Chapter 10:
Thunderstorms and
Tornadoes
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Thunderstorms
Tornadoes
Tornadic thunderstorms
Severe weather and doppler radar
Thunderstorms
Thunderstorms
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What do we already know about
thunderstorms?
Can produce intense rain, hail, wind, lightning
 Can make tornadoes
 Formed by rising air from:
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Uneven surface heating
 Uplift along a frontal boundary
 Topographical uplift
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Thunderstorms
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Definitions:
Supercell thunderstorms – large updraft
storms that can produce flash floods, severe
weather, tornadoes
 Severe Thunderstorm – has to have one of
following:
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Hail greater than or equal to ¾ inch
 A tornado
 Wind gusts 50 mph
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Ordinary Cell Thunderstorms
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Ordinary cell thunderstorm
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Form in region of low wind shear
Can form due to surface air convergence
Cumulus stage (growth stage) – due to rising
warm, humid air condensing into cumulus cloud
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Latent heat release keeps cloud warm and unstable
Grows quickly to towering cumulus
Usually no precipitation because of updrafts
No lightning or thunder
Ordinary Cell Thunderstorms
Ordinary Cell Thunderstorms
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Mature stage – marked by considerable
downdrafts
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Cloud particles grow larger, and begin to fall
Entrainment - Dry air is sucked into cloud, causing
evaporation and cooling
Heavier and cooler air descends…downdrafts
Formation of updraft and downdraft cells
Most intense time of thunderstorm
Storm may grow as high as stratosphere (anvil)
Heavy rain, lightning, small hail
Ordinary Cell Thunderstorms
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Mature stage
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Cloud top may overshoot into stratosphere
Ordinary Cell Thunderstorms
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Mature stage
Cloud top may overshoot into stratosphere
 Downdraft reaches ground and spreads along
surface as a gust front
 Sometimes rain may not reach ground, but
cold air does
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Ordinary Cell Thunderstorms
Ordinary Cell Thunderstorms
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Dissipating stage – thunderstorm weakens
Updrafts begin to weaken after 15 to 30 min.
 Gust front moves too far from the storm, so
updrafts have to weaken
 Light precipitation falls
 Only anvil remains
 All three stages in less than an hour
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Ordinary Cell Thunderstorms
Stepped Art
Fig. 10-1, p. 265
Severe Thunderstorms and
the Supercell
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So ordinary cell storms weaken because
the updraft weakens
What if the updraft doesn’t weaken
quickly?
If moderate wind shear pushes downdraft
downwind, updraft is not cut off
 If downdraft then undercuts updraft, a
multicell storm forms
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Severe Thunderstorms and
the Supercell
Severe Thunderstorms and
the Supercell
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Multicell storms
Cells of varying age co-mingling
 Top of cloud well into stratosphere
 Updrafts allow hail to grow large
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Severe Thunderstorms and
the Supercell
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Supercells
If wind shear is strong and changes direction
with height, downdraft may not undercut
updraft
 Updraft remains strong for long time (> 1
hour)
 One rotating vertical column
 Can create tornadoes
 Hail size of grapefruit
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Severe Thunderstorms and
the Supercell
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At surface, open wave with
warm, humid air ahead of
cold front
Overrunning warm air just
above surface
Dry air at 700 mb level
Trough of low pressure at
500 mb
Divergence at 300 mb
Severe Thunderstorms and
the Supercell
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Severe thunderstorms form
in light green area because:
 Warm air below cold air is
conditionally unstable
 Strong wind shear creates
severe thunderstorms
 In morning, atmos is
stable, no thunderstorms
 Surface heating creates
thunderstorms
Squall Lines and Mesoscale
Convective Complexes
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Squall line – line of multicell t-storms extending
many kilometers
Mesoscale convective complex- cluster of
storms in one spot
Squall Lines and Mesoscale
Convective Complexes
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Squall lines often form ahead of cold fronts (prefrontal squall-line thunderstorms)
MCCs are long-lasting and provide a lot of rain,
and a ton of severe weather
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Form during summer
Dryline Thunderstorms
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Dryline – narrow zone with a sharp moisture
contrast. Where do the thunderstorms form?
Gust Fronts, Microbursts and
Derechoes
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Gust front – leading edge of the thunderstorm
downdraft
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Passage can feel like a cold front passage
Shelf cloud – warm air rising above gust front
creates cloud
Roll cloud – cloud just behind the gust front that
spins horizontally
Outflow boundary – a huge gust front formed by
numerous thunderstorm gust front
Downbursts – localized downdraft like water
from a tap
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Microbursts – downbursts that extend 4 km or less
Gust Fronts, Microbursts and
Derechoes
Gust Fronts, Microbursts and
Derechoes
Fig. 10-11, p. 271
Microburst
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Microbursts can be
dangerous to
aircrafts
Plane encounters
uplift at (a), and pilot
puts nose down
Unexpected
downdraft crashes
plane
Has caused several
airplane crashes
Some airports have
microburst detection
Bow Echo and Derecho
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Bow echo – a line of
thunderstorms often
form a bow echo on
radar
Derecho – winds
associated with
downdrafts that
exceed 104 mph
and can cause a lot
of damage…20
derechoes each
year in U.S.
Floods and Flash Floods
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Flash floods – floods that rise rapidly with
little to no advance warning
Thunderstorms
cause flash floods
in two ways:
Hovering above one
area for a long time
Training –
thunderstorms keep
passing over same
area
Distribution of
Thunderstorms
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18 millions thunderstorms worldwide
annually
Need to have a
combination of
moisture and
warmth
So do
thunderstorms
form in the
Arctic? ITCZ?
Gulf?
Lightning and Thunder
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Lightning – simply a discharge of
electricity, usually in mature thunderstorms
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Heats air up to 54,000F (5x hotter that surface
of sun
Thunder – the explosive expansion of the
hot air creates a sound wave that travels
in all direction
• According to the Guinness Book of World Records,
Roy Sullivan holds the world’s record for being struck
by lightning seven times between 1942 and his death
in 1983.
Lightning and Thunder
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Light travels faster than sound, so we see
the lightning before we hear the thunder
Sound takes 5 seconds to travel 1 mile
 So if we hear thunder 5 seconds after we see
the lightning, the stroke was 1 mile away
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Lightning and Thunder
Electrification of Clouds
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Must have charging of the cloud to have
lightning
Electrification of clouds not fully
understood
Theory 1: Supercooled droplets fall
through cloud and collide with warm
hailstone. Latent heat is released (warm
or cold?)
Electrification of Clouds
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Net transfer of positive ions from warm to cold
object
Falling hailstone is negatively charged, falls to
bottom of cloud
Light positively charge particle is lifted to top of
cloud
Small area of positively charge particles near
melting level
Electrification of Clouds
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Theory 2: When precip forms, it has a neg
in the upper portion and pos charge in the
lower
As droplets collide, the large droplets
become neg and fall
The small droplets are lifted by updrafts
and rise
The Lightning Stroke
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Basics of lightning
Opposite charges attract
 Positive charges on ground follow negative
charges at base of cloud
 Electrical current will not flow because air is
good insulator
 Charge must be large (< 1million volts per
meter) to create a lightning bolt
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The Lightning Stroke
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Cloud-to-ground lightning
Discharge of electrons from cloud to ground
as a stepped leader (many times)
 Positive charges race back up from elevated
object as a return stroke
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The Lightning Stroke
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Cloud-to-ground lightning
Many electrons flow to ground and a stronger
return stoke follows (this is what you see)
 1/10,000 of a second so it looks like one
continuous flash
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The Lightning Stroke
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Dart leader – subsequent initial stroke that
follows same path as initial stepped leader
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Causes the multiple flash of the lightning
Types of Lightning
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Forked lightning – dart leader takes different
path than stepped leader
Ribbon lightning – wind blows charges into
ribbon-like lightning
Dry lightning – lightning that occurs in a dry
thunderstorm
Types of Lightning
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Heat lightning – lightning that is seen by not
heard (can be orange)
St. Elmo’s Fire – luminous green or blue halo
around the top of pointed objects (antennas,
masts of ships). Lightning may occur after this is
seen.
Lightning Detection and
Suppression
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Lightning direction-finder
Tornado Life Cycles
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Tornado – rotating column of air blowing
around a small low pressure that reaches
the ground
Tornado Life Cycles
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Funnel cloud
 Tornado that hasn’t reached the ground
Dust-whirl stage
 Swirling dust at the ground marks the tornadoes circulation
Mature stage
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Funnel at greatest width, most intense damage. Often vertical
Decay stage
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Funnel shrinks, damage becomes less, tornado becomes stretched
Tornado Outbreaks
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Tornado families – tornadoes spawned by
the same thunderstorms
Tornado outbreaks – many tornadoes that
form over same region
• Much ground-breaking research on tornadoes was
conducted by Professor Ted Fujita of the University
of Chicago. The “F-scale” of tornado intensity was
named after him.
Tornado Occurrence
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Tornado alley – part of the Central Plains from Texas
through Nebraska
Time of day – most often in the afternoon
Times of year – most often in Spring, lease often in
Winter
Tornado Winds
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Multi-vortex tornadoes
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A single tornado with multiple rotating
columns within it
Suction vortices
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The small rotating columns within multi-vortex
tornadoes
Seeking Shelter
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Tornado watch – tornadoes are likely to
develop within the next few hours
Tornado warning – issued once a tornado
is spotted
• It’s always a good idea to know what to do if a
tornado watch or warning is issued for your area.
The Fujita Scale
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Tornado classification based on damage
Supercell Tornadoes
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Mesocyclones – Rising, rotating column on the
south side of a supercell. Acts to increase the
updraft
Bounded weak echo region – area inside t-storm
where radar does not pick up precip
Hook echo – Radar representation of a tornado
(looks like a hook)
Rotating clouds – First sign tornado is about to
form
Wall cloud – rotating clouds that have lowered
below the base of a supercell
• A rotating wall cloud is an unforgettable sight - just
ask a successful storm chaser.
Fig. 10-35, p. 290
Nonsupercell Tornadoes
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Gustnadoes – tornadoes that flow along a
gust front
Landspouts – similar to water spouts and
form due to developing cumulus
congestus
Severe Weather and Doppler
Radar
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Doppler shift – Change in frequency in
sound waves for moving objects
Tornado vortex signature – rapidly
changing wind direction in mesocyclone
NEXRAD – Network of 150 radar stations
in U.S.
Waterspout – Tornado that forms over
water
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Chapter_10 - Weather Underground