Lightning is an atmospheric discharge of electricity accompanied by thunder occurring between clouds, or between a cloud and the ground.
Lightning is a powerful electrical discharge produced during a thunderstorm. The electric current is very hot and causes the air around it to expand very quickly, which in turn makes thunder. Sometimes it happens between clouds. Sometimes (in the rain) it goes from cloud to ground. If it goes from cloud to ground, it can strike a person. Around 2000 people are struck by lightning each year. About 50 to 100 lightning bolts strike the Earth every second.
Benjamin Franklin was interested in lightning. He discovered many things about it, and in 1772, he was the first to show that a thunderstorm lets out electricity. In his book he suggested an experiment to test it. According to report, Franklin went out with his son, William, in 1752, to a meadow during a thunderstorm, flew a kite high in the air, and electricity came down the kite's wet string.
Lots of people tried to follow Benjamin Franklin's experiment, but it was very dangerous, and could sometimes kill people. One person tried to fly a kite like Benjamin Franklin, but if the reports are true, ball lightning hit him on the head and killed him.
Lightning is like the electricity you experience when you rub your foot on a rug and touch a doorknob. Your foot rubs electrons from the rug that travel through you to your finger tips, and a spark of electricity shoots from your finger to the doorknob. In a thunderstorm the lower cloud has as much as 100 million volts of electricity. This electricity is given out either within the cloud, to the ground, to another cloud, or into the air. Lightning has even been known to travel from the ground upwards to the cloud. In 1993 scientits discovered lightning bolts that shot upward from the top of a cumulonimbus cloud.
Topics of Interest
Lightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 60,000 m/s (130,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground. There are some 16 million lightning storms in the world 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, friction, 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.
The irrational fear of lightning (and thunder) is astraphobia.
Historical scientific research
Benjamin Franklin (1706–1790) endeavored to test the theory that sparks shared some similarity with lightning by using a spire which was being erected in Philadelphia. While waiting for completion of the spire, he got the idea to use 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, others attempted to replicate it. However, experiments involving lightning are always risky and frequently fatal. One of the most well-known deaths 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. According to reports, while the experiment was under way, ball lightning appeared and collided with Richmann's head, killing him.
Although experiments from the time of Benjamin 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. In 1900, Nikola Tesla generated artificial lightning by using a large Tesla coil, enabling the generation of enormous high frequency voltages sufficient to create lightning.
An average bolt of negative lightning carries an electric current of 30 kiloamperes (kA), and transfers a charge of five coulombs and 500 MJ of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs. The voltage is proportional to the length of the bolt.
Charge separation in a cloud appears to require strong updrafts which carry water droplets upward, supercooling them to between -10 and -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 the less heavy ice crystals upwards, causing the cloud top to accumulate increasing positive charge. Gravity causes the heavier negatively charged graupel to fall toward 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 a lightning discharge, which occurs when the distribution of positive and negative charges forms a sufficiently strong electric field.
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.
Lightning requires the electrical breakdown of a gas, so it cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus, Jupiter and Saturn. Lightning on Venus is still a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the 1970s and '80s, signals suggesting lightning may be present in the upper atmosphere were detected. However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all. Despite this, it has been suggested that radio pulses recorded by the spacecraft Venus Express may originate from lightning on Venus.
Trees and lightning
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. In sparsely populated areas such as the Far East and Siberia, lightning strikes are one of the major causes of forest fires. The smoke and mist expelled by a forest fire can cause electric charges, multiplying the intensity of a forest fire. 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.
The two most frequently struck tree types are the oak and the elm. Pine trees are 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.
Lightning and Sound
Because the electrostatic discharge of terrestrial lightning superheats the air to plasma temperatures along the length of the discharge channel in a short duration, kinetic theory dictates gaseous molecules undergo a rapid increase in pressure and thus expand outward from the lightning creating a shock wave audible as thunder. Since the sound waves propagate not from a single point source but along the length of the lightning's path, the sound origin's varying distances from the observer can generate a rolling or rumbling effect. Perception of the sonic characteristics are further complicated by factors such as the irregular and possibly branching geometry of the lightning channel, by acoustic echoing from terrain, and by the typically multiple-stroke characteristic of the lightning strike.
Since light travels at a significantly greater speed than sound through air, an observer can approximate the distance to the strike by timing the interval between the visible lightning and the audible thunder it generates. At standard atmospheric temperature and pressures near ground level, sound will travel at roughly 343 m/s (1125 ft/sec); a lightning flash preceding its thunder by five seconds would be about one mile distant. A flash preceding thunder by three seconds is about one kilometer distant.
The movement of electrical charges produces a magnetic field. The intense currents of a lightning discharge create a fleeting but very strong magnetic field. Where the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized. This effect is known as lightning-induced remanent magnetism, or LIRM. These currents follow the least resistive path, often horizontally near the surface but sometimes vertically, where faults, ore bodies, or ground water offers a less resistive path. Lightning-induced magnetic anomalies can be mapped in the ground, and analysis of magnetized materials can confirm lightning was the source of the magnetization and provide an estimate of the peak current of the lightning discharge.
The earliest detector invented to warn of the approach of a thunder storm was the lightning bell. Benjamin Franklin installed one such device in his house. The detector was based on an electrostatic device called the 'electric chimes' invented by Andrew Gordon in 1742.
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), aboard the OrbView-1 satellite launched on April 3, 1995, and the subsequent Lightning Imaging Sensor (LIS) aboard TRMM launched on November 28, 1997.
Notable lightning strikes
Some lightning strikes have caused either numerous fatalities or great damage. The following is a partial list:
- A particularly deadly lightning incident occurred in Brescia, Italy in 1769. Lightning struck the Church of St. Nazaire, igniting the 100 tons of gunpowder in its vaults; the resulting explosion killed 3000 people and destroyed a sixth of the city.
- 1902: A lightning strike damaged the upper section of the Eiffel Tower, requiring the reconstruction of its top
- December 8, 1963: Pan Am Flight 214 crashed as result of a lightning strike, and 81 people were killed.
- July 1970, the central mast of the Orlunda radio transmitter collapsed after a lightning strike destroyed its basement insulator.
- December 24, 1971: LANSA Flight 508 crashed as a result of lightning in Peru, with 91 people killed.
- November 2, 1994, lightning struck fuel tanks in Dronka, Egypt and caused 469 fatalities.
Harvesting lightning energy
Since the late 1980s there have been several attempts to investigate the possibility of harvesting energy from lightning. While a single bolt of lightning carries a relatively large amount of energy, this energy is concentrated in a small location and is passed during an extremely short period of time (milliseconds); therefore, extremely high electrical power is involved. It has been proposed that the energy contained in lightning be used to generate hydrogen from water, or to harness the energy from rapid heating of water due to lightning.
A technology capable of harvesting lightning energy would need to be able to capture rapidly the high power involved in a lightning bolt. Several schemes have been proposed, but the low energy involved in each lightning bolt render lightning power harvesting from ground based lightning rods as impractical. According to Northeastern University physicists Stephen Reucroft and John Swain, a lightning bolt carries a few million joules of energy, enough to power a 100-watt bulb for 5.5 hours. Additionally, lightning is sporadic, and therefore energy would have to be collected and stored; it is difficult to convert high-voltage electrical power to the lower-voltage power that can be stored.
In the summer of 2007, an alternative energy company called Alternate Energy Holdings (AEH) tested a method for capturing the energy in lightning bolts. The design for the system had been purchased from an Illinois inventor named Steve LeRoy, who had reportedly been able to power a 60-watt light bulb for 20 minutes using the energy captured from a small flash of artificial lightning. The method involved a tower, a means of shunting off a large portion of the incoming energy, and a capacitor to store the rest. According to Donald Gillispie, CEO of AEH, they "couldn't make it work," although "given enough time and money, you could probably scale this thing up... it's not black magic; it's truly math and science, and it could happen."
According to Dr. Martin A. Uman, co-director of the Lightning Research Laboratory at the University of Florida and a leading authority on lightning, a single lighting strike, while fast and bright, contains very little energy, and dozens of lighting towers like those used in the system tested by AEH would be needed to operate five 100-watt light bulbs for the course of a year. When interviewed by The New York Times, he stated that the energy in a thunderstorm is comparable to that of an atomic bomb, but trying to harvest the energy of lightning from the ground is "hopeless".
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