A dirty thunderstorm is an expression that has been given to describe the electrical discharges of lightening that occur during a volcanic eruption.
National Geographic wrote about the natural phenomenon of volcanic lightening in 2007 and stated "the findings offer a rare glimpse of this poorly understood phenomenon, including evidence of one type of lighting never seen before by scientists". This occurrence, which until recently, was difficult for scientists to track, is beginning to have some light shed on this kind of event during a volcanic eruption.
Since the 2007 reporting, volcanic lightening has been followed and photographed when eruptions from volcanoes occur. It is said this exceptional type of lightening is a unique type of electrical discharge that comes from volcanic plumes.
In 2008, the Timesonline.co.uk reported that lightening is "sparked off" by infinite amounts of small fragments of ice that bang into one another inside the "turbulent thundercloud" and each time the ice fragments collide with each other, they generate static electricity that eventually grows large enough to create a massive spark which is then emitted as the lightening.
In the same article, it was stated that "smaller lightning bolts have also been discovered bursting out from the mouth of a volcano's crater as the gases and ash explode, but what generates the lightning is not known" (timesonline.co.uk).
National Geographic also reported that particles colliding into one another are what create the lightening. In that article, it was indicated that rock and ash are also a part of the ingredients that make up the recipe for a dirty thunderstorm.
In February 2010 National Geographic reported more on dirty thunderstorms. Leading up to this time a better ability to measure had been developed and various instruments were installed near the vents of an Alaskan volcano, Redoubt.
Volcanic seismologist, Steve McNutt, who studied the volcanic action in Alaska stated "Both types of bigger, more obvious bolts occur when water droplets and ice particles interact with the volcano's plume of electrically charged ash, creating a sort of "dirty thunderstorm," (NationalGeorgraphic.com)
To see a photo of what a dirty thunderstorm looks like you can see one on the National Geographic website which illustrates an Alaskan eruption on April 14, 2009. Ironically this is the same date that Eyjafjallajokull erupted in Iceland exactly one year later, spewing significant amounts of ash and a few days later, dirty thunderstorms were observed. The volcanic lightning produced from Eyjafjallajokull's activity can be seen on MSNBC.
Dirty thunderstorms while fascinating to watch and a most incredible sight, are very dangerous. Much is still unknown about this amazing volcanic activity and as more technological tools and measuring techniques are establishes, there is little doubt science will continue to study this remarkable activity caused by Mother Nature.