Lightning
• Lightning is a discharge of electrical energy.
• May occur:
– Between cells in the same storm (Inter-cloud
Lightning)
– Within a cloud (Intra-cloud Lightning)
– Cloud to Air
– Cloud to Ground (CG)
Intracloud (IC) vs. cloud to ground (CG)
World map showing frequency of total lightning strikes (IC +CG), in flashes per
km² per year. Lightning strikes most frequently in the Democratic Republic of
the Congo
IC vs. CG
Number of Lightning
Deaths in the United
States
Lightning Deaths in
the United States
Weighted by
Population
CG frequency in US
Positive CGS
Day vs. night
Lightning Type “Popularity”
-
-
-
+
+
-
-
+
+
+
+
-
+
+
+
Negative Cloud-to-Ground
(90 %)
-
-
+
-
Positive Cloud-to-Ground
(5 %)
Lightning Type “Popularity”
-
+
-
+
+
-
+
+
+
+
Positive Ground-to-Cloud
(3 %)
+
-
+
-
+
-
-
Negative Ground-to-Cloud
(2 %)
The atmosphere normally has a
voltage gradient of 100 volts/meter…
… which may
sound like a
lot, but what
happens when
you stand one
meter from a
110 volt outlet
?
It takes about
1,000,000 volts/meter
to cause a spark.
That’s 10,000 volts per
centimeter.
Lightning
• Air is a very good insulator.
• To have lightning:
– Need to have the charge centers very
close to each other
– Have very large differences in charge
“strength”
• In order to get lightning in a thunderstorm
you need to separate large amounts of
charge. How is this done?
Charging
by
Induction
Electrostatics
One way to explain the charge separation
Odd?
An alternative explanation
Graupel Method
• During a collision between heavy graupel
particles and lighter ice crystals:
– Negative charge is transferred to the
graupel
– Positive charge is transferred to the ice
crystals
– Graupel falls to the bottom of the cloud
bringing the negative charge with it
– Ice crystals are transported to the upper
levels of the thunderstorm
Takahashi 1978
Charge Separation
++ +++
- - - -- - - -+
+ + +
+
+
The strong negative charge
at the base of the thunderstorm
induces a positive charge at
the surface.
+
▬
+
Lightning (1)
Lightning (2)
Lightning (3)
Lightning (4)
Lightning (5)
Lightning (6)
The (CG) Lightning Stroke
• The lightning stroke begins when the electric
fields exceed breakdown voltage.
• Initially streams of electrons surge from the
cloud base toward the ground in steps of 50 to
100 m.
• Start and stop steps as the stepped leader
progresses toward ground.
Stepped Leader
• The stepped leader is:
–Very Faint
–Essentially invisible to the human
eye
–Produces an ionized channel that
will allow for the flow of charge
during the remainder of the
lightning stroke.
–Sometimes with branches
Stepped Leader
Stepped leader moving in ~50m steps.
© 1969 Martin Uman -- From: Uman, Lightning
The (CG) Lightning Stroke
• When the stepped leader gets near the
ground (~100 m or so):
– Positive charge moves from the ground up
toward the stepped leader -- these are
called streamers.
– The streamers may come from almost any
pointed object on the ground:
• Trees
Antennas
• Flagpoles
Telephone Poles
• Really Tall Towers
Etc.
Grass
People
Streamers
A streamer rising from a part of the
tree. The return stroke was created
when the stepped leader met with a
streamer from another part of the
tree.
© 1998 Oxford University Press -- From: MacGorman and Rust, The Electrical
Nature of Storms
Return Stroke
• One of the streamers will meet the stepped
leader -- not necessarily the one from the
tallest object!
• When they meet a pulse of energy flows up
toward the cloud (along the ionized path)
and toward the ground.
• This luminous pulse of electrical energy is
called the return stroke.
• Occurs very fast -- we see it as a flash!
Dart Leader
• Often a second series of stepped flow of
electrons moves from the cloud toward the
ground.
• Since the ionized channel already exists, the
stepped flow is much faster.
• This stepped flow is called the dart leader.
• Dart leader does not have branches
Return Stroke
• When the dart leader connects with a
streamer from the ground (usually along the
same path) another return stroke moves
toward the cloud and ground.
• May get several sets of dart leader/return
stroke pairs.
• Appears as if the lightning “flashes.”
Lightning Rods
• Lightning rods act as locations where streamers
can be launched toward descending stepped
leaders.
• It is better for the lightning rods to be struck,
where the electrical energy can be safely
transferred to ground, than the house.
• The “cone of safety” is about 45o under the
lightning rod.
Lightning
Rod
Simulator
Lightning striking the
Empire State Building
Thunder
• Lightning is very hot (~30 000C).
• Violently expanding air causes an audible
shock wave.
• Nearby Lightning
– Thunder sounds like a “crack” or a loud
bang.
• Distant Lightning
– Thunder sounds like a rumble. We are
hearing the sound from different parts of
the lightning channel.
Thunder
• How far is that lightning????
– Light travels much faster than sound.
– Count the time between when you saw the
flash and heard the thunder.
• 5 seconds per mile
• 3 seconds per km
• No thunder?
– Could be too far away
– Sound could be “bent” by the atmosphere
away from you.
Thunder
The man hears thunder from
the bottom of the lightning
channel in 6 seconds. He
will continue to hear the
thunder until sound from the
top of the channel (2.8 km
away) in 8.4 seconds.
The short sound of thunder
indicates a nearby strike.
2.8 km
2 km
2 km
Assignment #6