Tornadoes

Tornadoes

Tornado A violently rotating column of

air (vortex), hanging from a cumulonimbus cloud, with circulation that touches the surface of the earth

Other Names for Tornadoes

• Tornado comes from the Spanish word, tronada, which means “thunderstorm.”

• twisters, dust devils, whirlwinds, waterspouts, and cyclones

Waterspouts

A waterspout is a tornado that forms over a body of water, or a tornado that moves from land onto water

Tornado Facts• Tornadoes can occur almost anywhere in the world• Duration: a few minutes• Diameter (Avg.): 0.4 km• Length of path (Avg.): 6 km• Funnel can travel from 0 mph up to ~70 mph,

usually travels at 30 mph• 99% of all tornadoes in Northern Hemisphere

rotate counterclockwise• Texas is #1 for frequency of tornadoes per year• Between 1950 and 1995 Texas had 5,722

recorded tornadoes• Risk of death in a tornado in Texas: 1 in 1,054,267

How do tornadoes form?

1. Winds at 2 different altitudes blow at 2 different speeds creating “wind shear”

(Creates an invisible, horizontal spinning effect in the lower atmosphere) http://www.nssl.noaa.gov/edu/safety/tornadoguide.html

Wind Shear

How do tornadoes form?

2. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

http://www.nssl.noaa.gov/edu/safety/tornadoguide.html

How do tornadoes form?

An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation. http://www.nssl.noaa.gov/edu/safety/tornadoguide.html

Beginning Stage: Tornado begins as a rotating wall cloud which quickly evolves into a funnel

Early Stage: Tornado funnel

develops (may be transparent) and extends down from the cloud to the ground

Mature Stage: Tornado funnel reaches maximum width as well as maximum intensity then begins to shrink

Decay Stage:Tornado may remain stationary and take on a ropelike appearance before dissipating

Tornado Wind SpeedIn 1971, Dr. Fujita

developed a way of measuring the winds of a tornado. He reasoned that there was a link between wind speed and the damage caused by a tornado. There are 6 categories of tornados

(F0 – F5)*** IMPORTANT NOTE ABOUT F-SCALE WINDS: Do not use F-scale winds literally. These precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place -- even from building to building. Without a thorough engineering analysis of tornado damage in any event, the actual wind speeds needed to cause that damage are unknown.

*** IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS: The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed.

Fujita vs Enhanced Fujita

http://www.spc.noaa.gov/faq/tornado/ef-scale.html

Weak Tornadoes• 69% of all tornadoes • Less than 5% of

tornado deaths • Lifetime 1-10+

minutes • Winds less than 110

mph

F0 Category

• (Weak) winds (40-72) mph , little damage• Damage: tree branches snapped,

chimneys toppled, signs torn down

Strong Tornadoes• 29% of all

tornadoes • Nearly 30% of all

tornado deaths • May last 20

minutes or longer • Winds 110-205

mph

F3 Category

• (Strong) winds: (158-206) mph, severe damage

• Damage: most trees uprooted, trains overturned, roofs torn off, walls demolished

Violent Tornadoes• Only 2% of all

tornadoes • 70% of all

tornado deaths • Lifetime can

exceed 1 hour

F5 Category

• (Violent) winds: (261- 319) mph, incredible damage; rare

• Damage: bark peeled off trees, houses lifted off foundations, vehicles travel greater than 100 m through the air

Tornado Occurrence by Category

Tornado Deaths by Category

Tornado Indicators• Greenish colored sky• Mammatus clouds • Sudden drop in barometric pressure• Large hail of at least .75 in. diameter• Strong winds > 60 mph• Frequent and intense lightning• Rotating wall cloud or a cloud that appears to

hang from the sky• A loud rumbling noise- seek shelter!

Mammatus clouds Green sky

Tornado Frequency• Southern states: peak March - May, • Northern states: Summer. • Time of day: peak 3 and 9 p.m

Where Tornados Occur

“Tornado Alley” covers the Great Plains states

Did Dorothy from the Wizard of Oz live in “Tornado Alley?”

Dopplar Radar• Uses radio

waves to measure intensity of precipitation and how fast the rain or hail is moving toward or away from the radar

Tornados on Radar

Doppler image of a rain-wrapped tornado

Tornado Watches & Warnings

• Watch - conditions are favorable for severe weather to develop

• Warning - information provided by weather radar, and spotters (law enforcement) on the ground, severe weather is imminent.

• Severe thunderstorm warnings are passed to local radio and television stations and are broadcast over local NOAA Weather Radio stations serving the warned areas. These warnings are also relayed to local emergency management and public safety officials who can activate local warning systems to alert communities.

What to do…If a Warning is issued or if threatening weather approaches:• In a home or building, move to a pre-designated shelter, such

as a basement. • If an underground shelter is not available, move to an interior

room or hallway on the lowest floor and get under a sturdy piece of furniture.

• Stay away from windows. • Get out of automobiles. • Do not try to outrun a tornado in your car; instead, leave it

immediately. • Mobile homes, even if tied down, offer little protection from

tornadoes and should be abandoned.

Tornado History• “Tri-State Tornado” - the

most violent tornado on record

• March 18, 1925, the tornado formed in Missouri and traveled 219 miles across Illinois into Indiana

• The funnel was up to ¾ of a mile across and traveled as fast as 73 mph.

• It killed approximately 635 people

Tornado Myths• A highway overpass is a safe place to

take shelter under during a tornado

• Opening windows during a tornado will help balance the pressure between the inside and outside of the house and may prevent destruction of the structure

• One should seek shelter in the southwest corner of a house or basement.

Tornado OdditiesTornados are reported to routinely carry objects many miles and have:

– sucked the frogs out of a pond and dropped them on a town

– carried a necktie rack with 10 ties attached 40 miles

– carried a flour sack 110 miles from a millTornados also drive objects into other objects and have:

– Driven splinters into an iron fire hydrant– Driven straw and grass into telephone poles

View from the air of a tornado path in central OK