Lightning and Thunder

Tornadoes

The Development and Occurrence of Tornadoes

Tornado Forecasting

4/10/2003

Outline for Lecture 22Thunderstorm Review

cumulus stage: strong updrafts act to build the storm.

Thunderstorm development

mature stage: heavy precipitation and cool downdrafts in part of the storm. dissipating stage: warm updrafts disappear, rain stops, cloud dissipates

updraftsdowndrafts

lower stratosphere

Well-developed supercell

Strong localized downdrafts below thunderstorms are called downbursts.

Downbursts (microbursts)

(<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 airwhich leads to rapid acceleration.

Wind speeds can exceed 100 mph. The windsexpand out from the centerof the burst.

Microbursts—small downbursts

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.

Lightning and Thunder

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.

icewater

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How do we separate all that charge?

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

Sprite!

Sprite

Blue Flash

lightningCloud

Lightning is a massive discharge of electricity. Electricity is made up of electrons. When lightning strikes, massive amountsof 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) expandsthe air in and around it so rapidly that it results in a loud noise weknow as thunder. THUNDER IS AN EXPLOSION!!

Thunder

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?

Tornadoes

1032mb/996mbis ~ 3% difference…...OVER 1500 miles!

1032mb/928mb isa 10% difference.

The distance in a tornado is ~5 miles.

Pressure gradients and winds

Suction vortices are 10–30 meters wide

Strong tornadoes are made up of several suction vortices

Less than one percent of thunderstorms produce tornadoes.

Tornadoes can form in any situation that produces severe weatherincluding cold fronts, squall lines, and hurricanes. The most intense tornadoes are typically from supercell thunderstorms.

For a tornado to occur, first a mesocyclone (a vertical cylinder of rotating air 2-6 miles wide) must develop. Tornadoes often form about 30 minutes after mesocyclone formation.

Tornadoes

Wind speed shear and strong vertical updrafts lead to the rotatingcolumn of air.

Development of Tornadoes and Mesocyclones

Average annual tornado incidence per 10,000 square miles for a 27 year period.

mT

cP

Interaction of cold, dry cP polar air masses with the warm, humid mTtropical air masses results in majority of tornadoes over the U.S.

Most tornadoes occur in the spring.

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

•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 thenortheast (recall winds inadvance of a cold front arefrom the southwest).

Profile of a Tornado

Tornado Intensity: Fujita Scale

• F0 <72 mph Light damage

• F1 72-112 Moderate

• F2 113-157 Considerable

• F3 158-206 Severe

• F4 207-260 Devastating

• F5 >260 Incredible

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 fastercomputers have greatly improved our ability to warn the public about tornadoes

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

The wavelength of the radar echo changes is such a way to determine movement of the thunderstorm.

Christian Johann Doppler in 1842 explained the process.

higher frequency

lower frequency

Doppler Effect

Doppler image—2 mesocyclones

The tornado itself! (F3)

Damage from it

College Park Tornado

511 Reflectivity 516 Reflectivity 521 Reflectivity

526 Reflectivity 531 Reflectivity 526 Velocity

Tornado Safety

IN HOMES OR SMALL BUILDINGS:

• 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.

IN SCHOOLS, HOSPITALS, FACTORIES, OR SHOPPING

CENTERS • 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.

IN HIGH-RISE BUILDINGS:

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

IN CARS OR MOBILE HOMES

• 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.

IF NO SUITABLE STRUCTURE IS NEARBY:

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

• 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.

Exam on Tuesday