What are Dirty Thunderstorms and When Do They Appear?

A dirty thunderstorm is a rare and awe-inspiring phenomenon caused by powerful volcanic eruptions. Bolts of lightning shoot out of an erupting volcano in a dirty thunderstorm, making it one of nature’s most terrifying yet breathtaking spectacle to witness. More than two hundred cases of volcanic lightning have been documented over the past two centuries, but scientists still have not fully understood the dynamics of this unique phenomenon. Interested to know more about this mysterious electrical phenomenon that continues to puzzle volcanologists and has become the favorite scenic spot of photographers? Stay with us as we answer the most frequently asked questions about dirty thunderstorms and unravel their many mysteries. 

How is a Dirty Thunderstorm Formed? 

To generate lightning, we need one key component: charge separation. In an ordinary thunderstorm, ice particles in clouds collide with one another and produce static electricity, which results in lightning. 

In a dirty thunderstorm, however, lighting erupts from the ash-rich volcanic plume. It can occur in two places, either close to the ground in dense ash clouds or high up near the stratosphere.

column rising over Redoubt Volcano in Alaska. 

A volcanic plume, also known as an eruption column, is a cylinder-shaped cloud of super-hot ash emitted during some explosive volcano eruptions. Volcanic ash that makes up the eruption plume is a mixture of fragmented rocks, glass, and minerals formed when gases dissolved in magma expand and escape violently into the atmosphere. These densely packed particles collide and rub against each other when violently ejected into the eruption plume. This collision results in the generation of static charge — volcanic ash becomes electrically charged as some ash particles gain electrons while others lose elections. 

As the charged particles ascend from a compressed environment beneath the volcano to a less dense environment aboveground in the volcanic plume, charge separation occurs: positively charged ash particles separate from the negatively charged particles. The plume continues to rise and further separate the different charge regions until eventually there is an electric discharge as lightning tears through the volcanic plume to connect the oppositely charged areas. An intense lightning activity near the ground follows, featuring sparks and fiery thunderbolts. 

For volcanic lightning in the upper layer of the atmosphere, ice plays a crucial role. As the plume of ash and water rises upwards, it condenses to ice due to the decreasing temperature. From there, these ice particles generate electricity the same way they do in an ordinary thunderstorm: their collisions build up enough static charge to stimulate a lightning bolt.

Where Can You See a Dirty Thunderstorm? 

Instances of volcanic activity have been reported above many volcanoes around the globe in recent years, such as Mount Augustine volcano in Alaska, Eyjafjallajökull volcano in Iceland, and Taal volcano in the Philippines. However, in the ash plume of the Sakurajima volcano in Japan – one of the most active volcanoes in the world – volcanic lightning occurs more often than on any other volcano.

Kagoshima-shi-sakurajima-japan

Since volcanic lightning arises from the collision of particles in the thick plumes of ash, volcanoes that do not disgorge an eruption plume rarely have a dirty thunderstorm. Some volcanoes – for instance, the ones in Hawaii – are more likely to spew fluid lava fountains than ash-rich plumes. Such volcanoes lack volcanic lightning. However, there have been reports of mighty lightning activity above some lava-spewing volcanoes – such as Anak Krakatau, Indonesia – suggesting that some other factors might also be at play. 

Are All Dirty Thunderstorms the Same? 

Lighting can travel in all directions and take many different shapes and forms. It can occur as broad bolts, St. Elmo’s fire (ball lightning), bolt lightning, sheet lightning, separate sparks, or branching displays. Lightning bolts may also appear as a combination of all these shapes – this was the case during the eruption of Mount St. Helens in 1980 when eyewitnesses reported a periodic display of sheet lightning accompanied by gigantic St. Elmo’s fire bouncing on the ground.

Does the Plume Height Affect Volcanic Lightning? 

Scientists speculate that the height of the erupting plume could affect the intensity of the dirty thunderstorm as the atmospheric temperature has a significant role in the generation of lightning. In taller erupting plumes, the colder ambient temperatures promote freezing of ash and vapor particles, thus leading to more ice charging inside the plume, which in turn leads to more vigorous electrical activity. 

Are Dirty Thunderstorms Dangerous?

Although dirty thunderstorms are mother nature’s most spectacular display of destruction and terror, they do not pose the same threat as other features of a volcanic eruption, such as boiling lava, choking dust clouds, and volcanic ash landslides. 

a flash of lightning. 

Volcanic lightning can be a hazard to life if it strikes the ground: there have been instances when lightning bolts struck people and animals miles away from the erupting site. Moreover, a shockwave shortly follows as a lightning bolt strikes the sky. The magnitude of the shockwaves varies – photographers capturing this terrific phenomenon have reported that often their camera and tripod would be shaken by the impact of the shockwave. While a dirty thunderstorm can be a sight to behold for most people, the roaring claps of volcanic lightning can trigger anxiety and panicked reactions in people with astraphobia. 

Can Dirt Thunderstorms Be Used to Monitor Volcanic Eruptions?

a volcanic eruption.

While volcanic lightning is a display of terror for most people, for volcanologists, the incandescent flashes of the lightning are a flash of warning. Since volcanic lightning occurs at the beginning of a volcanic eruption, it can warn people and volcanologists far away that a potentially dangerous volcanic eruption is on its way. Scientists are currently working on different ways to use volcanic lightning as a monitoring tool to track and measure the ever-shifting dangers of a volcanic eruption. Detecting light is easy and can be done even when lightning strikes hundreds of miles away. Moreover, light does not suffer so-called path effects, making it a valuable tool in the volcano monitoring arsenal. Although this method is only in the initial stages of modeling as of now, once developed, it could prove groundbreaking. 

The Takeaway

There are still many questions about dirt thunderstorms that are yet to be answered by volcanologists, but there is no doubt that the violent electrical storms that break as a volcano erupts are the most vivid demonstration of the power, beauty, and mystery the natural world has to offer.