Volcanic lightning(Redirected from Dirty thunderstorm)
Volcanic lightning is a atmospheric electrical discharge caused by a volcanic eruption, rather than from an ordinary thunderstorm. Volcanic eruptions producing lightning can be colloquially referred to as a dirty thunderstorm, but this term technically only refers to ash-rich plumes interacting with weather systems containing ice. Volcanic lightning can occur entirely independently of any meteorological interactions.
Volcanic lightning above an eruption of Mount Rinjani
The earliest recorded observations of volcanic lightning are from Pliny the Younger, describing the eruption of Mount Vesuvius in 79 AD, “There was a most intense darkness rendered more appalling by the fitful gleam of torches at intervals obscured by the transient blaze of lightning.” The first studies of volcanic lightning were also conducted at Mount Vesuvius by Professor Palmieri who observed the eruptions of 1858, 1861, 1868, and 1872 from the Vesuvius Observatory. These eruptions often included lightning activity.
Volcanic lightning arises from the ash, rock fragments, and other ejecta which generate static electricity within the volcanic plume. One study stated that “27-35% of eruptions are accompanied by lightning, assuming one eruption per year per volcano”, and indicated that volcanic lightning has been observed in 212 eruptions from 80 different volcanoes.
A famous image of the phenomenon was photographed by Carlos Gutierrez and occurred in Chile above the Chaiten Volcano. It circulated widely on the internet. Another notable image of this phenomenon is "The Power of Nature", taken by Mexican photographer Sergio Tapiro in Colima, Mexico, which won third place (Nature category) in the 2016 World Press Photo Contest. Other instances have been reported above Alaska's Mount Augustine volcano, Iceland's Eyjafjallajökull volcano and Mount Etna in Sicily, Italy.
Triboelectric (frictional) charging within the plume of a volcano during eruption is thought to be a major electrical charging mechanism. Electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, similar to the way that ice particles collide in regular thunderstorms. The convective activity causing the plume to rise then separates the different charge regions, ultimately causing electrical breakdown.
Fractoemission is the generation of charge through break-up of rock particles. It may be a significant source of charge near the erupting vent.
Naturally occurring radioisotopes within ejected rock particles may cause self-charging of volcanic plumes. During an eruption, a large amount of fragmented sub-surface rock is ejected into the atmosphere. In a study performed on ash particles from the Eyjafjallajökull and Grímsvötn eruptions, scientists found that both samples possessed a natural radioactivity above the background level, but that radioisotopes were an unlikely source of self-charging in the Eyjafjallajökull plume. However, there was the potential for greater charging near the vent where the particle size is larger.
Other factors affecting volcanic lightningEdit
Large amounts of water are released as vapor during volcanic eruptions; one study found that the water content of volcanic plumes is much greater than the water content of thunderstorms. This study also found “there may be a threshold of water substance concentration required in a plume for lightning to occur … this appears to be a function of the large amount of water in the magma and not the smaller amount in the entrained air”. Seasonal effects, showing that volcanic lightning is more common in winter than in summer, supported this hypothesis.
The height of the ash plume appears to be linked with the mechanism which generates the lightning. In taller ash plumes (7–12 km) large concentrations of water vapor may contribute to lightning activity, while smaller ash plumes (1–4 km) appear to gain more of their electric charge from fragmentation of rocks near the vent of the volcano (fractoemission). The atmospheric temperature also plays a role in the formation of lightning. Colder ambient temperatures will create greater amounts of ice inside the plume thus leading to more electrical activity.
It has been suggested that a by-product of volcanic lighting are lightning induced volcanic spherules, (LIVS). These small glass spherules are thought to be formed during high-temperatures processes such as cloud-to-ground lightning strikes. The temperature of a bolt of volcanic lightning can reach 30,000 °C. When this bolt contacts ash particles within the plume it will cause them to melt and then quickly solidify as they cool, forming orb shapes. The presence of volcanic spherules aids in providing evidence for the occurrence of volcanic lightning when the lightning itself was not observed directly.
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