NATS 101
Lecture 20
Lightning
Review: Thunderstorms
• A cumulonimbus with lightning and thunder!
• Deep layer of conditionally unstable air is
necessary to produce a thunderstorm.
• Several types of thunderstorms.
Single Cell, Multicell, Squall Line,
Mesoscale Convective Complexes, Supercells
• Pose major hazards to public and economy.
Lightning, Hail, Microburst Winds,
Flash
Flooding, Tornadoes
Lightning Basics
• What is lightning?
An electric discharge, or spark, that
occurs in thunderstorms (usually)
80% occurs within clouds
20% occurs between cloud and ground
• Lightning is ubiquitous, with more than
6,000 ground strikes per minute from
40,000 thunderstorms per day worldwide
Lightning Videos
• Examples were shown of
In-Cloud (IC) Lightning
Cloud-to-Cloud (CC) Lightning
Forked Lightning
IC Lightning Video from MetEd/UCAR
• MCC Lightning from Space Shuttle
Lightning from Space Video from NASA
Lightning Pictures
• Examples were shown of
Cloud-to-Ground (CG) Lightning
In-Cloud (IC) Lightning
Cloud-to-Cloud (CC) Lightning
Forked Lightning
Chuck Doswell's Lightning Pictures-Very Nice!
• Excellent photography tips can be found
at Chuck Doswell’s web site. He’s good!
Charge Separation
Lightning requires the separation of different
charges into different regions of a cloud.
How does charge separation in clouds occur?
We don’t know for certain, but we observe this:
Lightning only occurs in cold clouds with
supercooled droplets and temps below 5oF.
Thus, the ice crystal processes responsible for
precipitation in cold clouds likely plays an
critical role in charge separation.
Charge Separation: One Theory
Hailstones are covered by a layer of liquid water.
The thin layer of liquid is positively charged.
When hailstones and ice crystals collide, some of
liquid molecules stick to the ice crystals.
Along with the mass transfer, positive ions
transfer from the hailstones to the ice crystals.
The heavier, negative hail falls to cloud bottom.
The lighter, positive ice crystals drift to cloud top.
Produces negative lower, positive upper cloud.
+
Charge Separation
Polarization
E
–
• Top of cloud top has a
positive charge.
• Lower and middle of cloud
has a negative charge.
• Charge separation in cloud
maintains the earth’s fair
weather electric field
denoted by the arrow E
• E points toward positive
polarity
Williams, The Weather Book
Fair Weather Electric Field
• An electric potential
exists between the
ionosphere (positive)
and surface (negative)
• Potential varies
between 200,000 500,000 Volts
• Average current is
2x10-12 Amps/m2
• Power is  10-6 W/m2
www://thunder.msfc.nasa.gov/primer
Lightning Stroke
Cloud-Ground Sequence
1) Downward stepped leader.
Stepped leader is invisible.
2) Upward return stroke.
3) Downward dart leaders.
4) Upward return strokes.
Dart leaders-return strokes:
up to 25 cycles, 3-4 usually.
Ground strikes are usually
negative, that is electrons
flow from cloud to ground.
Williams, The Weather Book
Types of Discharges
www://thunder.msfc.nasa.gov/
Lightning Safety
Williams, The Weather Book
Thunder
•
•
•
•
Williams, The Weather Book
What Causes Thunder?
Lightning rapidly heats air
to more than 30,000oC.
The intense heating causes
the air to expand rapidly.
The expanding air cools,
then contracts rapidly.
The expansion-contraction
generates sound waves.
How Far Away Is It?
• We see lightning instantly.
• But sound travels 1,000 ft
every second. If you hear
thunder 10 seconds after
seeing lightning, the bolt is
2 miles (~10,000 ft) away.
• We hear thunder from
closest part of flash first,
farthest part last. This
causes the rumble sound.
Williams, The Weather Book
1 mile
Why Thunder Rumbles?
1 mile
5 seconds
• Assume that you are
one mile away from a
a one mile long bolt.
• You hear thunder
from the lower part
of flash in 5 seconds,
from the upper part
of flash 7 seconds.
National Lightning
Detection Network
thunder.msfc.nasa.gov/primer
Gallup
Flagstaff
Phoenix
Tucson
www.nssl.noaa.gov/western/kaney
Phoenix
Tucson
www.nssl.noaa.gov/western/kaney
Global Lightning Distribution
from Satellite, Take 2
www.msfc.nasa.gov
Let’s Play “Who Gets Toasted”
• What is the probability in Tucson of a
Cloud-to-Ground lightning stroke hitting
within a certain Radius R of you in an
“average” year?
• Guesses? No peeking!
Courtesy Prof. E.P. Krider
Dept. of Atmospheric Sciences
CG Lightning
over Tucson
(2000-2002)
• 65,000 flashes in
80 km  80 km
over 3 years
~3.3/km2 per year
• Much higher
during monsoon
~12/km2 per year
Courtesy Prof. E.P. Krider
Dept. of Atmospheric Sciences
Summary: Key Points
• Lightning - electric discharge in thunderstorms
80% within clouds, 20% cloud to ground
• Lightning is ubiquitous, with more than 6,000
cloud-to-ground strikes per minute from more
than 40,000 thunderstorms per day worldwide.
• Lightning requires the separation of different
charges into different regions of cloud.
• Charge separation maintains the earth’s fair
weather electric field.
Assignment for Next Lecture
• Topic - Tornadoes
• Reading - Ahrens, p277-290
• Problems - 10.25, 10.26, 10.29