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Lightning

Lightning is a massive electrostatic discharge caused by unbalanced electric charge in the atmosphere, either inside clouds, cloud to cloud or cloud to ground, accompanied by the loud sound of thunder.

A typical cloud to ground lightning strike can be over 5 km (3 mi) long.A typical thunderstorm may have three or more strikes per minute at its peak.Lightning is usually produced by cumulonimbus clouds up to 15 km high (10 mi) high, based 5-6 km (3-4 mi) above the ground. Lightning is caused by the circulation of warm moisture-filled air through electric fields.Ice or water particles then accumulate charge as in a Van de Graaf generator.Lightning may occur during snow storms (thundersnow), volcanic eruptions, dust storms, forest fires or tornadoes.Hurricanes typically generate some lightning, mainly in the rainbands as much as 160 km (100 mi) from the center.


When the local electric field exceeds the dielectric strength of damp air (about 3 million Volts/m), electrical discharge results, often followed by more discharges along the same path. Mechanisms that cause lightning are still a matter of scientific investigation.Fear of lightning is called astraphobia. The study or science of lightning is called fulminology.


Formation of Lightning
Initiation
Even assuming an electric field has been established, the mechanism by which the lightning discharge begins is not well known. Electric field measurements in thunderclouds are typically not large enough to directly initiate a discharge.Many hypotheses have been proposed, ranging from including runaway breakdown to locally enhanced electric fields near elongated water droplets or ice crystals.Percolation theory, especially for the case of biased percolation,describe random connectivity phenomena, which produce an evolution of connected structures similar to that of lightning strikes.

Discharge
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.

Re-strike
High speed videos (examined frame-by-frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 or 4 strokes, though there may be more.Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.Each successive stroke is preceded by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.[citation needed] The sound of thunder from a lightning strike is prolonged by successive strokes.

Types of Lightning

Some lightning strikes exhibit particular characteristics; scientists and the general public have given names to these various types of lightning. The lightning that is most-commonly observed is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. The majority of strokes occur inside a cloud so we do not see most of the individual return strokes during a thunderstorm.

Cloud-to-ground
Cloud-to-ground is the best known and second most common type of lightning. Of all the different types of lightning, it poses the greatest threat to life and property since it strikes the ground. Cloud-to-ground (CG) lightning is a lightning discharge between a cumulonimbus cloud and the ground. It is initiated by a leader stroke moving down from the cloud. 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 relatively 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 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 is a cloud-to-ground lightning (CG) strike which is a short-duration stroke that (often but not always) appears as a single very bright flash and often has considerable branching.These are often found in the visual vault area near the mesocyclone of rotating
thunderstorms and coincides with intensification of thunderstorm updrafts. A similar cloud-to-cloud strike consisting of a brief flash over a small area, appearing like a blip, also occurs in a similar area of rotating updrafts.

Ground-to-cloud
Ground to cloud lightning is an artificially initiated, or triggered, category of ground flashes. Triggered lightning goes from tall structures on the ground, such as towers on mountains, to clouds.
Local variations in cloud formations can cause the bottom of a cloud to accumulate a positive charge which will induce a negative charge on the ground. Lightning can occur with both positive and negative polarity. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs.[43] An average bolt of positive lightning carries an electric current of about 300 kA — about 10 times that of negative lightning. Unlike the far more common "negative" lightning, positive lightning occurs when a positive charge is carried by the top of
the clouds (generally anvil clouds) rather than the ground. Generally, this causes the leader arc to form in the anvil of the cumulonimbus and travel horizontally for several miles before veering down to meet the negatively charged streamer rising from the ground. The bolt can strike anywhere within several miles of the anvil of the thunderstorm, often in areas experiencing clear or only slightly cloudy skies; they are also known as "bolts from the blue" for this reason. Positive lightning makes up less than 5% of all lightning strikes. Because of the much greater distance they must travel before discharging, positive lightning strikes typically carry six to ten times the charge and voltage difference of a negative bolt and last around ten times longer.During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated. As a result of their greater power, as well as lack of warning, 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.[48] The standard in force at the time of the crash, Advisory Circular AC 20-53A, was replaced by Advisory Circular AC 20-53B in 2006,however it is unclear whether adequate protection against positive lightning was incorporated

Cloud-to-cloud
Lightning discharges may occur between areas of cloud without contacting the ground. When it occurs between two separate clouds it is known as inter-cloud lightning, and when it occurs between areas of differing electric potential within a single cloud it is known as intra-cloud lightning. Intra-cloud lightning is the most frequently occurring type. These are most common between the upper anvil portion 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 hearing any thunder. The "heat" portion of the term is a folk association between locally experienced warmth and the distant lightning flashes. Another terminology used for cloud–cloud or cloud–cloud–ground lightning is "Anvil Crawler", due to the habit of the charge typically originating from beneath or within the anvil and scrambling through the upper cloud layers of a thunderstorm, normally generating multiple branch strokes which are dramatic to witness. These are usually seen as a thunderstorm
passes over the observer or begins to decay. The most vivid crawler behavior occurs in well developed thunderstorms that feature extensive rear anvil shearing. Sheet lightning is an informal name for cloud-to-cloud lightning that exhibits a diffuse brightening of the surface of a cloud, caused by the actual discharge path being hidden or too far away. The lightning itself cannot be seen by the spectator, so it appears as only a flash, or a sheet of light. The lightning may be too far away to discern individual flashes.

Effects of Lightning

Lightning is the world's most underrated weather hazard. It's also the most unpredictable. When it comes to lethal weather, lightning is hard to beat. On average only floods kill more people than lightning. In the United States (and most other places), lightning routinely kills more people every year than tornadoes or hurricanes. Other weather hazards, such as hailstorms and windstorms, aren't even in the running.

Dangers of Lightning

  • It's hard to know just when and where it is likely to strike—or how it will behave when it does.
  • Lightning is so dangerous is because of the destructive power it carries. The average lightning bolt carries about 30,000 amps of charge, has 100 million volts of electric potential, and is hot, hot, hot at about 50,000 degrees Fahrenheit.
  • Lightning makes every thunderstorm a potential killer, whether the storm produces one lightning bolt or 10,000.
  • Lightning has been seen during volcanic eruptions and extremely intense forest fires. It has also occurred during hurricanes and heavy snowstorms. Lightning has even been seen during surface nuclear detonations.
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