S
Steve Wilson
Guest
On September 13, I gave a short reply to JL on the iniation of a
lightning stroke. Here is the text:
A stepped leader starts in the cloud and reaches towards the earth. When
it
gets close, a streamer reaches up from the earth. When it reaches the
leader, a conductive path is formed that allows the discharge from the
cloud to the ground. Here are some videos in slow motion:
Sprites, Jets, and Glowing Balls: The Science of Lightning
https://www.youtube.com/watch?v=fzNk4w2k2h0
How does lightning work? Where does it come from? | Weather Wise S2E2
https://www.youtube.com/watch?v=9K-v-RJ-z2A
Lightning Strike at 103,000 FPS
- in Singapore
https://www.youtube.com/watch?v=qQKhIK4pvYo
Lightning in Super Slow Motion
https://www.youtube.com/watch?v=RLWIBrweSU8
This left the question what is a stepped leader? I came across an article
that gives a good description. I\'ll give the text here, but check out the
link. It has good pictures that explain better.
\"Inside a thunderhead, electrical charges become separated. Warm
updrafts sweep positive charges aloft, leaving the bottom of the
cloud negatively charged. The attraction between the ground and the
negative charges in the bottom of the cloud creates the lightning
stroke, a brief current of negative charge that travels from cloud
to ground.
The awesome power of the lightning stroke originates in the
thunderstorm cloud where charges somehow become separated. There are
several complicated theories that try to explain the actual
mechanism of this charge separation, but no one really knows what
pulls the charges apart in a thunderstorm cloud. It is believed that
somehow water drops in the cloud become negatively charged and,
being heavier than the surrounding air, fall to the bottom of the
cloud. Meanwhile, the positive ions left behind are swept upward to
the top of the cloud by the warm updrafts within the thunderhead. As
more and more charges separate, parts of the cloud become so highly
charged that the electrical forces tear nearby air molecules apart,
making more charged fragments.
Since the ground beneath the cloud has far fewer negative charges on
it than the bottom of the cloud, there is an attraction between the
ground and the bottom of the cloud. Therefore, any electrons
liberated near the cloud are pulled down toward the ground. As these
electrons move, they bash into air molecules that are in their way,
breaking the molecules up and creating more charged fragments. All
the new negative fragments are dragged downward along with the
original electrons and we have the makings of an electrical
avalanche.
The avalanche would continue unabated were it not for the heavier
and more sluggish positive charges that are left behind. They tend
to attract the accelerating army of electrons back toward the cloud.
But more electrons are continually being liberated up in the cloud,
and they stream to the rescue of the slowing electrons below,
reinforcing their race downward. This process of electrons slowing
and then being rescued by reinforcements repeats itself over and
over again.
Stepped Leader
The initial party of electrons makes its way in jerky 150-foot steps
along a sinuous path toward the ground.
This initial exploratory mission forms what is called a \"stepped
leader,\" named for its start-stop motion. The stepped leader takes
about 5/1000 of a second, moving at about 240 miles per second, to
reach from cloud to ground. When the leader gets near the ground, it
may draw a stream of positive charges (called a streamer) up from
the ground to meet it. When either the stepped leader reaches the
ground or a streamer runs up to join the stepped leader, an
electrical connection is completed between the cloud and the ground.
The ionized air molecules of the leader conduct electricity quite
well, and the path of charged particles acts as a wire, connecting
the highly negative cloud and the positive ground. This ionized air
becomes the path of the main bolt of lightning.
The first charges to feel the connection are those near the ground.
The light and mobile negative charges quickly accelerate along the
wire of ionized air. In their mad rush to the ground the negative
charges collide with the air, causing it to glow like a neon
sign--only thousands of times brighter and with a bluish-white
color. The air near the ground is the first to start glowing, but as
the electrons further and further up feel the connection and begin
to accelerate, the air further and further up also starts to glow.
Even though the negative charges all move from cloud to ground, the
bright flash of lightning moves from ground to cloud in a speedy
1/10,000 of a second, moving 61,000 miles per second! The
super-heated air expands outward explosively, producing the shock
wave we hear as thunder. The bright flash of glowing air is called
the return stroke since it moves from ground to cloud, opposite to
the moving charges.
The return stroke discharges a region of the cloud, but the cloud
can reorganize quickly and as many as 40 strokes have been observed
to use the same charged channel. If you\'ve been told that lightning
never strikes twice in the same place, don\'t believe it! Lightning
usually strikes more than once!
There is quite a lot of energy in a lightning stroke, about 250
kilowatt-hours. At the current cost of energy, this would be about
$16.75 worth. Doesn\'t sound like much, but with that amount of
energy, you could lift a 2000-pound car 62 miles high!
Lightning doesn\'t always travel from cloud to ground. If two parts
of the cloud are charged highly (and oppositely), a lightning bolt
can actually occur inside the cloud. Lightning can also arc from one
cloud to another.
lightning strike
The typical type of lightning is called streak lightning, or forked
lightning. (Photo to the right from NASA - Marshall Space Flight
Center.) If the lightning channel is blown by the wind during a
multiple discharge, each succeeding stroke is displaced by a short
distance, making it appear as ribbon lightning. On rare occasions
the lightning seems to break up into beads that persist for as long
as one second, an unexplained form called bead or chain lightning.
Sometimes the lightning flash is obscured by clouds, which are then
brightly illuminated. During this sheet lightning, the flash seems
to come from everywhere. The most controversial form of lightning is
ball lightning. Ball lightning has never been observed
scientifically and many doubt its existence altogether. It is
reported to occur with or right after a nearby lightning stroke and
is described as a luminous ball of light that floats along fences,
rooftops, or through the open air. The jury is still out on ball
lightning.
https://www.exploratorium.edu/ronh/weather/weather.html
lightning stroke. Here is the text:
A stepped leader starts in the cloud and reaches towards the earth. When
it
gets close, a streamer reaches up from the earth. When it reaches the
leader, a conductive path is formed that allows the discharge from the
cloud to the ground. Here are some videos in slow motion:
Sprites, Jets, and Glowing Balls: The Science of Lightning
https://www.youtube.com/watch?v=fzNk4w2k2h0
How does lightning work? Where does it come from? | Weather Wise S2E2
https://www.youtube.com/watch?v=9K-v-RJ-z2A
Lightning Strike at 103,000 FPS
- in Singapore
https://www.youtube.com/watch?v=qQKhIK4pvYo
Lightning in Super Slow Motion
https://www.youtube.com/watch?v=RLWIBrweSU8
This left the question what is a stepped leader? I came across an article
that gives a good description. I\'ll give the text here, but check out the
link. It has good pictures that explain better.
\"Inside a thunderhead, electrical charges become separated. Warm
updrafts sweep positive charges aloft, leaving the bottom of the
cloud negatively charged. The attraction between the ground and the
negative charges in the bottom of the cloud creates the lightning
stroke, a brief current of negative charge that travels from cloud
to ground.
The awesome power of the lightning stroke originates in the
thunderstorm cloud where charges somehow become separated. There are
several complicated theories that try to explain the actual
mechanism of this charge separation, but no one really knows what
pulls the charges apart in a thunderstorm cloud. It is believed that
somehow water drops in the cloud become negatively charged and,
being heavier than the surrounding air, fall to the bottom of the
cloud. Meanwhile, the positive ions left behind are swept upward to
the top of the cloud by the warm updrafts within the thunderhead. As
more and more charges separate, parts of the cloud become so highly
charged that the electrical forces tear nearby air molecules apart,
making more charged fragments.
Since the ground beneath the cloud has far fewer negative charges on
it than the bottom of the cloud, there is an attraction between the
ground and the bottom of the cloud. Therefore, any electrons
liberated near the cloud are pulled down toward the ground. As these
electrons move, they bash into air molecules that are in their way,
breaking the molecules up and creating more charged fragments. All
the new negative fragments are dragged downward along with the
original electrons and we have the makings of an electrical
avalanche.
The avalanche would continue unabated were it not for the heavier
and more sluggish positive charges that are left behind. They tend
to attract the accelerating army of electrons back toward the cloud.
But more electrons are continually being liberated up in the cloud,
and they stream to the rescue of the slowing electrons below,
reinforcing their race downward. This process of electrons slowing
and then being rescued by reinforcements repeats itself over and
over again.
Stepped Leader
The initial party of electrons makes its way in jerky 150-foot steps
along a sinuous path toward the ground.
This initial exploratory mission forms what is called a \"stepped
leader,\" named for its start-stop motion. The stepped leader takes
about 5/1000 of a second, moving at about 240 miles per second, to
reach from cloud to ground. When the leader gets near the ground, it
may draw a stream of positive charges (called a streamer) up from
the ground to meet it. When either the stepped leader reaches the
ground or a streamer runs up to join the stepped leader, an
electrical connection is completed between the cloud and the ground.
The ionized air molecules of the leader conduct electricity quite
well, and the path of charged particles acts as a wire, connecting
the highly negative cloud and the positive ground. This ionized air
becomes the path of the main bolt of lightning.
The first charges to feel the connection are those near the ground.
The light and mobile negative charges quickly accelerate along the
wire of ionized air. In their mad rush to the ground the negative
charges collide with the air, causing it to glow like a neon
sign--only thousands of times brighter and with a bluish-white
color. The air near the ground is the first to start glowing, but as
the electrons further and further up feel the connection and begin
to accelerate, the air further and further up also starts to glow.
Even though the negative charges all move from cloud to ground, the
bright flash of lightning moves from ground to cloud in a speedy
1/10,000 of a second, moving 61,000 miles per second! The
super-heated air expands outward explosively, producing the shock
wave we hear as thunder. The bright flash of glowing air is called
the return stroke since it moves from ground to cloud, opposite to
the moving charges.
The return stroke discharges a region of the cloud, but the cloud
can reorganize quickly and as many as 40 strokes have been observed
to use the same charged channel. If you\'ve been told that lightning
never strikes twice in the same place, don\'t believe it! Lightning
usually strikes more than once!
There is quite a lot of energy in a lightning stroke, about 250
kilowatt-hours. At the current cost of energy, this would be about
$16.75 worth. Doesn\'t sound like much, but with that amount of
energy, you could lift a 2000-pound car 62 miles high!
Lightning doesn\'t always travel from cloud to ground. If two parts
of the cloud are charged highly (and oppositely), a lightning bolt
can actually occur inside the cloud. Lightning can also arc from one
cloud to another.
lightning strike
The typical type of lightning is called streak lightning, or forked
lightning. (Photo to the right from NASA - Marshall Space Flight
Center.) If the lightning channel is blown by the wind during a
multiple discharge, each succeeding stroke is displaced by a short
distance, making it appear as ribbon lightning. On rare occasions
the lightning seems to break up into beads that persist for as long
as one second, an unexplained form called bead or chain lightning.
Sometimes the lightning flash is obscured by clouds, which are then
brightly illuminated. During this sheet lightning, the flash seems
to come from everywhere. The most controversial form of lightning is
ball lightning. Ball lightning has never been observed
scientifically and many doubt its existence altogether. It is
reported to occur with or right after a nearby lightning stroke and
is described as a luminous ball of light that floats along fences,
rooftops, or through the open air. The jury is still out on ball
lightning.
https://www.exploratorium.edu/ronh/weather/weather.html
