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 , and can reach temperatures approaching , 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.
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.
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, United States. 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. He was 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 Georg 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 enormously high voltages sufficient to create lightning.
Lightning can occur with both positive and negative polarity. An average bolt of negative lightning carries an electric current of 30,000 amperes ("amps") — 30 "kiloamps" (kA), and transfers five coulombs of electric charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs. An average bolt of positive lightning carries an electric current of about 300 kA — about 10 times that of negative lightning.
The voltage involved for both is proportional to the length of the bolt. However, lightning leader development is not just a matter of the electrical breakdown of air, which is about 3 megavolts per meter (MV/m). The ambient electric fields required for lightning leader propagation can be one or two orders of magnitude (10−2) less than the electrical breakdown strength. The potential ("voltage") gradient inside a well-developed return-stroke channel is on the order of hundreds of volts per meter (V/m) due to intense channel ionization, resulting in a true power output on the order of one megawatt per meter (MW/m) for a vigorous return stroke current of 100 kA. The average peak power output of a single lightning stroke is about one trillion watts — one "terawatt" (1012 W), and the stroke lasts for about 30 millionths of a second — 30 "microseconds".
Lightning rapidly heats the air in its immediate vicinity to about 20,000 °C (36,000 °F) — about three times the temperature of the surface of the Sun. The sudden heating effect and the expansion of heated air gives rise to a supersonic shock wave in the surrounding clear air. It is this shock wave, once it decays to an acoustic wave, that is heard as thunder.
The return stroke of a lightning bolt follows a charge channel about a centimeter (0.4 in) wide.
Different locations have different potentials ("voltages") and currents for an average lightning strike. In the United States, for example, Florida experiences the largest number of recorded strikes in a given period during the summer season , has very sandy soils in some areas, and electrically conductive water-saturated soils 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.
NASA scientists have found that electromagnetic radiation created by lightning in clouds only a few miles high can create a "safe zone" in the Van Allen radiation belts that surround the earth. This zone, known as the "Van Allen Belt slot", may be a safe haven for satellites in "middle Earth orbits" (MEOs), protecting them from the Sun's intense radiation.