Tag Archives: thunder

For the first time, researchers record a volcanic thunder

A group of scientists achieved what many believed was impossible: recording a volcano’s thunder.

This satellite image shows Bogoslof volcano erupting on May 28, 2017. The eruption began about 18 minutes prior to this image and the cloud rose to an altitude greater than 12 kilometers (40,000 feet) above sea level. Image credits: Dave Schneider / Alaska Volcano Observatory & U.S. Geological Survey.

Not all volcanoes are made equal, and not all eruptions are the same. Depending on the chemistry and temperature of the lava, some eruptions are essentially a neat lava fountain, while others are more explosive, ejecting clouds of hot rock and ash that can reach the stratosphere. As they do so, these charged particles can create loud thunders, but these thunders tend to get lost in the overall cacophony of tumbles and rumbles in the eruption.

Now, for the first time, geoscientists have managed to isolate that thunder boom, digitally disentangling it from other sounds in the background.

“It’s something that people who’ve been at eruptions have certainly seen and heard before, but this is the first time we’ve definitively caught it and identified it in scientific data,” said Matt Haney, a seismologist at the Alaska Volcano Observatory in Anchorage and lead author of the new study set to be published in Geophysical Research Letters.

You can listen to the sound here:

This audio file contains 20 minutes of microphone data recorded during the March 8, 2017 Bogoslof eruption, sped up 60 times. Credit: Matt Haney / Alaska Volcano Observatory & U.S. Geological Survey.

Isolating the sound isn’t just interesting as a technical achievement, it can be used as a proxy for volcanic lightning (the stronger the lightning is, the stronger the thunder is). Then, the lightning itself can be used to assess how big the volcanic plume is — and how hazardous it is.

“Understanding where lightning is occurring in the plume tells us about how much ash has been erupted, and that’s something that’s notoriously difficult to measure,” Johnson said. “So if you’re locating thunder over a long area, you could potentially say something about how extensive the plume is.”

Bogoslof Volcano erupting on June 5, 2017. Image credits: NASA Earth Observatory.

The team was able to record the rumbling and thunder using a microphone array, a tool which is becoming more and more common in volcano monitoring. Although zoning in on the thunder alone was considered impossible by some geoscientists, Haney believes the technique might become more and more common (and useful) in the near future.

“If people had been observing the eruption in person, they would have heard this thunder,” Haney said. “I expect that going forward, other researchers are going to be excited and motivated to look in their datasets to see if they can pick up the thunder signal.”

Journal Reference: ‘Volcanic thunder from explosive eruptions at Bogoslof volcano, Alaska’. Geophysical Research Lettersonlinelibrary.wiley.com/doi/10 … 017GL076911/abstract

You're still looking at lightning. Thunder image below. Credit: Flickr Matjs

This is what thunder looks like (kind of)

What does lightning sound like? Thunder. Well, what does thunder look like then? It’s no trick question. Like all acoustic waves, thunder can also be visualized and Maher Dayeh from the Southwest Research Institute in San Antonio was the first to turn a thunderclap into an image. His findings were shown at a meeting of the American Geophysical Union.

You're still looking at lightning. Thunder image below. Credit: Flickr Matjs

You’re still looking at lightning. Thunder image below. Credit: Flickr Matjs

Every day, some four million lightning strikes hit the surface of the planet. Despite this, how lightning, and subsequently thunder, is formed is not completely understood at a physical level. We know one thing for sure: it comes from clouds (dust, water and ice). Ice inside the cloud rubs against each other becoming electrically polarized or charged (the exact mechanism is a bit fuzzy, which is why the whole thing is debatable). The lighter ice will move upwards, while the heavier ice will stay below separating the negative and positive charges. Just like the cloud, because there’s a lot of charge hovering around, the air below the clouds also become ionized. In turn, the ionized air charges air particle further below in a cascading effect until it eventually reaches the ground. This happens very quickly, and the sections of ionized air look very much like electrical sparks or the static electricity released when you rub your sweater against a balloon. The  ground is very conductive compared to air, and will give up a large amount of electric charge into this completed circuit (between the ground and the cloud) that causes a lot of charge to flow from the ground upwards to the cloud (this is called the return stroke and is basically what you see as lightning). This ionizes the air completely between the ground and the cloud, and this is the part you can see for miles around.

Left: long exposure photo of lightning event with downstream in green, and return stroke in purple. Right: audio signature for each return stroke. Image: Nature

Left: long exposure photo of lightning event with downstream in green, and return stroke in purple. Right: audio signature for each return stroke. Image: Nature

 

As for thunder, because the ionization of air described above happens so quickly over a large area, it causes air to move (acoustic pressure) just like a sonic boom or explosion. Now, this sound has been recorded and visualized using processing algorithms by researchers at Southwest Research Institute in Antonio, Texas. Dayeh and team first went to a military installation in Florida, then installed a launch system which would shoot a rocket with a long copper wire trailing behind. The rocket was fired into a thundercloud. Then, it was only a matter of waiting for the rocket to trigger the lightning strike and profit. The lightning traveled down the wire and eventually hit the launch platform which was surrounded 15 microphones spaced 1 meter apart. This helped build an acoustic map, which looks like a contemporary painting. In fact, Dayeh and crew were so stoked by the results they thought they had done something wrong.

“The initial constructed images looked like a colourful piece of modern art that you could hang over your fireplace. But you couldn’t see the detailed sound signature of lightning in the acoustic data,” Dr Dayeh said.

Top: lightning. Bottom: acoustic map of the thunder. Image: Nature

Top: lightning. Bottom: acoustic map of the thunder. Image: Nature

The map also provided a few insights into thunder formation, like the fact that thunderclap depends on the peak electric current flowing through the lightning bolt. [source: Nature]