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Chapter 10
Thunderstorms and Tornadoes
What are Thunderstorms?
• A storm containing lightning and thunder
• Sometimes produce gusty surface winds,
heavy rain and hail
• May be a single Cb or several may form a
cluster or line
• They occur when warm humid air rises in
a conditionally unstable environment
Ordinary “Air Mass” Thunderstorms
Cumulus Stage
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Simplified model depicting the life
cycle of an ordinary thunderstorm that
is nearly stationary. (Arrows show
vertical air currents. Dashed line
represents freezing level, 0°C
isotherm.)
During this stage there is insufficient
time for precipitation to form, and the
updrafts keep water droplets and ice
crystals suspended in the cloud.
No lightning in this stage
As cloud builds above freezing level,
the cloud particles grow larger and
become heavier.
Entrainment – drier air from around the
cloud is drawn into it, causing some of
the drops to evaporate, chilling the air.
The cooler heavier air descends as a
downdraft
Ordinary “Air Mass” Thunderstorms
Mature Stage
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The appearance of the downdraft
marks the beginning of the Mature
stage of the thunderstorm.
The downdraft and updraft within the
mature thunderstorm constitute a cell.
This is the most intense stage of a
thunderstorm.
Top takes on an anvil shape
Cloud itself may extend to over 40,000
ft and be several km in diameter at the
base.
Severe turbulence within the storm
Overshooting top may develop
Lightning, thunder, hail, heavy rain
possible
Often a cold downrush of air
associated with the onset of
precipitation
Ordinary “Air Mass” Thunderstorms
Dissipating Stage
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After the storm enters the mature
stage, it begins to dissipate in
about 15 to 30 minutes.
Dissipating stage occurs when the
updrafts weaken and downdrafts
tend to dominate throughout much
of the cloud.
Deprived of the rich supply of
warm humid air, cloud droplets no
longer form. Light precip falls
from the cloud, only weak
downdrafts remain.
Sometimes the lower part of the
storm evaporates leaving only the
cirrus anvil above.
This entire process cumulus,
mature, dissipating may take
place in an hour or less.
An ordinary thunderstorm in its mature stage. Note the distinctive anvil top.
A dissipating thunderstorm. Most of the cloud particles in the lower half of the storm
have evaporated.
Multicell Storms
•
A multicell storm complex. This storm is composed of a series of cells in successive
stages of growth. The thunderstorm in the middle is in its mature stage, with a welldefined anvil. Heavy rain is falling from its base. To the right of this cell, a
thunderstorm is in its cumulus stage. To the left, a well-developed cumulus congestus
cloud is about ready to become a mature thunderstorm.
A supercell near Spearman, Texas, has a tornado extending downward from its base.
Severe Thunderstorms
• Capable of producing large hail, strong,
gusty surface winds, flash floods, and
tornadoes.
• Form in moist air forced to rise into a
conditionally unstable atmosphere. But
also form in areas with strong vertical wind
shear.
Greater or equal to 50 KTS
Greater or equal 3/4” Hail
The lower half of an intense thunderstorm and some of the features associated with it.
A dramatic example of a shelf cloud (or arcus cloud) associated with a severe
thunderstorm.
Gust Fronts and roll clouds
• Gust front – the boundary separating the cold
downdraft of a thunderstorm from the warm
surface air. An observer on the ground might
mistake the gust front for the passage of a cold
front in that the wind often shift, gusts can
exceed 55 kts and the temperature drops
sharply and pressure often rises.
• Roll cloud – occasionally an elongated ominous
looking cloud forms just behind the gust front.
These clouds, which appear to slowly spin about
a horizontal axis are called roll clouds.
A roll cloud forming behind a gust front. (Copyright Howard B. Bluestein.)
Microbursts
• Beneath a severe thunderstorm, the
downdraft may become localized so that it
hits the ground and spreads horizontally in
a radial burst of wind – such downdrafts
are called downbursts.
• A downburst with winds extending only 4
kilometers or less is termed a microburst.
An intense microburst can induce
damaging winds as high as 146 kts.
Damage from straight line winds
Tornado like (shear) vortices along leading edge can inflict damage
TR 303c-34
Low level wind shear
Dust clouds rising in response to the outburst winds of a microburst north of
Denver, Colorado.
Derecho
• Strong downburst winds associated with a
cluster of severe thunderstorms can produce
damaging straight-line-winds that may exceed
90 kts. If the wind damage extends for at least
400 km along the storm’s path, the winds are
called a derecho (day-ray-cho).
• Most derechos form in the early evening and are
associated with severe thunderstorms that
extend outward in the shape of a bow echo on a
radar screen.
Bow echos
• Doppler radar display showing
a line of thunderstorms (a
squall line) bent in the shape
of a bow (colors red, orange,
and yellow) as they move
eastward across the San
Joaquin Valley of California.
Such bow echoes often
produce damaging surface
winds near the center of the
bow. Sometimes the left
(usually northern) end of the
bow will develop cyclonic
rotation and produce a
tornado.
Supercell and Squall line
Thunderstorms
• Supercell storm – an enormous rotating
thunderstorm whose updrafts and
downdrafts are sufficiently structured so
that it is able to maintain itself as a single
entity for hours on end. Can produce
updrafts in excess of 90 kts, hail the size
of grapefruit, damaging surface winds and
large long-lasting tornadoes.
Supercell
• View of a classic supercell
from above, looking down on
the storm. The area enclosed
by the dashed line represents
the radar image of the storm.
(See how it compares with the
previous figure.) The area
shaded gray represents the
rear flank downdraft; the area
shaded red, the main updraft.
The cold and warm fronts
represent the position of the
gust front.
Tornado is on interface between inflow and outflow
RFD and Updraft interact to form a Tornado
TR 303c-16
Bounded Weak Echo Region caused by intense updrafts (precip forms higher)
Largest hail falls to left of wall cloud
Funnel clouds are common with wall cloud
TR 303c-15
Squall Line
• The squall-line forms as a line of
thunderstorms. “A line of thunderstorms
that form along a cold front or out ahead of
it.”
• Can from as much as 100 to 300 km out
ahead of a cold front in the warm air.
• Pre-frontal squall line thunderstorms of
the middle latitudes represent the largest
and most severe type of squall line.
Squall Line
A model describing air motions and precipitation associated with a squall line that
has a trailing stratiform cloud layer
Pre-frontal Squall line
• Pre-frontal squall-line thunderstorms may form ahead of
an advancing cold front as the upper-air flow develops
waves downwind from the cold front.
Severe Thunderstorms and the
Dryline
• Severe thunderstorms may form along or just
east of a boundary called a dry line.
• The dryline represents a narrow zone where
there is a sharp horizontal change in moisture.
• Because dew-point temperatures may drop
along this boundary by as much as 9°C (16°F)
per km dry lines have been referred to as dewpoint fronts.
• Most frequently observed in western half of
Texas, Oklahoma and Kansas in the spring and
early summer.
Dry Line
• Surface conditions that can produce a dryline
with severe thunderstorms.
cT clashes with mT !
TR 303c-7
TR 303c-8
TR 303c-9
TR 303c-10
TR 303c-11
Mesoscale Convective Complex
• An enhanced infrared satellite
image showing a Mesoscale
Convective Complex (dark red
color) extending from central
Kansas across western
Missouri. This organized mass
of thunderstorms brought hail,
heavy rain, and flooding to this
area.
• MCC can be as much as 100
times larger than the ordinary
thunderstorm, often cover an
entire state (100,000 km)
• Move slowly and often exist for
periods longer than 12 hours
• Produce hail, high winds and
flash floods (1/4 of all MCCs
produce derechos
Galva, IA Tornado
TR 303c-37
MCC – How is it formed?
• Form during the summer in regions where
the upper-level winds are weak, often
beneath a ridge of high pressure.
• A weak cold front may stall beneath the
ridge, surface heating and moisture may
be sufficient to generate thunderstorms on
the cool side of the front.
• Moisture from the south is brought into the
system by a low level jet stream.
Flash Floods
• Flooding during the summer of
1993 covered a vast area of
the upper Midwest. Here,
floodwaters near downtown
Des Moines, Iowa, during July,
1993, inundate buildings of the
Des Moines waterworks
facility. Flood-contaminated
water left 250,000 people
without drinking water
• Flash floods – floods that rise
rapidly with little or no advance
warning.
• Often a result of stalled or slow
moving thunderstorms or when
storms “train” over the same
area for long periods of time
Flash Floods
• Heavy rains due to
regenerative
convective storms
• High surface dewpoints
• Winds aloft are weak;
thunderstorms stall or
move slowly
• Associated with
intense synoptic scale
cyclone or frontal
system
• A quasi-stationary
front over area
• Winds aloft parallel to
front
TR 303c-36
Big Thompson Canyon Flood
• July 31, 1976
• By late PM, low cumulus
clouds started punching
through the inversion.
• Moist southeasterly winds fed
the clouds and that developed
into gigantic multicell
thunderstorm (tops > 60,000 ft)
• 12 inches of rain fell over the
canyon between 6:30 pm and
10:30 pm. (Normal rainfall for
a year is 16 inches)
• Of 2,000 people in canyon that
day, 135 died.
Distribution of Thunderstorms
•
The average number of days each year on which thunderstorms are observed
throughout the United States. (Due to the scarcity of data, the number of
thunderstorms is underestimated in the mountainous west.)
Distribution of Hail
• The average number of days each year on which hail is
observed throughout the United States.
Lightning and Thunder
• Lightning – simply a discharge of electricity, a giant
spark, which occurs in mature thunderstorms.
• Lightning can take place in cloud, cloud to cloud, cloud
to air, and cloud to ground.
• Thunder – the sound due to rapidly expanding gases
along the channel of a lightning discharge.
• Thunder travels about I mile in 5 seconds. If we see
lightning and then hear thunder 15 seconds later, the
stroke occurred about 3 miles away.
• Sonic Boom – sound produced when aircraft exceeds
the speed of sound at the altitude at which it is flying.
Aircraft compresses the air, forming a shock wave, air
pressure changes rapidly over a short distance, the rapid
change of air pressure causes the sonic boom.
Electrification of Clouds
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The generalized charge
distribution in a mature
thunderstorm.
Different ideas on why clouds
have charges.
One theory proposes that clouds
become electrified when graupel
and hail fall through a region of
supercooled droplets and ice
crystals. When the warmer
hailstone comes in contact with a
colder ice crystal there is a net
transfer of positive ions from the
warmer object to the colder object.
(Hail gets a negative charge, ice
gets positive charge).
Top of cloud is generally positive,
middle negative, lower part
negative, with some positive.
The Lightning Stroke
• The development of a lightning
stroke.
(a) When the negative charge
near the bottom of the cloud
becomes large enough to
overcome the air's resistance,
a flow of electrons - the
stepped leader - rushes toward
the earth.
• Stepped leader - an initial
discharge of electrons that
proceeds intermittently toward
the ground in a series of steps
in a cloud to ground lightning
stroke
The Lightning Stroke
• As the electrons
approach the ground,
a region of positive
charge moves up into
the air through any
conducting object,
such as trees,
buildings, and even
humans.
The Lightning Stroke
• When the downward flow
of electrons meets the
upward surge of positive
charge, a strong electric
current - a bright return
stroke - carries positive
charge upward into the
cloud.
• Return Stroke – the
luminous lightning stroke
that propagates upward
from the earth to the base
of a cloud
Time exposure of an evening thunderstorm with an intense lightning display near
Denver, Colorado. The bright flashes are return strokes. The lighter forked flashes
are probably stepped leaders that did not make it to the ground.
More lightning terms
• Dart leader – the discharge of electrons that proceeds
intermittently toward the ground along the same ionized
channel taken by the initial lightning stroke.
• Heat lightning – distant lightning from thunderstorms
that is seen but not heard. So called because it
frequently occurs on hot summer nights when the sky
overhead is clear.
• St. Elmo’s Fire – named after the patron saint of sailors.
A bright electric discharge that is projected from objects
(usually pointed) when they are in a strong electric field
such as during a thunderstorm.
Ball Lightning/St. Elmo’s Fire
• This picture of the freak
weather phenomenon of
ball lightning was taken
by a wildlife ranger in
Queensland, Australia, in
1987.
• Ball lightning occurs so
rarely that few
photographs of it exist
and researchers have
had to rely on eyewitness
accounts, some of them
from previous centuries.
Lightning Rod
• The lightning rod extends
above the building,
increasing the likelihood
that lightning will strike
the rod rather than some
other part of the structure.
After lightning strikes the
metal rod, it follows an
insulated conducting wire
harmlessly into the
ground.
•
The four marks on the road surface represent areas where lightning, after
striking a car traveling along south Florida's Sunshine State Parkway,
entered the roadway through the tires. Lightning flattened three of the car's
tires and slightly damaged the radio antenna. The driver and a six-year-old
passenger were taken to a nearby hospital, treated for shock, and released.
Lightning Detection
• Cloud-to-ground lightning
strikes in the vicinity of
Chicago, Illinois, as
detected by the National
Lightning Detection
Network. (Global
Atmospherics, Inc.)
• Cloud to ground strokes
are detected by a
lightning direction finder,
which works by detecting
the radio waves produced
by lightning.
Tornadoes
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•
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Diameter of most tornadoes (300 – 2000 ft)
Tend to move from southwest toward northeast
Most last only a few minutes – at least one lasted
7 hours and moved along a path of 292 miles
•
A mature tornado with winds
exceeding 150 knots rips through
southern Illinois.
Tornadoes are rapidly rotating
winds that blow around a small
area of intense low pressure. A
tornado’s circulation is present on
the ground either as a funnel
shaped cloud or as a swirling
cloud of dust and debris.
Other names – twisters, cyclones
Funnel cloud – a tornado whose
circulation has not reached the
ground.
Gustnadoes – tornadoes that
form along the gust front of a
severe thunderstorm
A funnel cloud extends downward from the base of a non-supercell
thunderstorm over central California.
Tornado incidence by state. The upper figure shows the number of tornadoes
reported by each state during a 25-year period. The lower figure is the average
annual number of tornadoes per 10,000 square miles. The darker the shading,
the greater the frequency of tornadoes.
Tornado alley – tornado belt – central plains from central Texas to Nebraska
Most often occur from March to July
Tornado Outbreak – April 3, 1974
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Outbreak – 6 or more tornadoes
over a particular region
Map of 148 Tornadoes that
Occurred on April 3 & 4, 1974
315 people died, 5,000 injured
Tornado Winds
• The total wind speed of a
tornado is greater on one
side than on the other.
When facing an
onrushing tornado, the
strongest winds will be on
your left side.
• Most tornadoes probably
have winds less than 125
kts, but most powerful
twisters seldom exceed
220 kts.
TR 303c-26
TR 303c-29
A devastating F5 tornado about 200 m (650 ft) wide plows through Hesston,
Kansas, on March 13, 1990, leaving almost 300 people homeless and 13
injured.
Conditions leading to the formation
of severe thunderstorms that can
spawn tornadoes. Boxed red area
on surface map shows where
tornadoes are most likely to form.
Note: LLJ at 850 mb, cold air at
700 mb, trough at 500 mb, PFJ at
300 mb.
Some of the features associated with a tornado-breeding supercell
thunderstorm as viewed from the southeast. The storm is moving to the
northeast.
Observing Tornadoes
• Wall cloud – An area of rotating clouds that
extends beneath a severe thunderstorm and
from which a funnel cloud may appear
• Tornado watch – tornadoes are likely to form in
the next few hours – issued by Storm Prediction
Center in Norman
• Tornado warning – Once a tornado is spotted –
either visually or on radar – issued by local NWS
office
A wall cloud photographed southwest of Norman, Oklahoma.
Thunderstorm Rotation
• Spinning vortex tubes created by wind shear.
Thunderstorm Rotation
• The strong updraft in
the thunderstorm
carries the vortex
tube into the
thunderstorm,
producing a rotating
air column that is
oriented in the vertical
plane.
Doppler Radar
• Similar to a conventional radar in that is
can detect areas of precipitation and
measure rainfall intensity.
• Doppler can do more – actually measures
the speed at which precipitation is moving
horizontally toward or away from the radar
antenna.
• Doppler radar “peers into severe
thunderstorms and reveals its winds.
Doppler radar display
showing precipitation
inside a large supercell
thunderstorm that is
spawning an F4 tornado
(circled area) near Lula,
Oklahoma. The regions
in orange and red show
the area of heaviest
precipitation. The close
packing of the winds
indicates strong cyclonic
rotation and the
signature of a tornado.
Red and orange indicate
winds blowing away from
the radar. Green and
blue indicate winds
blowing toward the radar
Doppler on Wheels and NEXRAD
• Graduate students from
the University of
Oklahoma use a portable
Doppler radar to probe a
tornado near Hodges,
Oklahoma
• NEXRAD – not mobile –
Next Generation Weather
Radar – WSR -88D
Doppler radar and a set
of computers that help
process the data
Waterspouts
• A rotating column of air over a
large body of water. May be a
tornado that formed over land
and them moved over water.
• Fair Weather Waterspouts –
form over waters of Florida
Keys, Caribbean. Much
smaller than an average
tornado, (3 – 100 meters),
wind less than 45 kts, tend to
move slowly.
• A powerful waterspout moves
across Lake Tahoe, California.