GEL-1370

We are going to learn answers to the following questions:
• 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?

•
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

• Stages of ordinary thunderstorms:
– 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.
• 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.
• 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.
• 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 4kms or less
• Supercell and Squall-line thunderstorms:
– 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.
• 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
• 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:
– 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

• >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
• 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
• 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
• 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
• Cold, upper part becomes +ively charged & middle of the cloud becomes –ively charged

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
• 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
• 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.
• 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
• 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
• 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
(10-cm long) --- metal will produce many tiny sparks and prevent the electrical potential in the cloud from building to a point where lightning occurs

• 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:
– 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
• 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):
– 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
• Sometimes violent damage
– 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: A series of tornadoes that forms within a particular region, often associated with widespread damage and destruction; a region may include several states
• April 3, 1974: 16 hour period, 148 tornadoes cut through parts of 13 states, 307 people killed, >3700 people injured, damage >600 million $
• 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
• 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
• .
Category Mi./hr knots Expected damage
F3
F4
F5
F0 Weak
F1
F2 Strong
Violent
40-72
73-112
113-157
158-206
207-260
261-318
35-62
63-97
Light; tree branches broken; sign boards damaged
Moderate; trees snapped; windows broken
98-136
137-209
Considerable; large trees uprooted, weak struc. Destroyed
Severe trees leveled, cars overturned, walls removed from bldg.
180-226
227-276
Devastating frame houses dstroyed
Incredible; structure the size of autos moved over 100 m

• 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

• 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
• 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
• 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
• 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
1950-1959
1960-1969
1970-1979
1980-1989
1990-1999
Tornadoes/year Deaths/year
480 148
681
858
94
100
819
1,220
52
56

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