Outline for lecture 22
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Outline for Lecture 22. Thunderstorm Review. Lightning and Thunder. Tornadoes. Tornado Forecasting. The Development and Occurrence of Tornadoes. 4/10/2003. Thunderstorm development. cumulus stage : strong updrafts act to build the storm. .

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Outline for Lecture 22

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Outline for Lecture 22

Thunderstorm Review

Lightning and Thunder


Tornado Forecasting

The Development and Occurrence of Tornadoes


Thunderstorm development

cumulus stage: strong updrafts act to build the storm.

mature stage: heavy precipitation and cool downdrafts in part of the storm.

dissipating stage: warm updrafts disappear, rain stops, cloud dissipates

Well-developed supercell

lower stratosphere



Downbursts (microbursts)

Strong localized downdrafts below thunderstorms are called


Microbursts—small downbursts

(<4km wide)

The colder air is, the more dense, the more dense the

faster it will fall.

In microbursts, evaporative

cooling helps to cool the air

which leads to rapid


Wind speeds can exceed

100 mph. The winds

expand out from the center

of the burst.

Lightning and Thunder

Lightning is the result of charge separation (part of the cloud develops an excess of negative charge, whereas another part

acquires a positive charge).

The rapid vertical motion, present in mature cumulonimbus clouds, is key to charge separation.

How do we separate all that charge?















The formation of ice in clouds is important.

Charge separation within the freezing crystal is where it begins.

  • Interior of shell freezes

  • Expands & shatters

  • Leaves differently charged

  • fragments.

Discharge via cloud-to-ground lightning (10%)



Blue Flash




Lightning is a massive discharge of electricity. Electricity is

made up of electrons. When lightning strikes, massive amounts

of electrons shoot through the air at once. In the process, they

bump into a lot of air molecules.

When lightning strikes, intense heat (8,000°C - 33,000 °C) expands

the air in and around it so rapidly that it results in a loud noise we

know as thunder. THUNDER IS AN EXPLOSION!!


Tornadoes (twisters, cyclones) are violent windstorms that take the form of a rotating column of air

Air flows from higher to lower pressure

Wind speed is determined by the pressure gradient force

The pressure at the center of a tornado is as much as 10 percent lower than just outside it

How significant is a 10 percent drop?

Pressure gradients and winds


is ~ 3% difference…

...OVER 1500 miles!

1032mb/928mb is

a 10% difference.

The distance in a

tornado is ~5 miles.

Strong tornadoes are made up of several suction vortices

Suction vortices are 10–30 meters wide


Less than one percent of thunderstorms produce tornadoes.

Tornadoes can form in any situation that produces severe weather

including cold fronts, squall lines, and hurricanes. The most

intense tornadoes are typically from supercell thunderstorms.

For a tornado to occur, first amesocyclone (a vertical cylinder

of rotating air 2-6 miles wide) must develop. Tornadoes often

form about 30 minutes after mesocyclone formation.

Development of Tornadoes and Mesocyclones

Wind speed shear and strong vertical updrafts lead to the rotating

column of air.

Average annual tornado incidence per 10,000 square

miles for a 27 year period.



Interaction of cold, dry cP polar air masses with the warm, humid mT

tropical air masses results in majority of tornadoes over the U.S.

Average number of tornadoes and tornado days each month in the U.S.

Most tornadoes

occur in the spring.

Profile of a Tornado

  • Average tornado has a

  • diameter of 500-2000 ft.

  • Tornadoes move at

  • about 45 mph

  • The typical path is approximately 16 miles long.

Most tornadoes move to the

northeast (recall winds in

advance of a cold front are

from the southwest).

Tornado Intensity: Fujita Scale

  • F0<72 mphLight damage

  • F172-112Moderate

  • F2113-157Considerable

  • F3158-206Severe

  • F4207-260Devastating

  • F5>260Incredible

Fujita Scale

Tornadoes are classified according to the worst damage that they did anywhere along their path.

College Park tornado was an F3:

Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown

Doppler Radar

Doppler radars and faster

computers have greatly

improved our ability to

warn the public about


We now have our own!Well, OK, so it belongs to Channel 9

Doppler Effect

higher frequency

Christian Johann Doppler

in 1842 explained the process.

lower frequency

The wavelength of the radar echo changes is such a way to

determine movement of the thunderstorm.

Doppler image—2 mesocyclones

The tornado itself! (F3)

Damage from it

College Park Tornado

521 Reflectivity

516 Reflectivity

511 Reflectivity

526 Reflectivity

531 Reflectivity

526 Velocity

Tornado Safety


  • Go to the basement (if available) or to an interior room on the lowest floor, such as a closet or bathroom. Wrap yourself in overcoats or blankets to protect yourself from flying debris.


  • Go to interior rooms and halls on the lowest floor. Stay away from glass enclosed places or areas with wide-span roofs such as auditoriums and warehouses. Crouch down and cover your head.


  • Go to interior small rooms or halls. Stay away from exterior walls or glassy areas.


  • ABANDON THEM IMMEDIATELY!! Most deaths occur in cars and mobile homes. If you are in either of those locations, leave them and go to a substantial structure or designated tornado shelter.


Lie flat in the nearest ditch or depression and use your hands to cover your head.

Tornado damage: F0

Tornado damage: F1

Tornado damage: F2

Tornado damage: F3

Tornado damage: F4

Tornado damage: F5

Exam on Tuesday

  • Will cover all of Chapters 6-10

  • There won’t be much from Chapters 1-5, but the principles are all over the place, since Chapters 6-10 use stuff from the first five.

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