METEOROLOGY

GEL-1370

Chapter TenChapter Ten

Thunderstorms andThunderstorms andTornadoesTornadoes

Goal for this ChapterWe 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?

Thunderstorms• 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• 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 4-kms 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 stormsSummer 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• >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• 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 homesMarch 18, 1925 tornadoes: 695 people died, 7 tornadoes

traveled a total of 703 km across portions of Missouri, Illinois and Indiana

Tornado outbreak• 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

• .Scale Category Mi./hr knots Expected damage

F0

F1

Weak 40-72

73-112

35-62

63-97

Light; tree branches broken; sign boards damagedModerate; trees snapped; windows broken

F2

F3

Strong 113-157

158-206

98-136

137-209

Considerable; large trees uprooted, weak struc. DestroyedSevere trees leveled, cars over-turned, walls removed from bldg.

F4

F5

Violent 207-260

261-318

180-226

227-276

Devastating frame houses dstroyedIncredible; structure the size of autos moved over 100 m

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• 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

• .Decade Tornadoes/year Deaths/year

1950-1959 480 148

1960-1969 681 94

1970-1979 858 100

1980-1989 819 52

1990-1999 1,220 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