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Amazing Lightning Pictures Definition

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Lightning is a massive electrostatic discharge between electrically charged regions within clouds, or between a cloud and the Earth's surface. The charged regions within the atmosphere temporarily equalize themselves through a lightning flash, commonly referred to as a strike if it hits an object on the ground. There are three primary types; from a cloud to itself (intra-cloud or IC); from one cloud to another cloud (CC) and finally between a cloud and the ground (CG). Although lightning is always accompanied by the sound of thunder, distant lightning may be seen but be too far away for the thunder to be heard.

Lightning occurs approximately 40–50 times a second worldwide, resulting in nearly 1.4 billion flashes per year.[1]

Many factors affect the frequency, distribution, strength, and physical properties of a "typical" lightning flash to a particular region of the world. These factors include ground elevation, latitude, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC, CC and CG lightning may also vary by season in middle latitudes.
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Sound of a thunderstorm

Because human beings are terrestrial and most of their possessions are on the Earth, where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types, even though IC and CC are more common. Lightning's relative unpredictability limits a complete explanation of how or why it occurs, even after hundreds of years of scientific investigation. A typical cloud to ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km (3 mi) tall, from within the cloud to the ground's surface. The actual discharge is the final stage of a very complex process.[2] A typical thunderstorm has three or more strikes to the Earth per minute at its peak.[3]
Four-second video of a lightning strike, Island in the Sky, Canyonlands National Park, Utah, United States.

Lightning primarily occurs when warm air is mixed with colder air masses resulting in atmospheric disturbances necessary for polarizing the atmosphere. However, it can also occur during dust storms, forest fires, tornadoes, volcanic eruptions, and even in the cold of winter, where the lightning is known as thundersnow.[4][5] Hurricanes typically generate some lightning, mainly in the rainbands as much as 160 km (100 mi) from the center.[6][7][8]

The science of lightning is called fulminology. The fear of lightning is called astraphobia.
Establishing conditions necessary for lightning

In order for an electrostatic discharge to occur, two things are necessary: 1) a sufficiently high electric potential between two regions of space must exist; and 2) a high-resistance medium must obstruct the free, unimpeded equalization of the opposite charges.

    It is well understood the thunderstorm is able to separate and aggregate charges in certain regions of the cloud, however the exact processes by which this occurs are not fully understood;[17][18]

        Main article: thunderstorm

    The atmosphere provides the electrical insulation, or barrier, that prevents free equalization between charged regions of opposite polarity. This is overcome by "lightning", a complex process referred to as the lightning "flash". The details of a lightning flash will follow.

Establishing the electric field in CG lightning

    As a thundercloud moves over the surface of the Earth, an equal electric charge, but of opposite polarity, is induced on the Earth's surface underneath the cloud. The induced positive surface charge, when measured against a fixed point, will be small as the thundercloud approaches, increasing as the center of the storm arrives and dropping as the thundercloud passes. The referential value of the induced surface charge could be roughly represented as a bell curve.

    The oppositely charged regions create an electric field within the air between them. This electric field varies in relation to the strength of the surface charge on the base of the thundercloud – the greater the accumulated charge, the higher the electrical field.

View of lightning from an airplane flying above a system.
A lightning flash or strike: a very complex process

The best studied and understood form of lightning is cloud to ground (CG). Although more common, intracloud (IC) and cloud to cloud (CC) flashes are very difficult to study given there are no "physical" points to monitor inside the clouds. Also, given the very low probability lightning will strike the same point repeatedly and consistently, scientific inquiry is difficult at best even in the areas of high CG frequency. As such, knowing flash propagation is similar amongst all forms of lightning, the best means to describe the process is through an examination of the most studied form, Cloud to Ground.
Downward leader formation for negative CG lightning
A downward leader travels towards earth, branching as it goes.

In a process not well understood, a channel of ionized air, called a leader, is initiated from a negatively charged region in the thundercloud. Leaders are electrically conductive channels of partially ionized gas that travel away from a region of dense charge. Negative leaders propagate away from densely-charged region of negative charge, and positive leaders propagate from positively-charged regions.

The positively and negatively charged leaders proceed in opposite directions, positive upwards within the cloud, the negative towards the earth. Both ionic channels proceed, in their respective directions, in a number of successive spurts, similar to two kids hopping randomly in opposite directions. Each leader "pools" ions at the leading tips, shooting out one or more new leaders, momentarily pooling again to concentrate charged ions, then shooting out another leader.

Leaders often split, forming branches in a fashion similar to a tree.[19] In addition, negative leaders travel in a discontinuous fashion. The resulting jerky movement of these "stepped leader(s)" can be readily observed in slow-motion videos of negative leaders as they head toward ground prior to a negative CG lightning strike. The negative leaders continue to propagate and split as they head downward, often speeding up as they get closer to the Earth's surface.

About 90% of ionic channel lengths between "pools" are approximately 45 m (148 ft) in length.[20] The establishment of the ionic channel takes a comparatively long amount of time (hundreds of milliseconds) in comparison to resulting discharge which occurs within a few microseconds. The electric current needed to establish the channel, measured in the tens or hundreds of amperes, is dwarfed by subsequent currents during the actual discharge.

    Initiation of the outward leaders is not well understood. The electric field strength within the thundercloud is not typically large enough to initiate this process by itself.[21] Many hypotheses have been proposed. A promising theory proposes that showers of relativistic electrons are created by cosmic rays. The electrons are then accelerated to even higher velocities via a process called runaway breakdown. As these relativistic electrons collide and ionize neutral air molecules, they initiate leader formation. Another theory invokes locally enhanced electric fields being formed near elongated water droplets or ice crystals.[22] Percolation theory, especially for the case of biased percolation,[23] describe random connectivity phenomena, which produce an evolution of connected structures similar to that of lightning strikes.

Upward streamers
A lightning flash terminates [& discharge occurs] on a tree while an un-attached streamer is visible on the earth surface projection to the left.

When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. The electric field is strongest on grounded 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 positively charged ionic channel, called a positive or upward streamer, can develop from these points. This was first theorized by Heinz Kasemir.[24][25]

As negatively charged leaders approach, increasing the localized electric field strength, grounded objects already experiencing corona discharge exceed a threshold and form upward streamers.