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 the
following animation. The long bolt that strikes to the left of the storm cloud is an
example of anvil lightning:
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 Intracloud.
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.
Flash- Referring to a lightning discharge. (See Stroke)
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).
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 in 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)
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.