Volcanic lightning — dirty thunderstorms

Volcanic lightning also called dirty thunderstorms is a lightning that appear during a volcanic eruption, so it happens not only in clouds. In this new lesson of the Windy.app Meteorological Textbook (WMT) for better weather forecasting, you will learn what volcanic lightning is and how it forms.

What do a volcano and a balloon have in common?

As you know, everything we see consists of tiny particles: atoms. Atoms are made up of smaller particles, including protons and electrons. While protons are securely “stuck” in an atom’s nucleus, the electrons are much less fixed, leading to atoms exchanging electrons regularly.

Often physical bodies have the same number of protons and electrons. But if you rub two surfaces against each other — for example, a rubber balloon against your hair — an exchange would happen, and one of the surfaces will end up with more electrons than protons.

Eruption of Calbuco, a volcano in Chile, in 2015. Picture credit: Martin Bernetti/Agence France-Presse — Getty Images

Everything in the world pursues balance, so in this situation, the extra electrons would “want” to get to a place where there isn´t enough of them. Of course, the electrons don´t precisely “want” anything, as they do not have their own will, but they tend to get there by the laws of physics. Electrons, as well as protons, have a characteristic called charge. Protons are positively charged, and electrons are negatively charged; opposite charges neutralize each other.

But if a particular object has more protons than electrons, it has an overall positive charge. And when there are more electrons, the charge ends up being negative. The exchange happens when objects with different charges meet.

If you rub a balloon against your hair, and then touch your hair, you will receive a weak electric shock with a short flash: that is, electrons are rushing between your fingers and your hair to the place where there aren’t enough of them. This exchange is called electric discharge.

And what does it have to do with volcanoes?

Quite a lot, actually. The mechanism of spectacular lightning happening directly in an eruption column made of hot volcanic ash seems to be pretty much the same. With the exception of the fact that, instead of your hair and the balloon surface rubbing together, it is particles of rocks, ash, water, steam, and volcanic gases that are propelled from the depths of the earth at a mind-blowing speed.

The exchange of electrons between differently-charged parts of the eruption column is observed during the eruption of Colima, a volcano in Mexico, in 2017. Picture credit: Mercury Press

There are two types of volcanic lightning: one that happens between ash particles close to the ground, and one that occurs higher, when the steam is rising from the volcano starts freezing.

In the second case, the lightning is very similar to the lightning in a cumulonimbus — dense vertical clouds that bring heavy rains; ice crystals, which have collected extra electrons and therefore gained a negative charge, become heavier than those that lost their electrons and are lower.

When this mass of negatively charged snowflakes manages to “grab” a positively charged object, be it a higher mass of snowflakes, or a rock on the ground (the same happens when your electrified hairtries to reach your fingers), a discharge occurs.

Eruption of Calbuco in 2015. Picture credit: Francisco Negroni

Since the variety of particles in an eruption column is much higher than in a regular cumulonimbus or during an eruption, they move, to put it mildly, slightly faster; volcanic lightning can happen much more often than it does in a regular thunderstorm.

Because of the ash, these thunderstorms are called “dirty”. This phenomenon isn´t infrequent: by some estimations, about one out of three eruptions cause volcanic lightning. But it is not a danger on its own, as the eruption is a much more significant threat. In other words, if you ever see lava at the gate of your town, some lightning far in the background should be the least of your concerns.

Text: Windy.app team

Cover photo: Marc Szeglat / Unsplash