping technology will be increasingly important to detect the effects of climate change: ‘It’s like measuring a child’s growth by marking their height on a door. If you only have one measurement, you can’t say whether or not the child is growing. The more that we monitor climate and weather, in particular extreme events, the clearer picture emerges as to how our climate is changing.’