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The sight of lightning is fascinating, the destruction it can cause can be devastating. Earle Williams has been studying the effects of lightning for 25 years and talked to FirstScience about his work and his experiences.

 

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.



NOAA

The power of lightning can range from ten to several hundred megavolts


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?



NOAA

Cloud-to-ground lightning is the most dangerous for humans


EW: One can make a broad separation between lightning that contacts earth (cloud-to-ground lightning) and lightning that does not (intracloud lightning). Generally, the lightning type of concern for human safety, the initiation of forest fires and the destruction of electronic equipment is the cloud-to-ground flash, and as a consequence, this type of lightning is targeted by operational networks for lightning detection. Within these two broad categories, there are other subcategories. Cloud-to-ground lightning can transfer either negative or positive charge to earth. The great majority of ground flashes are negative, but the most powerful and most dangerous are the positive ground flashes that can also produce sprites in the mesosphere. Positive flashes occur in the decaying phases of large thunderstorms and in the very active stages of severe storms. Intracloud lightning is far more prevalent than cloud-to-ground lightning, and the mean peak currents in intracloud lightning tend to be smaller. Intracloud flashes that leave cloud base and approach ground, but do not reach ground, are called air discharges. Intracloud lightning that leaves cloud top and heads upwards are sometimes referred to as cloud-to-ionosphere flashes. Intracloud flashes that bridge two thunderstorms are called intercloud flashes. In the active stages of severe storms, intracloud flashes can outnumber ground flashes by 10-100 to one.

FS: What research are you currently carrying out?

EW: Two areas are receiving most of my attention at the moment: studies of the Earth's Schumann resonances and lightning in severe Florida thunderstorms.



NOAA

Every strong lightning flash in the world can be detected at the station in Rhode island


The measurements of Schumann resonances provides access to many things on a global basis. These are electromagnetic standing waves trapped in the Earth-ionosphere cavity and maintained by global lightning. We record these signals in the remote woods of western Rhode Island. The fundamental resonant frequency is 8 Hz. Every lightning radiates a small amount of energy at 8 Hz and so contributes to the quasi-continuous background signal of Schumann resonances. Every few minutes or so, a single extraordinarily energetic lightning will single-handedly 'ring' the Earth-ionosphere cavity and such events can be located from the Rhode Island station on a global basis. The most energetic events of all are large ground flashes with positive polarity and accompanying horizontally extensive 'spider' lightning. Many of these events also trigger sprites high in the mesosphere (75 km altitude), a luminous phenomenon shaped like jellyfish but as large as a thunderstorm itself. The global mapping capability for large lightnings thereby provides indirectly a global map for sprites. The electromagnetic signals can be used to compute the dipole moment change of the lightning and from this one can determine whether the field increase over the storm is large enough to produce a sprite.

The analysis of the electromagnetic signal for individual lightnings can also be used to explore the properties of the ionosphere which is the upper boundary of the Earth-ionosphere waveguide. Recent comparisons of measurements between Hungary (by our collaborator Dr. Gabriella Satori) have shown small but systematic increases in the Schumann resonance frequencies at both stations which can be attributed to the changes in ionization in the upper D region of the ionosphere on the 11 year solar cycle. We are presently near the maximum of this cycle, and so the resonant frequencies are also near their peak values.



NOAA

Tornadic storms may be predicted in future by the lightning patterns in the preceding storm clouds


Studies on severe weather in Florida are also underway with MIT Lincoln Laboratory and NASA Marshall Space Flight Center. The main interest at present is the behavior of total lightning flash rate that may signal the occurrence of a tornado. We have found already that strong upsurges in total flash rate, dominated by intracloud lightning, tend to precede all forms of severe weather on the ground (hail, wind, tornadoes) by 5-15 minutes. One challenge at present is to identify features that may distinguish a tornado from another form of severe weather.

FS: How do you measure the volts in lightning?

EW: Measuring the voltage of lightning itself is probably a near impossible task, because in general we are unable to predict where a lightning will occur to make the appropriate measurement. One can however estimate the voltage difference developed in a thundercloud before it produces lightning by measuring the vertical variation of electric field from the ground to the center of electric charge in the storm with balloon-borne measuring equipment. The integral of this vector electric field is the total voltage available to drive the lightning. The values one obtains are in the range of ten to several hundred megavolts.

FS: Can we make use of lightning in terms of power?

EW: If lightning occurred in one place in a predictable manner day in and day out, the harnessing of its energy might be seriously considered. Such is not the case. Lightning does strike twice in the same place, but extremely infrequently. To harness the energy, the lightning must strike an electrode connected to a very robust bank of electrical capacitors. One captured strike would deliver at most 10^8 joules. This would provide enough energy to power one electric hairdryer for about ten hours. So clearly, a large number of captured flashes would be needed to supply the energy needs of just a single household.

FS: What is the ‚€˜global circuit‚€™ and what have you learnt from this?



NOAA

Lightning is more common in warmer conditions but its sensitivity to global warming is still unknown


EW: The global circuit is the framework set up by the Earth, the ionosphere and all the electrified weather in the insulating air layer in between. Electrified storms pump current from the Earth to the upper atmosphere and thereby maintain a 250 kV voltage difference between Earth and ionosphere - a giant spherical capacitor. The same pair of concentric conductors (Earth and ionosphere) make up the electromagnetic waveguide that contains the Earth's Schumann resonances which we discussed earlier. Both aspects of the global circuit - the 'DC' version with the 250 kV ionospheric potential and the 'AC' version with Schumann resonances, provide integrated estimates of global weather measurable at single locations on the Earth's surface. This situation provides a natural framework for studying global change. For several years we have been intrigued with the idea of monitoring temperature variations with the global circuit, following well recognized local observations indicating that, on average, warmer conditions favor more lightning. Within the past decade, both the 'DC' and 'AC' global circuits have been examined for sensitivity to temperature on a variety of time scales - the diurnal, the intraseasonal (20-60 days), the semiannual, the annual and the El Nino time scale - all with some indication of a correlation with temperature and with a sensitivity of some tens of percent per degree centigrade. The global circuit sensitivity to global warming is still highly uncertain, the available record lengths are short and the natural variability of the global circuit is large. This work is now in progress along with notable gains in our ability to measure the global circuit, a capability that has greatly lagged our conceptual understanding.

FS: We need batteries and motors to produce electricity, how is lightning produced in our atmosphere?



NOAA

Electrical storms pump current from the Earth to the upper atmosphere


EW: The details of charge separation leading to lightning generation are still poorly understood, particularly in comparison with conventional batteries and motor-generators that routinely produce electricity. The inhospitable environment of the thunderstorm has greatly slowed progress toward this solution. We do know that mixed phase conditions, in which water substance in all three of its phases (vapor, liquid, and solid) is an essential aspect of vigorous charge separation. Millimeter- to centimeter- sized graupel particles grow by accreting supercooled droplets which subsequently freeze on the graupel surface to add to its mass. The graupel particles develop appreciable fall speeds relative to the surrounding air and collide with ice crystals and smaller graupel particles. It is widely believed that microphysical differences in these colliding particles lead to selective transfer of negative charge to the larger particles and positive charge transfer to the smaller particles. The subsequent differential motions under gravity result in the large scale vertical dipole of a thundercloud with positive charge uppermost and negative charge in the lower portion of the mixed phase zone.

FS: How many people each year are struck by lightening?

EW: As I recall, approximately 300 people per year are struck by lightning, or about one person per day worldwide. Recent surveys indicate a considerable underreporting of such strikes. A surprisingly large number of people live to tell about a lightning strike to their person. Several medical doctors specialize in lightning injury.

 


Earle Williams is a research scientist in the Center for Meteorology and Physical Oceanography, MIT. His research interests include thunderstorm electrification, radar meteorology and hydrological applications of radar, and the global electrical circuit.

This article is courtesy of the NASA Earth Observatory. If you would like to learn more please visit the Earth Observatory website.

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First Science 2014