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WHAT IS VOLCANIC LIGHTING AND THUNDER AND HOW DOES IT FORM? 

It is a phenomenon of terrifying beauty – the flash of lightning within the dark, brooding clouds of ash thrown out by erupting volcanoes.

Volcanic lightning forms in plumes of ash rather than thunder clouds, and it is followed by terrifying thunder.

The lightning follows chaotic paths and will often travel sideways or even upwards through an ash cloud. The lightning paths tend to be more chaotic than those seen in thunderstorms.

Scientists have long debated whether this lightning is caused by the electrostatic charge of the ash itself or of ice particles forming within the plumes. 

In 2016, a study found that friction created by ash particles as they are ejected as high pressure from crater led to a build-up of static electricity, leading to chaotic low level lightning bolts.

These often do not discharge downwards towards the ground as seen in traditional thunderstorms, but can travel sideways and even upwards.

A second study by a separate group also showed that when ash clouds extend high enough into the atmosphere, ice particles can build up within it and lead to a different sort of electrical discharge.

In this case the ice particles generate the electrical charge as they rub together and the resulting lightning follows the path of the ice particles through the ash cloud.

Together the findings suggest that volcanic lightning can form in even relatively small eruptions as the charge begins to build up as soon as ash is flung into the air.

But in very large eruptions when ash clouds reach high into the atmosphere, the development of ice can enhance these storms to produce some of the most dramatic lightning.

Dr Corrado Cimarelli, a volcanologist at the Ludwig Maximilian University in Munich, Germany, who led the first study, said these differences could help to predict the amount of ash being discharged by an eruption and so give warnings to nearby aircraft about the potential risks.

He said: 'Regardless of the size of the eruption, any ash emissions are associated with electrical discharges.

'This we could measure and use to estimate the amount and distribution of the ejected ash - and from a safe distance of several kilometers and even in poor visibility.

'So we could estimate the ash distribution in the atmosphere early on and warn the aviation authorities.'

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