Electric Skies - Thunderstorms and Lightning

FirstScience: How long have you been studying the effects of lightning?

Earle Williams: I have been involved with research on thunderstorms and lightning for about 25 years, with field projects in Massachusetts, Alabama, North Carolina Colorado, New Mexico, Florida, Australia, and Brazil.

FS: What is the worst storm that you have experienced?

EW: I have experienced many active storms and it is difficult to identify a grand winner. I recall a storm that came off an escarpment in the Top End of Australia, flashed continuously, and produced a nearby ground flash that vaporized a TV antenna perhaps 10 meters away. That got our attention. In tropical Brazil last year, an unusual storm with rotating updraft was continuously illuminated by intracloud lightning. The radar cloud top was an exceptional 19km high. This was probably the most active storm I have ever seen in the tropics.

What may seem surprising is that I have never witnessed firsthand a tornadic storm, though have studied the data for a large number of them.

The great majority of tornadic thunderstorms in the world occur in the Great Plains of North America, but I have not been an active 'chaser' in that region.

FS: What causes lightning?

EW: Lightning is a giant electrical spark and is caused by the separation of electrical charge in a thunderstorm. The mechanism of charge separation is still poorly understood but there now seems little doubt that ice particle collisions are involved and the conditions favorable to ice particle collisions are strong updrafts which invigorate a thunderstorm's ice factory. Charge separation by particle collisions occurs until the voltage difference in the cloud reaches 10 to 100 megavolts and the local electric field reaches several hundred kilovolts per meter. The latter field is the approximate dielectric strength of the thundercloud medium and its attainment is required to initiate ionization, the initial stage of the large spark that is lightning.

FS: How many different types of lightening are there and which type is the most powerful?