-Lightning-

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~Lightning~

• Lightning is an atmospheric discharge of electricity which usually accompanied by thunder that occurs during thunderstorms and sometimes during volcanic eruptions or dust storms.
  

• A leader of a bolt of lightning can travel at speeds of 60,000 m/s or 130,000 mph. The temperature can reach up to 30,000 degree Celsius or 54,000 Fahrenheit.

• There are some 16 million lightning storms in the world every year. It can also occur within the ash clouds from volcanic eruptions. Besides, it can be also caused by violent forest fires which generate sufficient dust to create a static charge.

• Fear of lightning or thunder is astraphobia.

• Scientist have studied root causes lightning ranging from atmospheric perturbations which are the wind, humidity, friction and atmospheric pressure to the impact of solar wind and accumulation of charged solar particles.

• Moreover, the ice inside the cloud might be a key element in lightning development that may cause a forcible separation of positive and negative charges within the cloud and thus forming lightning.

• Charge separation 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.

• The mechanism by which charge separation happens is still the subject of research. Another hypothesis is the polarization mechanism, which has two components:

1. Falling droplets of ice and rain become electrically polarized as they fall through the Earth's natural electric field;
2. Colliding ice particles become charged by electrostatic induction (see above).

There are several additional hypotheses for the origin of charge separation.

Illustration of a negative streamer (blue) meeting a positive counterpart (red) and the return stroke

Slow-motion footage of lightning

• As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.

• An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as leaders. The negatively charged leaders, generally a "stepped leader", proceed downward in a number of quick jumps (steps). Each step is on the order of 50 to 100 ft (15 to 30 meters) long but may be up to 165 ft (50 m).

• As it continues to descend, the stepped leader may branch into a number of paths. 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 when compared with the subsequent lightning channel.

• When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric fields. The electric field is strongest on ground-connected objects whose tops are closest to the base of the thundercloud, such as 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.

• Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.

There are few types of lightning

Cloud-to-ground lightning

This 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 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

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

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 a cloud to ground lightning strike which is a short-duration stroke that appears as a single very bright flash and often has considerable branching.

Forked lightning

Forked lightning is a name, not in formal usage, for cloud-to-ground lightning that exhibits branching called forked lighting.

Ground-to-cloud lightning

Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud initiated by an upward-moving leader stroke. It is much rarer than cloud-to-ground lightning. This type of lightning forms when negatively charged ions called the stepped leader rises up from the ground and meets the positively charged ions in a cumulonimbus cloud. Then the strike goes back to the ground as the return stroke.

Cloud-to-cloud lightning

Multiple paths of cloud-to-cloud lightning, Swifts Creek, Australia

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.

Sheet lightning

Sheet lightning is an informally applied name to cloud-to-cloud lightning that exhibits a diffuse brightening of the surface of a cloud caused by the actual discharge path being hidden.

Heat lightning

Heat lightning occurs too far away for the thunder to be heard. This occurs because the lightning occurs very far away and the sound waves dissipate before they reach the observer.

Dry lightning

Dry lightning is a term in the United States for lightning that occurs with no precipitation at the surface. Pyrocumulus cloud produce lightning for the same reason that it is produced by cumulonimbus clouds. When the higher levels of the atmosphere are cooler, and the surface is warmed to extreme temperatures due to a wildfire, volcano, etc, convection will occur, and the convection produces lightning. Therefore, fire can beget dry lightning through the development of more dry thunderstorms which cause more fires.

Rocket lightning

It 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.

Positive lightning

Positive lightning is a type of lightning strike that comes from apparently clear or only slightly cloudy skies; they are also known as "bolts from the blue" because of this trait. The positive charge is carried by the top of the clouds (generally anvil clouds) rather than the ground. It makes up less than 5% of all lightning strike. 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.

Ball lightning

A photo purportedly depicting natural ball lightning, taken in 1987 by a student in Nagano, Japan

Ball lightning may be an atmospheric electrical phenomenon, the physical nature of which is still controversial. The term refers to reports of luminous, usually sperical objects which vary from pea-sized to several meters in diameter. It is sometimes associated with thunderstorms, but unlike lightning flashes, which last only a fraction of a second, ball lightning reportedly lasts many seconds.