Picture yourself on a beach in Indonesia. As the seawater laps against your sandy toes, an earth-shattering boom shakes your body. A mountain in the distance now spews magma as it collapses into the sea. Nature reveals the full force of its might as a tsunami forms and lightning ripples across the sky.

If this sounds terrifying, then you fully appreciate the power of the Anak Krakatau volcanic eruption. Starting on December 22, 2018, this incident produced the deadliest tsunami of the 21^st century. But alongside the hundreds who lost their lives, Anak Krakatau was also unique because it produced lightning for six days straight. 

Although it’s normal for volcanoes to create lightning, these situations usually only last for a single day. To better understand what happened in this case, a research team from the Barcelona Supercomputing Center took a closer look. Andrew Prata, an atmospheric scientist, sat down with us to explain how molten rock can create lightning and why this research matters.

A natural Rube Goldberg machine

It’s hard to imagine that hot magma stuck in the Earth could create flashes of light in the sky, but it makes sense if you understand how lightning is formed.

As Prata explains, the volcanic eruption triggered a landslide that pushed a lot of magma into the sea. This created an abudnance of steam and vapor that shot upward.

“This huge amount of energy that was being put into the atmosphere encouraged convection, rising air or rising heat, around the atmosphere,” says Prata. “And all this vapor condensed as it cooled. And when it got high enough, it was converted into ice.”

Prata continues: “Because of this continual heat source at the base, these convective processes continued. These processes are typical in thunderstorms where you need a source of heat at the base. Usually it's afternoon surface heating that will trigger afternoon thunderstorms in the tropics. But in this case the convective trigger, or initiation trigger, was this constant source of magma and seawater interacting.” 

Thankfully, this process didn’t last forever. Eventually, the magma rising through the volcano got blocked up and closed the open vent, ending the thunderstorms. That said, the lightning was only a small part of what was going on. Prata and the rest of the team also investigated the source of the lightning: ice rapidly forming in the atmosphere.

The right tools for the job

An issue that Prata discusses has to do with observing volcanic activity from a satellite. When volcanoes erupt, they throw a lot of ash into the air. We currently have algorithms that, when applied to satellite data, can track these airborne particles. That said, eruptions like Anak Krakatau complicate things.

When there's lots of ice in the cloud, these algorithms don't indicate ash but ice—which is a real hazard for aviation.

“What we want to understand is can we distinguish volcanic ice particles from meteorological ice particles?” says Prata. “Are there some optical properties that we can leverage to distinguish between the two?”

To find out, the team leveraged information from a variety of sources. They collected geostationary data from the Advanced Himawari Imager aboard the Himawari 8 meteorological satellite. This allowed them to better understand the temperature flows behind the lightning. They also found that ice particles in the Anak Krakatau plume were notably smaller than those in the surrounding meteorological clouds. 

“The nice thing about the geostationary satellite data is that it measures thermal infrared radiation,” says Prata. “We can get measurements of what we call brightness temperature, which is an equivalent blackbody temperature measured by the satellite. And when you have really optically thick clouds it's a pretty good estimate of the actual physical temperature. So, it gives us an indication of how cold the clouds are.”

They also relied on data from Aqua and Terra, two polar orbiting satellites. Each of these were equipped with a MODIS instrument, which has a higher resolution than the Himawari and can get more accurate temperature measurements.

Working with such a unique event had a lot to teach. Prata and the rest of the team took this opportunity to learn all they could for future eruptions. 

 “We've started to collect observations for several different case studies of volcanoes with different kinds of dynamics,” says Prata. “Some with lots of ash. Some with lots of ice. And we want to test the empirical relationship between lightning flash rate and plume height to see how robust it is."

Read more:

• The mystery of Hawaii's volcanoes
• Measuring the earth from space