THEORIES OF LIGHTNING FORMATION

 

One of the most powerful forces in nature, lightning was once magical, mysterious, and misunderstood. Historically, myth and legend accounted for lightning with stories of angry gods and heroes. Today, meteorologists use lightning as a tool for analyzing and forecasting the intensity and movement of thunderstorms. While lightning is still a little mysterious, it is now understand enough to know how to prepare for it. Of course, every once in a while lightning does still put on a magic show.

 

Lightning occurs when there is a charge separation in the cloud.  As a thunderstorm grows, electrical charges build up within the cloud, with positive charges at the top, and negative charges at the bottom. Oppositely charged particles gather at the ground below the cloud. The attraction between positive and negative charges quickly grows strong enough to overcome the air's resistance to electrical flow.  Soon after, the rapid and staggered advancement of a shaft of this negatively charged air, called a “stepped leader,” begins.  As the leader approaches the ground in approximate 50 meter steps, a spark surges up from the ground toward it.  Racing toward each other, they connect in microseconds and complete the electrical circuit, allowing for the flow of electrons, with speeds close to 1/3 the speed of light, which will start the series of return strokes making up the lightning flash. 

Animation

Taken from http://www.nationalgeographic.com/features/96/lightning/

 

The leader is invisible to the naked eye, while the return stroke is the most luminous part of the strike, and the part that is really visible.  Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect.  There are many theories as to how the charge separation in the cloud occurs, some of which are listed in Table 1 below.

 

Table 1. Cloud Charge Separation Mechanism Theories

Separation by Physical Effects

Mechanism

Diagram

Description

Convection

Saunders

Positive ions from the surface are brought into the cloud by convective updraft. The growing positive charge in the cloud attracts negative ions from the surrounding air creating a screen layer. This screening layer is then thought to descend because of entrainment processes on the borders of the cloud. This would create a net positive charge toward the top of the cloud and a net negative charge at the bottom edges.

Diffusion/Drift of Ions

Beard

Ions present in the atmosphere (e.g. from cosmic rays) may not be neutrally distributed. These ions will move diffusively to water droplets, thus charging the droplet with the small charge bias that may be present in the ions. If an electric field exists in the cloud, then the movement of ions will be oriented according to the field and will move in a process called drift. This will enhance the charge separation.

Separation by Induced Charged Effects

Drop Breakup

Beard

The collision and coalescence of raindrops (1-6 mm) and drizzle (100-1000 µm) often results in the fragmentation of the combined drop. The existing electric field induces a charge separation in the drop that tends to put positive charge on the bottom and negative charge on the top. After breakup, large fragments will tend to come from the bottom and be positively charged, while small fragments will tend to be negatively charged. Gravity will causes different sized fragments to move at different rates, thus separating the charge.

Particle Rebound

Beard

An asymmetrical collision between two polarized drops can cause charge to be transferred from one particle to another. If the smaller particle approaches the larger one from the bottom, then positive charge will be moved to the small particle and negative charge will move to the large particle. Gravity will then cause a separation of these charges.

Selective Ion Capture

(Wilson Effect)

Beard

The cloud's electric field creates a dipole in a cloud droplet, which is falling at a certain rate. The falling droplet will catch negative ions from below, but may or may not catch positive ions from above depending on their relative speed. They will be caught if the ions are faster than the droplet, and won't if they are slower. For a field strength of 10 V m-1, the droplet needs to have a radius greater than 1 µm to fall faster than the positive charges and acquire a net negative charge.

Separation by Particle Collision Effects

Thermoelectric Effect

Pruppacher

Contact between droplets at different temperatures can create a temperature gradient across the particles. The temperature gradient causes an ion gradient and an electric field. With hail, the larger objects will tend to be frozen, while the smaller objects may be in the liquid phase. Contact between these would impart a negative charge to the large particle, and a positive charge to the smaller particle. Gravity would then separate the charges.

Contact Potential Effect

Pruppacher

The electric surface potential of two particles is likely to be different. If two particles with different potentials collide, then it would be expected that charge would be transferred between the two particles in an attempt to equalize the potential. It has been observed that the contact potential decreases with riming, and that the potential becomes more negative with decreasing temperature down to about -20°C. Thus a collision between an ice particle and a rimed ice particle would lead to a negatively charged rimed particle and a positively charge ice crystal.[Saunders, 1995]

Workman-Reynolds Effect

 Pruppacher

Freezing of an aqueous solution creates a potential difference between the ice and the solution. If the freezing process is interrupted and the liquid is removed (e.g. by a collision), then a charge difference will be created between the ice and the liquid droplet. The sign of this charging depends on the type of ion, with NH4+ creating positively charged ice and Cl- creating negatively charged ice.

Separation by Precipitation Breakup Effects

Breakup of Freezing Drop

Pruppacher

As a drop freezes, an ice shell maybe created around it. If the drop fragments at this stage, the main ice particle is often negatively charged and the splinters are positively charged.[Pruppacher, 2000]

Drop Breakup

 Pruppacher

This is similar to inductive drop breakup, except that there is no inductive charge separation prior to breakup. Instead, electrons are stripped off of the water during drop breakup leaving the remaining main body of water positively charged. As before, gravity will then cause a separation of charges.

Graupel Melting

 Pruppacher

Experiments have shown that drops formed from the melting of ice containing air bubbles will be positively charged. This happens because minute negatively charged droplets are produced as the air bubbles burst on the surface of the melt water, and thus take away negative charge.

Splintering During Riming

 Pruppacher

Collisions between small water droplets and riming ice particles tend to leave the ice particle negatively charged, while producing positively charged ice splinters.

 

 

Recently, two schools of thought have developed in regards to how lightning actually forms once the charge separation occurs.  Conventional thinking says that the flash is initiated when the electric field associated with the charge build up in the cloud exceeds the insulating ability of the surrounding air.  Once this point is reached, the current is allowed to flow.  However, as you can see in the diagram to the right, taken from Joeseph Dwyer’s article,  “A Bolt out of the Blue,” the electrons are moving relatively slowly because they are hampered by their constant collisions with air molecules.  More of their energy goes to kicking out other electrons than can be gained from the electric field.  These collisions create an effective drag force that is similar to what is felt when you stick your hand out a moving car window, the faster you go the stronger the force, making it more difficult to move.  An incredibly large electric field would be needed to overcome this energy barrier and form lightning.

 

The other proposed theory, runaway breakdown, was first suggested in 1961 by Alexander Gurevich.  It is rooted in the belief that if the electron velocities are high enough (at least 6 million meters per second), the drag force decreases, as you can see in the figure to the right, and the electrons go even faster since less energy is expended when ejecting other electrons.  If a strong electric field accelerates a high speed electron, the drag force becomes smaller, which allows the electron to move even faster, reducing the drag even more, and so on. 

 

These runaway electrons can accelerate to nearly the speed of light, gaining enormous amounts of energy and ultimately producing the discharge called runaway breakdown.  The drawback to this process, however, is that it requires a seed population of electrons with high initial energies.  It has been suggested that the decay of radioisotopes or the collisions of cosmic ray particles with air molecules could generate enough energetic electrons to satisfy this condition.  The next diagram shows how this could occur.

 

Taken from the article “A Bolt Out of the Blue” by Joseph Dwyer, printed in Scientific American, May 2005

 

Since the early 1900’s, it was thought that lightning could produce x-rays, but because the equipment of the day was not very sturdy, good storm based readings could not be established since the environment is so rough.  In 2001, however, scientists found a direct link between x-rays and lightning when Charles Moore and others at NMT reported observing energetic radiation from several natural lightning strikes.  Unlike their earlier balloon observations, the radiation they observed seemed to be produced by the lightning itself and not by the electric fields inside the cloud.  Intrigued by these findings Joseph Dwyer and his group at the Florida Institute of Technology decided to investigate. 

 

Over the next couple of years, they not only confirmed what the NMT group had seen, but also found that the emission seemed to be associated with the upward sparks, with energies equivalent to those of dental x-rays.  They also found something that they did not expect on the last launch of the 2003 season, a burst of gamma rays, which only added more confusion to the mix.  Currently, the lightning research field is experiencing an explosion of growth, as more and more researchers have begun investigating these phenomena.

 

Where does lightning usually strike?
Most commonly, lightning comes from a parent cumulonimbus cloud. However, lightning can also occur as a result of volcanic eruptions, which generate sufficient dust to create a static charge.  At any rate, these thunderstorm clouds are formed wherever there is enough upward motion, instability in the vertical, and moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing. These conditions are most often met in summer. In general, the US mainland has a decreasing amount of lightning toward the northwest. Over the entire year, the highest frequency of cloud-to-ground lightning is in Florida between Tampa and Orlando. This is due to the presence, on many days during the year, of a large moisture content in the atmosphere at low levels (below 5,000 feet), as well as high surface temperatures that produce strong sea breezes along the Florida coasts. The western mountains of the US also produce strong upward motions and contribute to frequent cloud-to-ground lightning. There are also high frequencies along the Gulf of Mexico coast westward to Texas, the Atlantic coast in the southeast US, and inland from the Gulf. Regions along the Pacific west coast tend to have the least cloud-to-ground lightning.

 

http://www.stormchasing.nl/lightning.html

http://thunder.msfc.nasa.gov/primer/primer2.html

http://www.nationalgeographic.com/features/96/lightning/

Beard, K.V.K., and Ochs, H.T., Charging Mechanisms in Clouds and Thunderstorms, in The Earth's Electrical Environment, pp. 114-130, National Academy Press, Washington D.C., 1986.

Pruppacher, H.R., and Klett, J.D., Microphysics of Clouds and Precipitation, pp. 792-852, Kluwer Academic Publishers, Norwell, Mass, 2000.

Saunders, C.P.R., Thunderstorm Electrification, in Handbook of Atmospheric Electrodynamics, Volume I, pp. 61-92, CRC Press, Inc., Boca Raton, 1995.