June 10, 2018: This weekend, a powerful mesoscale convective system (MSC) of thunderstorms over central Europe produced a furious outburst of sprites. “It was unreal,” says Martin Popek of Nýdek, Czechia, a veteran photographer of the upward directed bolts. “I recorded more than 250 sprites in only 4.5 hours of observation! That’s nearly as many as I typically see in the entire summer thunderstorm season.”
Many of the sprites during the outburst looked like this:
This is a jellyfish sprite–so called because it resembles the eponymous sea creature. Jellyfish sprites are typically very large, stretching as much as 50 km between the tops of their heads to the tips of their tentacles below. “Regular jellyfish sprites are associated with very strong positive cloud-to-ground lightning strokes in the underlying convective storms,” notes lightning scientist Oscar van der Velde of the Technical University of Catalonia, Spain.
However, not all of the jellyfish were regular. Some were “decapitated”–without heads. “I recorded about 20 sets of tentacles only,” says Popek. Here is one example of many:
“In my experience, this is quite rare,” he adds.
“It is rare,” agrees van der Velde. “We don’t know why they sometimes look like this.” He speculates that atmospheric waves called “gravity waves” sometimes interfere with the normal formation of jellyfish, leaving them headless. “Mesospheric gravity waves likely help focus the electric field to trigger downward streamers,” he says. “But note that sprite morphology is not fully understood–not even for regular jellyfish. We have a lot to learn.”
Another observer in the Czech Republic, Daniel Ščerba-Elza, also photographed the display. “It was extremely active,” says Ščerba-Elza. “I recorded about 69 sprites, much more than usual. The storms were about 250 – 300 km away in Austria and Hungary. This is a good distance because it allows you to see over the tops of the thunderheads.” He made a summary video of the outburst.
Such an outburst before summer even begins may be a good omen for sprite photographers as thunderstorm season gains steam. Stay tuned for more sightings.