What is lightning?
Lightning is a gigantic electrostatic discharge (the same kind of electricity that can shock you
when you touch a doorknob) between the cloud and the ground, other clouds, or within a cloud.
Scientists do not
understand yet exactly how it works or how it interacts with th
atmosphere or the earth
's electromagnetic field.
Lightning is one of the oldest observed natural phenomena on earth. It has been seen in volcanic
eruptions, extremely intense forest f
ires, surface nuclear detonations, heavy snowstorms, in large
hurricanes, and obviously, thunderstorms.
What causes lightning?
The creation of lightning is a complicated process. We generally know what conditions are
needed to produce lightning, but
there is still debate about exactly how lightning forms.
way a cloud builds up the electrical charges that lead to lightning is not completely understood.
Precipitation and convection theories both attempt to explain the electrical structure with
Precipitation theorists suppose that different size raindrops, hail, and
get their positive or
negative charge as they collide, with heavier particles carryin
g negative charge to the cloud
bottom. Convection theorists believe that updrafts transport positive charges near the ground
upward through the cloud while downdrafts carry negative charges downward. What follows is a
summary of what we know.
ms have very turbulent environments
strong updrafts and downdrafts occur often
and close together. The updrafts carry small liquid water droplets from the lower regions of the
storm to heights between 35,000 and 70,000 feet
miles above the freezing lev
el. At the same
time, downdrafts are transporting hail and ice from the frozen upper parts of the storm. When
these particles collide, the water droplets freeze and release heat. This heat keeps the surface of
the hail and ice slightly warmer than its surr
ounding environment, and a soft hail, or graupel
When this graupel collides with additional water droplets and ice particles, a key process occurs
: negatively charged electrons are sheared off the rising particles and
collect on the falling particles. The result is a storm cloud that is negatively charged at its base,
and positively charged at the top.
Opposite charges attract one another. As the positive and negative areas grow more distinct
within the cloud, an electric field is created between the oppositely
charged thunderstorm base
and its top. The farther apart these regions are, the stronger the f
ield and the stronger the
attraction between the charges. But we cannot forget that the atmosphere is a very good insulator
that inhibits electric flow. So, a HUGE amount of charge has to build up before the strength of the
overpowers the atmosphere's insulating properties. A current of electricity forces a
path through the air until it encounters something that makes a good connection. The current is
d as a stroke of lightning.
While all this is happening inside the storm, beneath the storm, positive charge begins to pool
within the surface of the earth. This positive charge will shadow the storm wherever it goes, and
is responsible for cloud
und lightning. However, the electric field within the storm is much
stronger than the one between the storm base and the earth 's surface, so about 75
lightning occurs within the storm cloud.
There are two categories of ground flashes: natural (those that occur because of normal
electrification in the environment), and artificially initiated or triggered. Artificially initiated lightning
includes strikes to very tall structures, airplanes, rock
ets and towers on mountains. Triggered
lightning goes from ground to cloud, while "natural" lightning is cloud to ground.
Terms used to describe ground flashes include forked lightning, which shows branching to the
ground from a nearly vertical channel
; ribbon lightning, when the horizontal displacement of the
channel by the wind appears as a series of ribbons; and bead lightning, when the decaying
channel of a ground flash will sometimes break into a series of bright and dark spots. Ball
lightning is a
luminous sphere whose physics is not well understood.
ground lightning (CG's)
A channel of negative charge, called a
, will zigzag downward in roughly 50
segments in a forked pattern. This step leader is invisible to the human eye, and shoots to the
ground in less time than it takes to blink. As it nears the ground, the negatively charged step
leader is a
ttracted to a channel of positive charge reaching up, a
, normally through
something tall, such as a tree, house, or telephone pole. When the oppositely
streamer connect, a powerful electrical current begins flowing. A return stroke of bright luminosity
travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps
as many as 20 return strokes. We see lightning flicke
r when the process rapidly repeats itself
several times along the same path. The actual diameter of a lightning channel is one
ground flash is a negative stepped leader that travels downward through
the cloud, followed by an upward traveling return stroke. The net effect of this flash is to lower
negative charge from the cloud to the ground. Less common, a downward travel
leader followed by an upward return stroke will lower positive charge to earth. These positive
ground flashes now appear to be linked to certain severe storms and are the focus of intense
research by scientists