Lightning Terminology:
The Lightning Dictionary.
Anvil Lightning- Often called a 'bolt from the blue' because it occurs in seemingly cloudless skies, a cloud-to-ground discharge reaching from the top of a thunderstorm and arcing away from the main cloud and striking the ground under sometimes clear skies. It is dangerous because it fools people into thinking the lightning danger is gone because the storm is not overhead. Watch this animated image. The long bolt that strikes to the left of the storm cloud is an example of anvil lightning.
Arc- Large electric current flowing through air, visible as an intensely bright, hot light. Arcs are similar to lightning in that the current flows through a channel of ionized air.
Ball Lightning- A rare phenomenon described as a floating, illuminated sphere that occurs during thunderstorms. It may move fast, slow or stay stationary, it may be quiet or produce a hissing or crackling noise, it may pass through windows, last from seconds to minutes, and disappear slowly or suddenly either quietly or with a loud bang. No photographs have ever been obtained of ball lightning, nor have any official accounts been made of it, therefore its existence is questionable. The only source of information on ball lightning comes from eyewitness accounts and no scientific information has been obtained, so ball lightning remains a mystery.
Branches- The illuminated parts of the stepped leader that are not part of the main channel. They are only visible during the first return stroke, and are somewhat dimmer than the main channel.
Channel- The path of ionized air in which the discharge current flows. It is illuminated brightly during the discharge.
Cloud-to-Air- Referring to a discharge from a cloud into a pocket of charge in the surrounding air.
Cloud-to-Cloud- Referring to a discharge between two seperate thunderstorms. Not to be confused with Intracoud.
Cloud-to-Ground- Referring to a discharge between cloud and ground initiated by a downward moving stepped leader.
Discharge- The flow of current down the ionized channel that equalizes the charge difference between two regions of opposite charge.
Fault- Another word for a short circuit.
Flash- Referring to a lightning discharge. (See Stroke)
Flashover- A short circuit of an electrical power line in the form of a bright arc.
Fulgurite- Brittle, glassy formations caused by a lightning strike to sandy soil. The lightning heats the soil and fuses the soil particles together surrounding the path of the channel, resulting in a hollow tube-like formation shaped like the section of lightning that formed it. Artificial fulgurites have been created using man-made lightning in laboratories. The word fulgurite comes from the Greek word fulgur, which means lightning.
Ground-to-Cloud- Referring to a discharge between cloud and ground initiated by an upward moving stepped leader.
Heat Lightning- Distant flashes of lightning barely visible on the horizon from faraway thunderstorms. Named so because it is often seen on hot, muggy nights, when conditions are favorable for thunderstorm development.
Intracloud- Referring to a discharge within a cloud, the most common type. The channel is normally obscured from view, and the discharge appears to the observer as a sheet of light in the sky, therefore it is often called Sheet Lightning. (See Sheet Lightning.)
Ionization- The process by which air becomes conductive. It is caused by a tremendous charge difference between two regions of opposite charge, and in the case of lightning it is the process that starts the discharge. The electrons in the negatively charged region are so strongly attracted to the positively charged region that they begin to move through the air towards the opposing charge and create a conductive channel. The ionized channel is a conducting path for the lightning discharge. Also called electrical breakdown. (See Leader, Stepped leader)
Leader- A forming channel of ionized air moving towards the opposing charge. It can move in steps and branch out, (stepped leader) or move continuously in a single path. (See Stepped Leader, Ionization)
Lightning Rod (Lightning Protection System)- A lightning protection device designed to intercept a strike and divert it safely to ground, avoiding structural damage to buildings, boats, and other vulnerable objects. A lightning rod system consists of metallic rods, heavy-duty cable, and a solid grounding terminal. The cable connects the lightning rods to ground. Lightning rods do not attract lightning, they simply provide a safe path for the lightning current to flow.
Power Flash- Bright electrical arcs from power lines, transformers, or other electrical equipment, usually used to describe those caused by lightning, tornadoes, high winds, or winter weather.
Return Stroke- The flow of current (discharge) through a ionized channel. The return stroke is brightly illuminated and is the source of thunder. Many times there will be more than one return stroke through the same channel , making the lightning seem to flicker. (See Streamers)
Sferics- Radio waves produced by a lightning discharge. They can be heard with an A.M. radio.
Sheet Lightning- Name for the sheet of light in the sky from an intracloud discharge.
Shock Wave- The rapid expansion if air caused by the sudden and extreme heating of the air in a lightning channel during a return stroke. The shock wave continues outward for a few hundred yards, moving faster than the speed of sound, and then slows to a sound wave, heard as thunder. The shock wave from an extremely close lightning strike can knock a person off his/her feet, and cause hearing damage and/or other injury. These shock waves can also damage objects directly struck or nearby objects. (See Thunder)
Sprites and Jets- Electrical discharges that occur high above active thunderstorms. They have been found to occur in conjunction with and/or as a reaction to a normal cloud-to-ground lightning discharge. They are swift and faintly lit, making them almost invisible to the naked eye. Observations of sprites and jets have been made by placing a telescope on a high mountain and aiming it above thunderstorms occuring hundreds of miles away. For more information and photos on sprites and jets.
St. Elmo's Fire- Name given to the blue-green glow around objects (particularly on airplanes or the masts of ships) dring a thunderstorm. St Elmo's fire is thought to be a form of corona discharge caused by the high electrical potential in a thunderstorm.
Stepped Leader- Name for the downward moving action of electrical breakdown that propagates from the base of the cloud towards the ground. It splits into more and more branches as it moves downward, and the branch that reaches an upward-moving leader from an object on the ground first becomes the path for the return stroke. The stepped leader is named so because its propagation moves in in steps, moving through the air in short bursts. The stepped leader illuminates dimly after each 'step', but it is not visible because the entire process occurs so fast and so close to the bright return stroke that the human eye cannot see it. However, it has been photographed with a streak camera. Stepped leaders can also start from the ground and move upward. (See Streak Camera, Ionization)
Streak Camera- a special camera used by researchers that can take photographs of several special lightning features on one piece of film. It works by moving a length of film rapidly behind the camera lens with high-speed motors, allowing each step of a lightning flash to be recorded side-by-side on the same photograph. Streak cameras have been used to photograph stepped leaders and multiple return strokes.
Streamers- small intracloud discharges that accompany a larger lightning flash that 'feed' additional charge to the larger flash channel, initiating another return stroke along the larger channel. This process can repeat many times and is the cause of a lightning flash appearing to 'flicker' on ond off.
Strike Point- referring to the object on the ground that was the location of the termination of the lightning channel. In layman's terms, it's what the bolt hit.
Stroke- referring to the flow of current through a lightning channel.
Thunder- The sound waves produced by the explosive heating of the air in the lightning channel during a return stroke. It originates as shock waves close to the channel, and moves radially away from the channel. Thunder changes in pitch with varying distances from the channel. The closer one is to the lightning flash, the more high-pitched and 'crackle-sounding' the thunder. The further away, the more low-pitched and 'boom-rumble' sounding it is. Thunder rumbles and crackles because the lightning channel is crooked and jagged, causing the sound waves to arrive at the hearer at different times and directions. If lightning strikes closer than around 300 feet, the observer will hear one loud, startling, high-pitched bang which is not 'sound wave' thunder, but the shock wave, sometimes preceded by a faint crackling noise from a yet to be determined source. (See Shock Wave)
Voltage Gradient- The surge of voltage through the ground raidially outward from the lightning strike point. The voltage gradient can electrocute anyone standing on the ground close to a lightning strike. This is often how lightning injuries occur to people or animals who are near a lightning strike but are not directly hit. The reason that standing under or near a tree during a storm is dangerous is due mainly to the voltage gradient. Even if the main lightning channel flows entirely through or along the tree and does not jump over and hit whoever is standing there, the resulting huge surge of current through the ground surrounding the tree will give a nasty shock to anyone touching the ground nearby. The reason the term 'gradient' is used is because the voltage in the ground is lower with increasing distance from the strike point.

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