A dirty thunderstorm is a weather

Weather

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

 phenomenon that occurs when lightning

Lightning

Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 is produced in a volcanic plume

Eruption column

An eruption column consists of hot volcanic ash emitted during an explosive volcanic eruption. The ash forms a column rising many kilometres into the air above the peak of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km, penetrating the stratosphere...

.

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. Other instances have been reported above Alaska's Mount Augustine volcano, and Iceland's

Iceland

Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

 Eyjafjallajökull

2010 eruptions of Eyjafjallajökull

The 2010 eruptions of Eyjafjallajökull were volcanic events at Eyjafjöll in Iceland which, although relatively small for volcanic eruptions, caused enormous disruption to air travel across western and northern Europe over an initial period of six days in April 2010. Additional localised disruption...

 volcano.

Causes

A study in the journal Science

Science (journal)

Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

indicated that electrical charges are generated when rock fragments, ash, and ice particles in a volcanic plume collide and produce static charges, just as ice particles collide in regular thunderstorms.

As the plume started going downwind, it seemed to have a life of its own and produced some 300 more or less normal [lightning bolts] ... The implication is that it has produced more charge than it started with. Otherwise [the plume] couldn't continue to make lightning.

—Martin Uman, co-director of the University of Florida Lightning Research program

Volcanic eruptions also release large amounts of water, which may help fuel these thunderstorms.