How it is formed
Double lightning.The first process in the generation of lightning, overlooked by some lightning investigators, is the ejection of particles from the sun in what is called the solar wind. The Earth, in its orbit around the Sun, acquires a charge from the particles that are ejected from the Sun, as Earth orbits the sun through this solar wind. Most of this charge collects in our outer atmospheric layers, especially the ionosphere. This builds up a charge in the atmosphere that will neutralize itself through any available path. This causes the forcible separation of positive and negative charge carriers within a cloud or air. The mechanism by which this happens is still the subject of research, but one theory is the polarisation mechanism. This theoretical mechanism has two components: the first is that falling droplets of ice and rain become electrically polarised as they fall through the atmosphere's natural electric field, and the second is that colliding ice particles become charged by electrostatic induction. Another theory is that opposite charges are driven apart by the above mechanism and energy is stored in the electric fields between them. The positively charged crystals tend to rise to the top, causing the cloud top to build up a positive charge, and the negatively charged crystals and hailstones drop to the middle and bottom layers of the cloud, building up a negative charge.
The Earth collects charged particles as it moves through the solar wind, which is composed of ejecta from the Sun. This results in a potential difference between the Earth's atmosphere and its surface. The charged particles will follow any available path to the Earth's surface to neutralize this potential difference.
It has been observed that the top of clouds are often seen to have electrical activity, from current induced from above. Cloud-to-cloud lightning can also appear at this point. Cloud-to-ground lightning is the result of the charge in the cloud reaching the point where the path to ground becomes ionized until it breaks down, and allows a current flow. We know, however, that when sufficient negative and positive charges gather, and when the electric field becomes sufficiently strong, an electrical discharge (the bolt of lightning) occurs within the clouds or between the clouds and the ground. In fact, the sheer number of lightning strikes from one cloud during very high lightning activity in the central states does not support the accepted theory that the charge is caused by above-described polarisation mechanism while the ice crystals and rain droplets are moving up and down in the cloud. The charge supply must be larger, probably through the ionospheric charge just described.
Some suggest that these discharges are triggered by cosmic ray strikes which ionise atoms, releasing electrons that are accelerated by the electric fields, ionising other air molecules and making the air conductive by a runaway breakdown, then starting a lightning strike. The mainstream theory is that during the strike, successive portions of air become conductive as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions (stepped channels called step leaders); this conductive filament grows in length, and at the same time, electrical energy stored in the electric field flows radially inward into the conductive filament.
According to this theory, when a charged step leader is near the ground, opposite charges appear on the ground and enhance the electric field. The electric field is higher on trees and tall buildings. If the electric field is strong enough, a discharge can initiate from the ground. This discharge starts as a positive streamer and, if it develops as a positive leader, can eventually connect to the descending discharge from the cloud. In actuality, however, probably what really happens is that the charge in the cloud above forms one side, or "plate" of a condensor, and the Earth below forms the other. Therefore, as the cloud progresses over the Earth's surface, an equal but opposite charge is induced in the Earth below the cloud, and moves along with the cloud. When the voltage differential in this air "capacitor" exceeds the resistance of the air between, the air "breaks down" (becomes conductive), and allows current flow in the form of a lightning strike.
Lightning can also occur within the ash clouds from volcanic eruptions[1][2], or can be caused by violent forest fires which generate sufficient dust to create a static charge.
Negative C-G lightning with two visible non-connected streamersAccording to mainstream theory, again, it usually begins when an invisible negatively-charged stepped leader stroke is sent out from the cloud. Also according to this theory, as it does so, a positively charged streamer is usually sent out from the positively charged ground or cloud. When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the strike, and is the part that is really visible.
However, it has now been proven experimentally using "stop action" movies of lightning strikes (single frame analysis of movies of strikes), that most lightning strikes consist of several succeeding discharges (i.e, a single, bright discharge, followed by a short time (a few dark frames at 24 fps) later, by another discharge that is not quite as intense, followed by a larger number of dark frames, and another discharge that is weaker yet, followed again by an even longer time (more dark frames), and so on, for as many as 8 to 12 or more separate discharges), causing the "flickering" effect commonly seen during a lightrning discharge. This discharge rapidly superheats the leader channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble is caused by the of heating and cooling of the discharge channel by successive lightning strokes. The variations in successive discharges are the result of what is known in the electronic world as "damped oscillation", caused by the very large magnetic field suddenly being set up by the large current flow while the discharge lasts, then collapsing of the magnetic field when the current drops off at the end of that particular discharge. This causes further current flow beyond the point at which the charge is dissipated, possibly causing the next discharge to be the opposite direction (though this has not yet been proven).[3]
It is also possible for streamers to be sent out from several different objects simultaneously, with only one connecting with the leader and forming the discharge path. Photographs have been taken on which non-connected streamers are visible such as that shown on the right.
An average bolt of lightning carries a current of 30-to-50 kiloamperes, and transfers a charge of 5 coulombs and 500 megajoules (enough to light a 100 watt light bulb for 2 months). However, it has been observed from experiments that different locations in the US have different potentials (voltages) and currents, in an average lightning strike for that area. For example, Florida, with the largest number of recorded strikes in a given period, has a very sandy ground saturated with salt water, and is surrounded by water. California, on the other hand, has fewer lightning strikes (being dryer). Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 6,000-7,000 feet above ground level, and gets very long, thin, purplish discharges, which crackle; while Oklahoma, with cloud bases about 1,500-2,000 feet above ground level and fairly soft, clayey soil, has big, blue-white explosive lightning strikes, that are very hot (high current) and cause sudden, explosive noise when the discharge comes. The potential difference in each case may consist of different levels of voltages, for example a few kilovolts accompanied by a very high current in Oklahoma; but 1,000,000 volts and higher, with a much lower current, in Arizona. Research on this is still going on.
Positive lightning makes up less than 5% of all lightning. It occurs when the stepped leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike several kilometers or miles distant from the clouds — the reason being, that the voltage difference has to be much higher, due to the tens of thousands of additional feet the strike must travel. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.
As a result of their power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[4]
Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707. Subsequently, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the chances of a similar occurrence.
Positive lightning has also been shown to trigger the occurrence of upper atmospheric lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.
An average bolt of positive lightning carries a current of 300-to-500 kiloamperes (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a thousand million volts), and dissipates enough energy to light a 100 watt lightbulb for up to 95 years, and lasts for an observed hundreds to thousands of milliseconds.
Intracloud or possibly cloud-to-cloud lightning.Heinz Kasemir first hypothesised that a lightning leader system actually develops in a bipolar fashion, with both a positive and a negative branching leader system connected at the system origin and containing a net zero charge. This process provides a means for the positive leader to conduct away the net negative charge collected during development, allowing the leader system to act as an extending polarised conductor (or capacitor). Such a polarised conductor would be able to maintain intense electric fields at its ends, supporting continued leader development in weak-background electric fields.
During the eighties, flight tests showed that aircraft can trigger a bipolar stepped leader when crossing charged cloud areas. Most scientists now think that positive and negative lightning in a cloud are actually bipolar lightning.
To spontaneously ionise air and conduct electricity across it, an electric field of field strength of approximately 2500 kilovolts per metre is required. However, measurements inside storm clouds to date have failed to locate fields this strong, with typical fields being between 100 and 400 kilovolts per metre. While there remains a possibility that researchers are failing to encounter the small high-strength regions of the large clouds, the odds of this are diminishing as further measurements continue to fall short.
A theory by Alex Gurevich of the Lebedev Physical Institute in 1992 proposes that cosmic rays may provide the beginnings of what he called a runaway breakdown. Cosmic rays strike an air molecule and release extremely energetic electrons having enhanced mean free paths of tens of centimeters. These strike other air molecules, releasing more electrons which are accelerated by the storm's electric field, forming a chain reaction of long-trajectory electrons and creating a conductive plasma many tens of meters in length. This was initially considered a fringe theory, but is now becoming mainstream because of the lack of other theories.
It has been recently revealed that most lightning emits an intense burst of X-rays and/or gamma-rays which seem to be produced during the stepped-leader and dart-leader phases just before the stroke becomes visible. The X-ray bursts typically have a total duration of less than 100 microseconds and have energies extending up to nearly a few hundred keV. The presence of these high-energy events match and support the "runaway breakdown" theory, and were discovered through the examination of rocket-triggered lightning, and from satellite monitoring of natural lightning.
NASA's RHESSI satellite typically reports 50 gamma-ray events per day, and many of these are strong enough to fit the theory. Additionally, low-frequency radio emissions detected at ground level can detect lightning bolts from upwards of 4000 km away; combining these with gamma-ray burst events detected from above show overlapping positions and timing.
There are problems with the "runaway breakdown" theory, however. While there seems to be a strong correlation between gamma-ray events and lightning, there are insufficient events detected to account for the amount of lightning occurring across the planet. Another issue is the amount of energy the theory states is required to initiate the breakdown, and the sheer number of strikes observed from one cloud during high lightning activity. Cosmic rays of sufficient energy strike the atmosphere on average only once per 50 seconds per square kilometre. Measured X-ray burst intensity also falls short, with results indicating particle energy 1/20th of the theory's value.