Chapter 11 – Lightning, Thunder and
Tornadoes
Understanding Weather and Climate
Aguado and Burt
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Bolt From the Blue
Photo Courtesy of John Schroeder
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Lightning
• Lightning occurs when a voltage gradient in
the cloud(s)/ground overcomes the
electrical resistance of the air.
• Types:
– Cloud to cloud (CC)
– Cloud to ground (CG)
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Charge Separation
• Positive charges accumulate in the upper
portions and negative charges in the lower
portions of the cloud.
– Exact cause unknown!
– Lightning only occurs in clouds that extend
above the freezing level.
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Lightning Facts
• Electrical current – 20,000 A
• Temperature – 54,000°F
• Total Power Transfer – 100-watt light bulb
for a minute
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Cloud to Ground Lightning
• Stepper Leader – invisible downward leader
• Spark from the Ground – completing
pathway for flow of electrons
• Return Stroke – brightly illuminated,
transfer positive charge from surface to
cloud base
• Dart Leader – repetitive process to remove
remaining negative charge at cloud base
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Thunder
• Thunder is caused as air explosively
expands in response to the extreme heat of
lightning.
• The speed of sound is slower than the speed
of light, thus there is often a lag between the
lightning and the resultant thunder.
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Air Mass Thunderstorms
• Life Cycle
– Cumulus Stage
– Mature Stage
– Dissipative Stage
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Severe Thunderstorms
• Wind speeds in excess of 58 mph
• Hail larger than 0.75 inches
• Funnel cloud or tornado
• Types:
– Mesoscale Convective Systems (MCS)
• Squall Lines, Mesoscale Convective Complexes (MCC)
– Supercell Thunderstorms
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Severe Thunderstorms
• Severe thunderstorms require:
– Wind shear
– High water vapor content in the lower
troposphere
– A trigger mechanism
– Potential instability
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Potential Instability
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Mesoscale Convective Complexes
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Squall Lines on Radar
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Squall Lines
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Bowing
Squall
Line
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Supercell Thunderstorm
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Mesocyclone Formation
• Dependent on vertical
wind shear
• Updrafts tilt horizontal
vortex tubes vertically
• Stretching of the vortex
tube provides a narrowing
column of rotating air
• Can lead to wall clouds,
funnel clouds and
tornadoes
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Mesocyclone Formation
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Supercell Thunderstorm
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Supercell
Thunderstorm
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Doppler Radar Detection of
Supercell Thunderstorms
http://www.usatoday.com/weather/tg/wsupercl/wsuprcl1.htm
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Mesocyclone Detection
Air Motion Towards the Observer
Air Motion Away From the Observer
Incoming = Green Colors
Outgoing = Red Colors
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Supercell Structure
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Upper Level Flow
Anvil
Back Sheared Anvil
Updraft
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Photo Courtesy of Jeremy Massey, Ralls/Crosbyton May 27, 2002
Updraft
Rain Free
Updraft Base
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Forward Flank
Downdraft
Wall Cloud
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Rear Flank
Downdraft
Updraft
Forward Flank
Downdraft
Wall Cloud
John Schroeder, Muleshue, Texas June 3, 2003
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Rear Flank
Downdraft
Forward Flank
Downdraft
Updraft
Wall Cloud
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John Schroeder, Near Dalhart Texas, May 15, 2003
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Thunderstorm Climatology
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Detailed Reflectivity Data
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Detailed Reflectivity Data
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John Schroeder, Near Dalhart Texas, May 15, 2003
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Tornado Climatology
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Tornado Climatology
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Tornado Climatology
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Tornado Climatology
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Tornado
Climatology
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Tornado
and
Supercell
Thunderstorm
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Tornado
and
Supercell
Thunderstorm
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Tornado Damage
• Buildings do not explode
• Winds and windborne debris are primary
causes for property damage and loss of life.
– Do not open your window
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Tornado Damage
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Watches and Warnings
• Watch – the atmosphere is conducive to
producing severe weather and/or tornadoes
– Issued by Storm Prediction Center (SPC)
• Warning – Doppler radar or a storm spotter
has indicated that severe weather and/or a
tornado is occurring or probable
– Issued by local National Weather Service
(NWS) office.
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