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Lightning is an atmospheric discharge of electricity, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. A bolt of lightning can travel at a speed of 220,000 km/h (136,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse soil or sand into glass channels. There are over 16 million lightning storms every year.
Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge.
How lightning initially forms is still a matter of debate: Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles. Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.
Early ideas and research about lightning
The ancient Greeks believed that their chief deity Zeus was in command of the natural phenomena of lightning and thunderbolts. In the Book of Job God asks "Will lightning flash at your command?". In his Dream Pool Essays written in AD 1088, the Song Dynasty polymath Chinese scientist Shen Kuo (1031-1095) wrote that when a house belonging to one Li Shunju had been struck by lightning, everyone assumed that the house would be burnt to the ground. To everyone's surprise, some of the wooden walls were merely blackened and lacquerwares untouched, while metal objects such as a steel sword were melted into liquid. Kuo compared this phenomenon to the equally strange effects of water being unable to douse Greek fire (which had been known to the Chinese since the Arabs had traded it, or a chemical composition fairly equal to it, in the 10th century). For these strange effects of lightning, Kuo wrote:
“Most people can only judge of things by the experiences of ordinary life, but phenomena outside the scope of this are really quite numerous. How insecure it is to investigate natural principles using only the light of common knowledge, and subjective ideas”
Thus was the frustration of learned men in his time of the desire to know the nature of lightning and other such common phenomena. However, in the Western world details of its force would become known by the 18th century.
Lightning strikes the Eiffel Tower in 1902.
Benjamin Franklin (1706-1790) endeavored to test the theory that sparks shared some similarity with lightning using a spire which was being erected in Philadelphia. While waiting for completion of the spire, he got the idea of instead using a flying object, such as a kite. During the next thunderstorm, which was in June 1752, it was reported that he raised a kite, accompanied by his son as an assistant. On his end of the string he attached a key, and he tied it to a post with a silk thread. As time passed, Franklin noticed the loose fibers on the string stretching out; he then brought his hand close to the key and a spark jumped the gap. The rain which had fallen during the storm had soaked the line and made it conductive.
Franklin was not the first to perform the kite experiment. Thomas-François Dalibard and De Lors conducted it at Marly-la-Ville in France a few weeks before Franklin's experiment. In his autobiography (written 1771-1788, first published 1790), Franklin clearly states that he performed this experiment after those in France, which occurred weeks before his own experiment, without his prior knowledge as of 1752.
As news of the experiment and its particulars spread, people attempted to replicate it. However, experiments involving lightning are always risky and frequently fatal. The most well-known death during the spate of Franklin imitators was that of Professor George Richmann of Saint Petersburg, Russia. He created a set-up similar to Franklin's, and was attending a meeting of the Academy of Sciences when he heard thunder. He ran home with his engraver to capture the event for posterity. While the experiment was under way, ball lightning appeared, collided with Richmann's head, killing him and leaving a red spot.
Although experiments from the time of Franklin showed that lightning was a discharge of static electricity, there was little improvement in theoretical understanding of lightning (in particular how it was generated) for more than 150 years. The impetus for new research came from the field of power engineering: as power transmission lines came into service, engineers needed to know much more about lightning in order to adequately protect lines and equipment.
Properties of lightning
An average bolt of lightning carries a negative electric current of 40 kiloamperes (kA) (although some bolts can be up to 120 kA), and transfers a charge of five coulombs and 500 MJ, or enough energy to power a 100 watt lightbulb for just under two months. The voltage depends on the length of the bolt, with the dielectric breakdown of air being three million volts per meter; this works out to approximately one gigavolt (one billion volts) for a 300 m (1000 ft) lightning bolt. With an electric current of 100 kA, this gives a power of 100 terawatts.
Lightning heats nearby air to about 10,000 °C (18,000 °F) nearly instantly, which is almost twice the temperature of the Sun’s surface. The heating creates a shock wave that is heard as thunder.
Different locations have different potentials (voltages) and currents for an average lightning strike. For example, Florida, with the United States' largest number of recorded strikes in a given period during the summer season, has very sandy ground in some areas and conductive saturated mucky soil in others. As much of Florida lies on a peninsula, it is bordered by the ocean on three sides. The result is the daily development of sea and lake breeze boundaries that collide and produce thunderstorms. Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 1800-2100 m (6,000-7,000 ft) above ground level, and gets very long and thin purplish discharges which crackle; while Oklahoma, with cloud bases about 450-600 m (1,500-2,000 ft) above ground level and fairly soft, clay-rich soil, has big, blue-white explosive lightning strikes that are very hot (high current) and cause sudden, explosive noise when the discharge comes. The difference in each case may consist of differences in voltage levels between clouds and ground. Research on this is still ongoing.
Formation
Positive lightning (a rarer form of lightning that originates from positively charged regions of the thundercloud) does not generally fit the following pattern.
Charge separation
The first process in the generation of lightning is charge separation.
Polarization mechanism theory
The mechanism by which charge separation happens is still the subject of research, but one theory is the polarization mechanism, which has two components:
- Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field;
- Colliding ice particles become charged by electrostatic induction.
Electrostatic induction theory
Another theory is that opposite charges are driven apart by the above mechanism and energy is stored in the electric field between them. Cloud electrification appears to require strong updrafts which carry water droplets upward, supercooling them to -10 to -20 °C. These collide with ice crystals to form a soft ice-water mixture called graupel. The collisions result in a slight positive charge being transferred to ice crystals, and a slight negative charge to the graupel. Updrafts drive lighter ice crystals upwards, causing the cloud top to accumulate increasing positive charge. The heavier negatively charged graupel falls towards the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate lightning discharges, which occurs when the gathering of positive and negative charges forms a sufficiently strong electric field.
There are several additional theories for the origin of charge separation.
Leaders
As a thundercloud moves over the Earth's surface, an equal but opposite charge is induced in the Earth below, and the induced ground charge follows the movement of the cloud.
An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. The discharge ionized channels are called leaders. The negative charged leaders, called a "stepped leader", proceed generally downward in a number of quick jumps, each up to 50 meters long. Along the way, the stepped leader may branch into a number of paths as it continues to descend. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible compared to the subsequent lightning channel.
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. The electric field is highest on trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points. This was first theorized by Heinz Kasemir. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible. 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 lightning discharge.
Discharge
Lightning sequence (Duration: 0.32 seconds)
When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.
The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke.
Double Lightning
Types of lightning
Some lightning strikes take on particular characteristics; scientists and the public have given names to these various types of lightning. Most lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.
The return stroke of a lightning bolt, which is the visible bolt itself, follows a charge channel only about a half-inch (1.3 cm) wide. Most lightning bolts are about a mile (1.6 km) long.
Positive lightning
Positive lightning, also known colloquially as a "bolt from the blue" makes up less than 5% of all lightning. It occurs when the 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 tens of kilometres/miles from the clouds. The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/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 greater 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.
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 atmosphere 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 up to 300 kA (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 (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300 GJ (gigajoules) (a billion joules).[citation needed] With this voltage, a positive lightning bolt may dissipate enough energy to light a 100-watt lightbulb for up to 95 years, as opposed to 2 months by a standard (negative) lightning bolt.
One special type of cloud-to-ground lightning is anvil-to-ground lightning. It is a form of positive lightning, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (often ahead) of the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and if one occurs in a largely clear sky, it is known colloquially as a "Bolt from the blue."
Cloud-to-cloud lightning, Steinenbronn, Germany
Multiple paths of cloud-to-cloud lightning, Swifts Creek, Australia
Multiple paths of cloud-to-cloud lightning, Swifts Creek, AustraliaLightning discharges may occur between areas of cloud having different potentials without contacting the ground. These are most common between the anvil and lower reaches of a given thunderstorm. This lightning can sometimes be observed at great distances at night as so-called "heat lightning". In such instances, the observer may see only a flash of light without thunder. The "heat" portion of the term is a folk association between locally-experienced warmth and the distant lightning flashes.
Dry lightning
Dry lightning is a folk misnomer in common usage in the United States for thunderstorms which produce no precipitation at the surface. This type of lightning is the most common natural cause of wildland fires.
Rocket lightning
Rocket Lightning, Queanbeyan, Australia
Rocket Lightning, Queanbeyan, AustraliaIt is a form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently.
The movement has been compared to that of a skyrocket, hence its name. It is also one of the rarest of cloud discharges.
Cloud-to-ground
Cloud-to-ground lightning is a great lightning discharge between a cumulonimbus cloud and the ground initiated by the downward-moving leader stroke. This is the second most common type of lightning, and poses the greatest threat to life and property of all known types.
Bead lightning
Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections, which last longer than the usual discharge channel. It is fairly rare. Several theories have been proposed to explain it; one is that the observer sees portions of the lightning channel end on, and that these portions appear especially bright. Another is that, in bead lightning, the width of the lightning channel varies; as the lightning channel cools and fades, the wider sections cool more slowly and remain visible longer, appearing as a string of beads.
Ribbon lightning
Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.
Staccato lightning
Staccato lightning is nothing more than a leader stroke with only one return stroke.
Ground-to-cloud lightning
Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke.
Ball lightning
Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists.
The engineer Nikola Tesla wrote, "I have succeeded in determining the mode of their formation and producing them artificially". There is some speculation that electrical breakdown and arcing of cotton and gutta-percha wire insulation used by Tesla may have been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials. Some later experimenters have been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.
Several theories have been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and also more contemporary research. Japanese research shows that several instances have been reported of ball lightning without any connection to stormy weather or lightning.
Ball lightning is typically 20 – 30 cm (8-12 inches) in diameter, but ball lightning several meters in diameter has been reported. Ball lightning has been seen in tornadoes, and has also been seen to split apart into two or more separate balls and recombine, and vertically-linked fireballs have been reported. Ball lightning has carved trenches in the peat swamps in Ireland. Because of its strange behavior, ball lightning has been mistaken for a UFO by many witnesses. One theory that may account for this wider spectrum of observational evidence is the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex').
Ball lightning apparently is created when lightning strikes silicon in soil, and has been created in a lab in this manner.
Trees and Lightnings
Eucalyptus tree that was blown apart by a lightning strike
Trees are frequent conductors of lightning to the ground. Since sap is a poor conductor, its electrical resistance causes it to be heated explosively into steam, which blows off the bark outside the lightning's path. In following seasons trees overgrow the damaged area and may cover it completely, leaving only a vertical scar. If the damage is severe, the tree may not be able to recover, and decay sets in, eventually killing the tree. It is commonly thought that a tree standing alone is more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.
Lightning damage to tree in Maplewood, NJ
After the two most frequently struck tree types, the Oak and the Elm, the Pine tree is also quite often hit by lightning. Unlike the Oak, which has a relatively shallow root structure, pine trees have a deep central root system that goes down into the water table. Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to its being targeted are a high resin content, loftiness, and its needles which lend themselves to a high electrical discharge during a thunderstorm.
Trees are natural lightning conductors, and are known to provide protection against lightning damages to the nearby buildings. Tall trees with high biomass for the root system provide good lightning protection. An example is the teak tree (Tectona grandis), which grows to a height of 45 metres (147.6 ft). It has a spread root system with a spread of 5 m and a biomass of 4 times that of the trunk; its penetration into the soil is 1.25 metres (4.10 ft) and has no tap root. When planted near a building, its height helps in catching the oncoming lightning leader, and the high biomass of the root system helps in dissipation of the lightning charges.
Lightning currents have a very fast risetime, on the order of 40 kA per microsecond. Hence, conductors of such currents exhibit marked skin effect, causing most of the currents to flow through the conductor skin. The effective resistance of the conductor is consequently very high and therefore, the conductor skin gets heated up much more than the conductor core. When a tree acts as a natural lightning conductor, due to skin effect most of the lightning currents flow through the skin of the tree and the sap wood. As a result, the skin gets burnt and may even peel off. The moisture in the skin and the sap wood evaporates instantaneously and may get split. If the tree struck by lightning is a teak tree (single stemmed with branches) it may not be completely destroyed since only the tree skin and a branch may be affected; the major parts of the tree may be saved from complete destruction due to lightning currents. But if the tree involved is a coconut tree it may be completely destroyed by the lightning currents.
Lightning Detection
Lightning discharges generate a wide range of electromagnetic radiations, including radio-frequency pulses. The times at which a pulse from a given lightning discharge arrive at several receivers can be used to locate the source of the discharge. The United States federal government has constructed a nation-wide grid of such lightning detectors, allowing lightning discharges to be tracked in real time throughout the continental U.S.
In addition to ground-based lightning detection, several instruments aboard satellites have been constructed to observe lightning distribution. These include the Optical Transient Detector (OTD) and the subsequent Lightning Imaging Sensor (LIS).
Records and locations
On average, lightning strikes the earth about 100 times every second. For most landmasses, lightning strikes most often during the summer, limiting the strike numbers. This is not the case in equatorial Africa, where summer is year-round, and lightning is a way of life. The spot with the most lightning lies deep in the mountains of eastern Democratic Republic of the Congo, near the small village of Kifuka which has an elevation of 3,200 feet (975 m). Thunderbolts pelt this land, and each year on average, 158 bolts occur over each square kilometer (equivalent to 10 city-blocks square). Singapore has one of the highest rates of lightning activity in the world. The city of Teresina in northern Brazil has the third-highest rate of occurrences of lightning strikes in the world. The surrounding region is referred to as the Chapada do Corisco ("Flash Lightning Flatlands").[68] In the US, Central Florida sees more lightning than any other area. For example, in what is called "Lightning Alley", an area from Tampa, to Orlando, there are as many as 50 strikes per square mile (about 20 per km²) per year. The Empire State Building is struck by lightning on average 23 times each year, and was once struck 8 times in 24 minutes.
Roy Sullivan held a Guinness World Record after surviving 7 different lightning strikes across 35 years.
In July 2007, lightning killed up to 30 people when it struck a remote mountain village Ushari Dara in northwestern Pakistan. Also, in Deerfield Beach, Florida lightning struck a diver's air tank as he surfaced off Florida's Atlantic coast, killing him. He surfaced about 10 metres (32.8 ft) from the boat, when lightning struck his tank.
Lightning can also strike indoor pools, directed into the pump by electrical circuits from outdoor power poles. Such strikes could potentially kill people who are swimming or walking on wet floors around a pool. In 2000, lightning killed two boys in an outdoor pool in Florida.
A single lightning strike can have a potential of a billion volts and deliver 100,000 amperes of current. If a bolt directly hits a marine animal swimming on the surface, it will undoubtedly hurt or kill the animal. Lightning strikes have killed or injured people on the surface more than 30 yards away.
When lightning strikes the ocean or other large water bodies, it spreads out over the conducting surface. Lightning also penetrates down into the water, and can kill fish in the nearby region. Fish down deep are safe.
Lightning, however, rarely strikes most of the open ocean, although some sea regions are lightning "hot spots." The Gulf Stream, for example, where fish abound, has as many lightning strikes as the southern plains of the USA. Winter storms passing off the east coast often erupt with electrical activity when they cross the warm waters of the Gulf Stream.
1 comment:
very informative... nvr knew there were different types of lightenin... nice article...
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