Lightning: An Overview
Developed By
Greg Seroka in a
Guest Lecture for ATMO 352
March 29, 2010
Worldwide Lightning
Distribution
-about 100 strikes per second on planet
-most flashes occur over land
NASA
U.S. Lightning Distribution
• Avg. of 20 million
cloud-to-ground
flashes annually
• 30 million locations
struck each year
• From 2002-2006
(NCDC):
– Nearly 50 killed
each year by
lightning in US
– 300 injured
– 84% male
– Most in FL
– As high as $4-5
billion annual
damages (fires)
Orville and Huffines
(2001)
Lightning and Forest Fires
• Lightning started 12,863
wildfires annually from
2001-2006
• Human activity started
62,609
• Annual average acreage
burned from lightning: 4.6
million (2 million more than
that due to humans)
• National Lightning Detection
Network (NLDN)
– Detects cloud-to-ground
(CG) strikes
– Time and location
– Useful for fire-fighting
• Protect property or
allowed to burn?
NASA
The Lightning Stroke
• Lightning: electrical discharge in atmosphere; form
of static electricity
• Length: about 5 km; Width: about 2-3 cm diameter
– Visible flash 10-20 times wider
• Current ranges from usually 15-40 or 50 kA, but
can be much higher
– Strongest peak currents up to 300 kA
• Temperature reaches 30,000 C
• 1-10 billion J from typical 5 km channel
– 0.5 to 5.5 months of power for house
• Only 10 million J electrical energy
– Only 6 hours!
Electricity and Charge
• Ion: # protons ≠ # electrons (molecule or atom)
– Electrons mobile, protons immobile
• Current: direction positive charge moves (opposite
flow of electrons)
• Electric field: present in any region exposed to
charges
– Very strong one needed for lightning
• Volts: measure of magnitude of force of attraction
• Volts/meter: measure of electric field strength
Electricity and Charge
• Atmosphere (positive)
• Earth’s surface (negative)
• Electric field
– Fair Weather: ~100 V/m
– Prior to strike: ~1 million
V/m
– How does atmosphere build
enough charge to move
from 100 to 1 million
V/m???
Cloud Charge Distribution
• Distribution of charge
in thunderstorm and on
ground prior to
lightning strike
• Another question: -40ºC
– What can cause this
charge distribution to
result?
+24 C
ice
-20 C
mixed
0ºC
• Two theories:
– Non-inductive (interface)
charging
– Inductive charging
++ + ++ + ++
liquid
+4 C
Secondary
accumulation of
positive charge
Non-Inductive (Interface)
Charging
• Conductors (metals, ice, water) may be electrically
neutral
– But surface distribution of electrons differs
• Two conductors (different materials):
– Brought close  electric field
– If contacted  current
Non-Inductive Charging (cont’d)
• Two conductors in our case:
– Graupel and hail: grow by collection,
freezing of supercooled water
droplets
– Ice crystals: form by vapor deposition
• Voltage (charge potential) exists
between two ice particle types
• Collision 
– large hail, graupel: negative
– small ice crystals: positive
• Updraft takes up small ice crystals
• Larger particles remain below
• The likely initial charging
mechanism
Inductive Charging
• Each particle influenced
by electric field within
whole cloud
• Top of each ice particle
negative
• Bottom positive
• Collision transfers
electron from top of ice
crystal to hailstone
• Reinforces electric field in
storm (feedback
mechanism)
More on Charge Distribution
• Screening layer: thin
layer of opposite charge
along sides of clouds
– Results from background
positive (negative) ions
attracted to cloud
boundary in lower
(upper) region of cloud
• Image charge: during a
storm, opposite charge
reflected at Earth’s
surface
Lightning in Hurricanes
• Very little lightning in eyewall
• More in outer storm bands
• Freezing level above 5 km
– Charging mechanisms require ice particles
• Upward velocities
– In mid-lat. storms: rarely below 10 m/s, often higher
– In eyewall: rarely above 10 m/s, usually much weaker
• Not strong enough to produce supercooled water,
small hailstones
• Inefficient charging mechanisms
The Lightning Flash
•
•
•
•
Intracloud (IC)
Cloud-to-cloud (CC)
Cloud-to-ground (CG)
80% or more of all flashes IC or CC
– Can occur between positive and negative charge
regions
• CG strike begins when electric field reaches
about 3 million V/m (air is conductor!)
Schematic of the Lightning
Flash
1) Stepped leader
– Each step 50-100 m long
– Pause of a few millionths of a second between steps
– Find path of least resistance
2) Attachment process (traveling spark)
– Spark from ground meets descending stepped leader
• E field so great, charged molecules “jump”
3) Return Stroke
– Along found path of least resistance
– Ionization begins w/ traveling spark upward along channel
• Flash of light upward, current upward (flow of electrons downward)
• Steps 1-3 take about 10 microseconds
(1 millionth of a second)!
Schematic of the Lightning Flash
(cont’d)
4) Dart leader
-more rapid than stepped leader because channel already
ionized
5) Attachment process
6) Return stroke
• Time between each return stroke only few hundredths of
a second
7) Repeat steps 4-6 if more return strokes
• Over 20 strokes can occur in single “flash”
Schematic of the Lightning Flash
(cont’d)
Schematic of the lightning flash (cont’d)
The formation of the stepped leader from cloud to ground (a)-(f), is followed by
the return stroke from ground to cloud (g) and (h), brief K and J streamers (i), the
dart leader from cloud to ground (j), followed by the second return stroke.
(Adapted from M. Uman)
http://www.youtu
be.com/watch?v=
JVXy-ZqqZ-g
Cloud-to-ground (bush)
lightning 200 feet from
the camera1.
Note:
•upward branching from tree
•downward branching towards
tree
•bush back-lighted on left;
•bush front-lighted on right
•lightning follows tree trunk to
ground
•branch luminosity decreases
towards tip
Richard Orville
1. Orville, R. E., 1968:
"Photograph of a close
lightning flash", Science, 162,
666-667
(a) Intracloud
flash
(b) First cloud-toground stroke
(c) Subsequent
cloud-to-ground
stroke
Negative vs. Positive Polarity
• Negative flashes
– Most common
– Brings negative charge
from lower negative charge
region of cloud to ground
• Positive flashes
– Less common (8% of
flashes)
– Brings positive charge from
upper positive charge
region (anvil) to ground
– Stronger (discharge must
occur over longer pathhigher current!)
– One stroke (normally)
Bolt from the blue
• Often initially IC flash
• Excess of negative charge
remains in cloud after
flash
• Discharge continues
toward positive charge
region at ground
• Often follows screening
layer, then stepped
leader, etc.
• Can be over 10 km from
cloud boundary
Thunder
• Lightning heats air up to 30,000 C
– Air expands explosively, creating a shockwave, crashing sound waves
(thunder)
• Sound travels ~330 m/s
– 3 sec to travel 1 km, 5 sec to travel 1 mile
• Temperature decreases with height  sound waves bent
upward
– Higher frequencies bent more upward, deeper sounds stay lower
•
•
•
•
Close to stroke: sharp explosive bang
Farther: bang followed by rumble
Farther still: deep rumble
>5 km (3 mi): no thunder at all
Thunder diagram
Other phenomena
• Heat lightning: misnomer
– Light from distant flash scattered/reflected to appear as light flash
overhead
• Bead lightning: deionization of lightning channel, or
obscuration of parts of channel by clouds, rain
• Sheet lightning: lightning illuminates exterior of cloud
uniformly from behind or within cloud
• Ball lightning: luminous small glowing ball of gas
– 10-40 cm in diameter, avg. 25 sec in duration, see p. 418
• St. Elmo’s Fire: named after patron saint of sailors
– Early sign of impending lightning
– Bluish green halo appears from continuous sparking on antennas, ice
picks, sharp metal objects caused by positive charge concentrating on
tips
Bead lightning
Sheet lightning
Ball lightning
St. Elmo’s Fire
Lightning effects in the upper atmosphere
Lightning effects in the upper
atmosphere (cont’d)
• All occur between tops of storms and mesosphere
• Red sprites: large, red weak luminous flashes, typically over
anvils
–
–
–
–
Brightest 65-75 km above Earth
Above, faint red glow or wispy structure to ~90 km
Below, blue tendrils to ~30 km
Nearly always with positive flashes
• Elves: disk-shaped regions above sprites
– Lasts less than a thousandth of a second
– Occur high above energetic CG (positive) lightning
– Ionosphere
• Jets: blue, undetectable by naked eye
– Observed with low-light TV systems
– Narrow cones extending upward from cloud top
– Speeds of 100 km/sec
Effects at the ground: Lichtenberg figure in the grass
30
Cadaver: Lichtenberg figure
31
Fulgurite
Lightning Safety
• 30/30 rule – seek shelter if the time between lightning
flash and the rumble of thunder is 30 seconds or
less. You should not resume activities until 30
minutes after the last audible thunder
• Estimate lightning distance: After you see lightning,
count the number of seconds until you hear thunder.
Divide the number of seconds by 5 to get the
distance in miles to the lightning flash.
• The safest place is indoors or in a vehicle.
• Indoors: Stay away from electrical equipment, metal
objects, and phones (cordless and battery operated
are ok.)
• If caught outdoors, STAY AWAY FROM TREES and
other tall objects.
• If you can see it, flee it!
• If you can hear it, clear it!