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METEOROLOGY
GEL-1370
Chapter Ten
Thunderstorms and
Tornadoes
Goal for this Chapter
We are going to learn answers to the following questions:
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What atmospheric conditions produce thunderstorms?
How severe thunderstorms are produced?
Why severe thunderstorms are not common in polar latitude?
How lightning are produced?
How thunders are produced?
What are tornadoes and how they are produced?
What is Fujita scale?
Major characteristics of a tornadoe?
Why highest frequency of thunderstorms occur in US?
Thunderstorms
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Thunderstorm: A storm that contains lightning and thunder
Birth occurs when warm humid air rises in a conditionally unstable environment
What can trigger the birth of thunderstorm – unequal heating of the surface, terrain, lifting of warm air along a
frontal zone
Ordinary thunderstorms (or air-mass thunderstorms): Develop in warm, humid air masses away from weather
fronts; usually short-lived and rarely produce strong winds or large hail
Severe thunderstorms may produce high winds, flash floods, changing hail & tornadoes
Thunderstorms
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Stages of ordinary thunderstorms:
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Cumulus Stage: Humid air rises, cools, & condenses in to cumulus clouds
Transformation of water-vapor into liquid or solid cloud particles releases large quantities of latent heat; this keeps the air
inside the cloud warmer than the surrounding air
During cumulus stage, insufficient time for precipitation to form, and the updrafts keep water droplets and ice crystals
suspended within the cloud; no lightning or thunder during this stage
As the cloud builds well above the freezing level, cloud particles grow larger and heavier; drops begin to fall; drier air around
the cloud is being drawn into it; entrainment of drier air leads to evaporation of raindrops; air becomes colder & heavier; air
begins to descend as a downdraft
Thunderstorms – contd.
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Appearance of the downdraft marks the beginning of the mature thunderstorms; downdraft & updraft within the
mature thunderstorm constitute a ‘cell’
In most storms, there are several cells, each of which may last for an hour or so
Updrafts & downdrafts reach their greatest strength in the middle of the cloud, creating severe turbulence
Overshooting: Intrusion of the updraft above the cloud top in to the stable atmosphere
Dissipating stage: When updrafts weaken & downdrafts tend to dominate throughout much of the cloud
Three stages: Cumulus stage, maturing thunderstorm stage, & dissipating stage
Thunderstorms – contd.
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A single ordinary thunderstorm may go through its three stages in an hour or less
The cold downdraft may force warm, moist surface air upward; this air may condense and can gradually build into a
new thunderstorm – multicell thunderstorms
Most ordinary thunderstorms are multicell storms
Severe Thunderstorms: Capable of producing large hail, strong, gusty surface winds, flash floods, and tornadoes
Can form from moist air when it is forced to rise into a conditionally unstable atmosphere; severe thunderstorms
also form in areas with a strong vertical wind sheer
Air motions associated with thunderstorms; severity depends on the intensity of the storm’s circulation
pattern
Ordinary thunderstorm in its mature stage
A multicell thunderstorm; in the middle is in its mature stage; to its right of the cell, a thunderstorm is its
cumulus stage
A simplified model describing air motions & other features associated with a severe thunderstorm; severity depends on
the intensity of the storm’s circulation pattern
Severe Thunderstorms – contd.
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The storm in the previous figure, moves from left to right & the upper-level winds cause the system to tilt so that
the updrafts move up and over the downdrafts
The updrafts in a severe thunderstorm may be so strong that the cloud top is able to intrude well into the stable
atmosphere; top of the cloud may even extend to more than 18 km above the surface
Gust Front: The boundary separating the cold downdraft from the warm surface air
Along the leading edge of the gust front, the air is turbulent; strong winds here can pick-up loose dust and soil and
lift them into a huge tumbling cloud
Gust Front & Microburst
Downbursts: A severe localized downdraft that can be experienced that fall slowly and reduce visibility more than
light rain
Microburst: A downburst with winds extending only 4-kms or less
Supercell and Squall-line thunderstorms:
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Supercell Storm: An enormous severe thunderstorm whose updrafts (can exceed 90 knots) and downdrafts are nearly in
balance, allowing it to maintain itself for several hours. It can produce large tornadoes & hail (> grapefruit size); most
supercell storms move to the right of the steering winds aloft
Squall-line storms form as a line of thunderstorms along a cold front or out ahead of it
The lower half of a severe squall-line type thunderstorms and some of the features associated with it
Dust clouds rising in response to the outburst winds of a microburst north of Denver, CO
Doppler radar display showing a line of thunderstorms bent in the shape of a bow (Red, orange, and yellow)
Supercell near Spearman, TX has a tornado extending downward from its base
Some of the features of a classic supercell thunderstorm, viewed from southeast
Diagram of the thunderstorm from above, looking down on the storm; shaded red: updraft; shaded gray: downdraft
Severe Thunderstorms – contd.
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Dry Line (dew-point fronts): A zone of instability along which thunderstorms form; dew point temp may drop
along this boundary by as much as 9°C/km
Mesoscale Convective Complexes: A large organized convective weather system comprised of a number of
individual thunderstorms; size of an MCC ~ 1000 times larger than individual thunderstorm
Surface conditions that can produce a dryline with severe thunderstorms; A developing mid-latitude cyclone with a
cold front, warm front, and three distinct air masses (cP, cT & mT)
IR image showing a Mesoscale Convective Complex extending from central Kansas across western
Missouri
Floods & Flash Floods
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Flash floods: Floods that rise rapidly with little or advance warning; results when thunderstorms stall or move
slowly, causing heavy rainfall over a relatively small area
Causes for Flash Floods:
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Thunderstorms stall or move slowly
Thunderstorms move very quickly but keep passing over the same area (phenomenon called ‘training’)
Heavy rain and melting of snow taking place in spring
Torrential rains from tropical storms
Summer of 1993 rain in the upper Midwest caused the worst flood 6.5 billion $ crop lost; 43 human lives; 45,000
homes were lost; evacuation of 74,000 people
Distribution of Thunderstorms
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>40,000 thunderstorms/day (14 millions/yr) in the world
14 million/year
Conducive conditions for thunderstorm formation: Combination of warmth and moisture
Where thunderstorms are prevalent: i) Southeastern states along the Gulf Coast with a maximum in Florida (mainly
during summer); ii) Central Rockies; iii) Over water along the intertropical convergence zone where the low-level
convergence of air helps to initiate uplift
Where thunderstorms are rare: i) Dry regions such as polar regions and the desert areas of the subtropical highs; ii)
Pacific coastal and interior valleys
Average number of days each year on which thunderstorms are observed in US; mountainous west has
sparse data
Average number of days each year hail observed
Thunderstorms and Lightning
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Lightning: A giant spark discharging electricity that occurs in mature thunderstorms; can take place within a cloud,
from one cloud to another, cloud to surrounding air or cloud to ground (~20%); 80% within the clouds
Lightning stroke can heat the air surrounding it to 30,000°C which in turn causes the air to expand, thus initiating a
shock wave that becomes a booming sound wave-thunder
Light travels faster than sound (345 m/s @25 °C)
Time difference between the light and sound can be utilized to determine how far away the stroke took place
Lightning & Thunder – contd.
Close distance lightning: Clap sound or crack followed immediately by a loud bang
Farther away: rumbling sound due to sound emanating from different areas of the stroke
Lightning, but no thunder: Thunder waves were refracted and the sound waves got attenuated, making the thunder
inaudible
Sonic boom: Produced when an aircraft exceeds the speed of sound at the altitude at which it is flying
Condition for lightning to occur: Separate regions containing opposite electrical charges must exist within the
cumulonimbus cloud
Electrification of clouds
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Several theories to explain the formation of lightning
When hail fall through supercooled droplets, the droplets freeze and release latent heat; this heat warms the
hailstone; contact of warmer hailstone and colder ice crystal leads to a net transfer of positive ions from the warmer
object to the colder object --- hailstone is negatively charged and ice crystals +ively charged
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Positively charged ice particles carried to the upper part of the cloud by updrafts & larger haldstones with –ive
charge fall toward the bottom of the cloud
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Cold, upper part becomes +ively charged & middle of the cloud becomes –ively charged
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Electrification of the Clouds – contd.
Another school of thought: Regions of separate charge exist within tiny cloud droplets and larger precipitation
particles during the formation of precipitation --- Negative charge in the upper part of these particles & +ive charge
in the lower part of the particles --- when falling precipitation collides with smaller particles, larger precipitation
particles become negatively charged and the smaller particles positively charged --- updraft sweeps smaller sized
particles leading to net +ive charge
Generalized charge distribution in a mature thunderstorm
The Lightning Source
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Negative charge at the bottom of the cloud causes a region of the ground beneath to become +ively charged; as the
thunderstorms move, the positive charge moves along with it; the positive charge is most dense on protruding
objects; charge separation causes electric field existence; electric potential difference between cloud and the ground
--- when electric potential builds up, current flow results and lightning occurs
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Cloud-to-ground lightning begins when the localized electric potential gradient >3 million volt/m --- leads to the
discharge of electrons toward the cloud base and then to the ground
Lightning source – contd.
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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
Return Stroke: The luminous lightning stroke that propagates upward from the earth to the base of a cloud
Dart Leader: Discharge of electrons that proceeds intermittently toward the ground along the same ionized
channel taken by the initial lightning stroke
Different types of Lightning: Forked lightning (crooked or forked in shape), ribbon lightning (ribbon hanging in the
cloud), bead lightning (series of beads tied to a string), ball lightning (sphere appears to float in the air) & sheet
lightning (cloud appears like a white sheet)
Development of stepped leader: when –ive charge near the bottom of the cloud becomes large enough to overcome
air’s resistance, flow of electrons rushes to the earth
As the electrons approach the ground, a region of + charge moves up into the air through any conducting
object, such as trees, buildings
When the downward flow of electrons meets the upward surge of +ive charge, a strong electric current – a bright return
stroke – carries +ve charge upward into the cloud
Lightning rod extends above the building: when lightning strikes, it follows an insulated conducting wire
into the ground
Four marks on the road surface represent areas where lightning, after striking a car entered the roadway; 3 tires were
flattened
Lightning Detection & Suppression
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Heat Lightning: Distant lightning from thunderstorms that is seen, but not heard
As the electric potential near the ground increases, a current of +ive charge moves up pointed objects, such as
antennas
Lightning rods (made of metal with a pointed tip) are placed that extend well beyond the height of the structure
Lightning Direction-finder: It detects the direction of lightning by measuring the radiowaves produced by lightning
Damages by lightning in US
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10,000 fires/yr in US are started by lightning
50 million $ worth of timber is destroyed per yr
Can we reduce the cloud-to-ground lightning?? Seeding cumulonimbus clouds with hair-thin pieces of Al wire (10cm long) --- metal will produce many tiny sparks and prevent the electrical potential in the cloud from building to a
point where lightning occurs
Tornadoes
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Tornadoes: A product of thunderstorms; rapidly rotating winds that blow around a small area of intense low
pressure
Tornadoe’s circulation is present on the ground either as a funnel-shaped cloud or as a swirling cloud of dust &
debris; majority rotate counterclockwise
Other shapes:
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Twisting ropelike funnels
Cylindrical-shaped funnels
Massive black funnels
Funnels that resemble an elephant’s trunk hanging from a large cumulonimbus cloud
Tornadoes – Features and stages
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Diameter (most): 100-600 m (few meters – 1,600 m rare)
Most last only a few minutes & average path length of ~7 km (largest one: 470 km; lasted for 7 hrs) in Illinois and
Indiana in 1917
Stages of a Tornadoes (most common):
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Dust-Whirl stage: Dust swirling upward from the surface – damage is light
Organizing Stage: Tornado increases in intensity with an overall downward extent of the funnel
Mature Stage: funnel reaches its greatest width & is almost vertical; damage is most severe
Shrinking stage: Overall decrease in the funnel’s width & increase in the funnel’s tilt; still capable of intense &
Tornadoes – Features and stages
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Sometimes violent damage
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Decay Stage: The final stage, usually finds the tornado stretched into the shape of a rope
Minor tornadoes may evolve only through certain stages
Damages:
~ 100 people/year killed (11/10/02 – 37 people died on a single day)
45% mortalities in mobile homes
March 18, 1925 tornadoes: 695 people died, 7 tornadoes traveled a total of 703 km across portions of Missouri, Illinois and
Indiana
Tornado outbreak
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Tornado Outbreak: A series of tornadoes that forms within a particular region, often associated with widespread
damage and destruction; a region may include several states
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April 3, 1974: 16 hour period, 148 tornadoes cut through parts of 13 states, 307 people killed, >3700 people injured,
damage >600 million $
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Occurrence: Most numbers in US; average: >1,000/yr; 1,424 during 1998
Tornado alley: Tornado belt, Central Plains, stretches from central Texas to Nebraska
A mature tornado with winds >150 knots rips through southern illinois
Tornado incidence by State; upper:number by each state (25 yrs); lower: average annual number/100,000
square miles; darker: greater frequency
Tornadoes and their impact
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Lifting railroad coach with 117 passengers and dumping it 25 m away
Schoolhouse was demolished and 85 students inside were carried over 100 m without one of them being killed
Most tornadoes have winds of less than 125 knots
Pressure in the center of a tornado may be more than 100 mb lower than the surrounding & there is a momentary
drop in outside pressure when tornado is above a structure
When confronted with a tornado, take shelter immediately (basement, stay away from windows, small bathroom,
closet, interior hallway)
Fujita scale for damaging wind
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Fujita Scale – contd.
Fujita Scale: Theodore Fujita in late 1960s --- classifying tornadoes according to their rotational wind speed based
on the damage done by the storm
Majority of tornadoes are F0 and F1 (weak ones) and only a few % are above the F3(violent) with ~ 1 F5/yr
Tornado Formation: Tornadoes tend to form with intense thunderstorms and a conditionally unstable atmosphere is
essential for their development
Most strong and violent tornadoes develop near the right rear sector of a severe thunderstorm
In order for a tornado to spawn a tornado, the updraft must rotate
Total wind speed of a tornado is greater on one side than on the other
A powerful multi-vortex tornado with three suction vortices
Conditions leading to the formation of severe thunderstorms that can spawn tornadoes; red boxed area:
tornadoes are likely to form
Where tornadoes are common
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Greatest tornado activity shifts northward from winter to summer
Winter: contrast between warm and cold air masses are the greatest over the southern Gulf states & tornadoes are
most likely to form in this region
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Spring: humid Gulf air surges northward, jet stream also moves northward; tornadoes more prevalent from the
southern Atlantic states westward into the southern Great Plains
Summer: contrast between air masses lessens & the jet stream is normally near the Canadian border; tornado
activity tends to be concentrated over the northern plains
Features associated with tornado-bearing thunderstorm; thunderstorm moves northeast; tornadoes form in
the southwest part
Mesocyclone
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Mesocyclone: A vertical column of cyclonically rotating air within a severe thunderstorm
Severe thunderstorms form in a region of strong vertical wind sheer; most strong and violent tornadoes form within
the mesocyclone
Existence of the swirling winds of the mesocyclone inside tornado-producing thunderstorms were observed 1970s
(first time) using Doppler Radar
30% of all mesocyclones produce tornadoes & 95% produce severe weather
Time between mesocyclone identification & tornado touching the ground is ~20 minutes
Tornadoes – contd.
Gustnadoes: Tornadoes that form along the gust front
Wall cloud: An area of rotating clouds that extends beneath a severe thunderstorm and from which a funnel cloud
may appear
Tornado Watch: Issued by Storm Prediction Center in Norman, Oklahoma
Doppler radar can detect areas of precipitation & measure rainfall intensity
Tornado Vertex signature (TVS): An image of a tornado on the Doppler radar screen that shows up as a small
region of rapidly changing wind directions inside a mesocyclone
A computer model illustrating motions inside a severe tornado-generating thunderstorm
Waterspouts
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Doppler Lidar: uses a light beam (instead of microwaves) to measure the change in frequency of falling
precipitation, cloud particles, and dust
Waterspout: A rotating column of air over a large body of water; tend to move slowly than tornadoes; last for only
10-15 minutes
Doppler radar display of large supercell thunderstorm that is spawning an F4 tornado (circled are) near
Lula, OK
Average annual number of tornadoes & tornado deaths by decade
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Summary – Chapter - 10
Stages of a thunderstorm and a tornado
Air-mass thunderstorm, multicell & supercell thunderstorm
Gust front, causes for downdraft, microburst
Squall line, dry line
Suitable time for the formation of thunderstorm
Lightning and thunder – formation and features
Funneling cloud, mesocyclone, wall cloud
Fujita scale
Direction of movement of tornadoes, conditions for its formation, waterspout