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Researchers are a step closer to understanding volcanic lightning, one of the most spectacular atmospheric phenomena, which can be seen playing among the clouds of smoke and ash during an eruption. The intensity is extreme: the Hunga Tonga-Hunga Ha‘apai eruption, in the Tongan archipelago in 2022, produced more than 2,600 lightning flashes a minute stretching up to 19 miles (31km) above sea level.

We know that storm clouds become electrically charged as a result of collisions between ice crystals rising in updraughts and falling particles of graupel, or soft hail. The ice picks up positive charge and the hail negative. What has puzzled scientists is how a volcanic plume, which is dry and consists of ash and rock fragments, could pick up charge. Particles made from the same rocky material should not do that during collisions.

New research published in Nature from the Institute of Science and Technology Austria shows the secret lies in a fine coating of carbon-rich molecules. Perfectly clean particles of silica did not tend to pick up charge, but where there was a carbon coating, charge transfer happened in collisions. The effect could be produced simply by heating the silica, as there are enough carbon-containing molecules in normal air to produce surface contamination.

The heat and updraught of a volcanic plume create just the right conditions for charging, hence the spectacular lightning shows.