Tornadoes: What and why Tornadoes: What and why they occurthey occur

Kevin Scott Kevin Scott

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Kevin Fitzgerald Kevin Fitzgerald

What is a Tornado?What is a Tornado? A local storm of short A local storm of short

duration.duration. Violent wind storms Violent wind storms

that take the form of a that take the form of a rotating column of air rotating column of air extending down from a extending down from a cumulonimbus cloud.cumulonimbus cloud.

Produces severe Produces severe thunderstorms, heavy thunderstorms, heavy winds, rainfall, & often winds, rainfall, & often damaging hail. damaging hail.

Occurrence & DevelopmentOccurrence & Development

Meteorologists are not sure what triggers Meteorologists are not sure what triggers tornado formation. tornado formation.

Can form in any situation that produces Can form in any situation that produces severe weather. severe weather.

Important precondition linked to tornado Important precondition linked to tornado development in severe thunderstorms is the development in severe thunderstorms is the development of a mesocyclone. development of a mesocyclone. – Mesocyclone is a vertical cylinder of rotating air, Mesocyclone is a vertical cylinder of rotating air,

about 2 to 6 miles across, that develops in the about 2 to 6 miles across, that develops in the updraft of a severe thunderstorm. updraft of a severe thunderstorm.

Atmospheric Conditions.Atmospheric Conditions.

Where cold dry Where cold dry continental polar air continental polar air meets the warm humid meets the warm humid tropical air. tropical air.

The greater the The greater the contrast when these contrast when these air masses meet, the air masses meet, the more intense storm. more intense storm.

Tornadoes occur Tornadoes occur slightly ahead of the slightly ahead of the cold front, in the zone cold front, in the zone of southwest winds.of southwest winds.

Where Tornadoes OccurWhere Tornadoes Occur

Most tornadoes occur Most tornadoes occur in the central part of in the central part of the United States the United States known as “Tornado known as “Tornado Alley.”Alley.”– Climatologists can only Climatologists can only

conclude that the conclude that the weather conditions in weather conditions in this area are particularly this area are particularly suited for tornado suited for tornado developmentdevelopment

Watches, Warnings, & DopplerWatches, Warnings, & Doppler

Tornado watchesTornado watches– For areas already identified in severe weather For areas already identified in severe weather

outlooks. Watch covers about 25,000 square outlooks. Watch covers about 25,000 square miles for 4-6 hours. miles for 4-6 hours.

Tornado warnings Tornado warnings – When a tornado has actually been spotted. When a tornado has actually been spotted.

Last for about 30-60 minutes. Last for about 30-60 minutes.

Doppler radarDoppler radar– Can detect the initial formation and Can detect the initial formation and

development of a mesocyclone. development of a mesocyclone.

Fujita Tornado Damage Scale.Fujita Tornado Damage Scale. F0 -Light Damage (<73 mph); Some damage to F0 -Light Damage (<73 mph); Some damage to

chimneys; branches broken off trees; shallow-chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged. rooted trees pushed over; sign boards damaged.

F1 - Moderate Damage (73-112 mph); Peels F1 - Moderate Damage (73-112 mph); Peels surface off roofs; mobile homes pushed off surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off foundations or overturned; moving autos blown off road. road.

F2 - Considerable Damage (113-157 mph); Roofs F2 - Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homes demolished; torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars uprooted; light-object missiles generated; cars lifted off ground. lifted off ground.

Fujita Scale Continued…Fujita Scale Continued… F3 - Severe Damage (158- 206 mph); Roofs and F3 - Severe Damage (158- 206 mph); Roofs and

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

F4 - Devastating Damage (207- 260 mph); Well-F4 - Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure with weak constructed houses leveled; structure with weak foundations blown off some distance; cars thrown foundations blown off some distance; cars thrown and large missiles generated. and large missiles generated.

F5 - Incredible Damage (261- 318 mph); Strong F5 - Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations and swept frame houses lifted off foundations and swept away; automobile sized missiles fly through the air away; automobile sized missiles fly through the air in excess of 100 meters (109 yards); trees in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur. debarked; incredible phenomena will occur.

What the Fujita Scale Really MeansWhat the Fujita Scale Really Means

Tornadoes Tornadoes Average of 770 tornadoes reported annually in the Average of 770 tornadoes reported annually in the

United States. United States. Occur most April through June. Occur most April through June. Lowest occurrences December and January. Lowest occurrences December and January.

Where Tornadoes OccurWhere Tornadoes Occur

•This graph shows that in Hattiesburg, Mississippi has no discernable annual tornado cycle.

•This graph, taken from Lubbock, TX, (located in the heart of “Tornado Alley”) shows a definite annual tornado cycle.

Profiling Tornadoes Profiling Tornadoes

Average tornado has a Average tornado has a diameter of 500 – 2000 diameter of 500 – 2000 feet. feet.

Travels at about 30 miles Travels at about 30 miles per hour.per hour.

On average cuts a path On average cuts a path about 6 miles long. about 6 miles long.

Maximum winds beyond Maximum winds beyond 310 miles per hour.310 miles per hour.

Typically tornadoes move Typically tornadoes move towards the northeast. towards the northeast.

Continued…Continued… 63% of tornadoes are 63% of tornadoes are

weak (F0-F1)weak (F0-F1) 2% of tornadoes are 2% of tornadoes are

classified as violent classified as violent (F4-F5)(F4-F5)– Account for 70% of Account for 70% of

tornado related tornado related deaths. deaths.

Tornadoes most Tornadoes most difficult natural difficult natural phenomena to phenomena to forecast precisely. forecast precisely.

Two Days of DevastationTwo Days of Devastation

April 3-4, 1974, super tornado outbreak. It was the worst April 3-4, 1974, super tornado outbreak. It was the worst tornado outbreak in U.S. history with 148 twisters touching tornado outbreak in U.S. history with 148 twisters touching down in 13 states. Before it was over 16 hours later, 330 down in 13 states. Before it was over 16 hours later, 330 people were dead and 5,484 were injured in a damage people were dead and 5,484 were injured in a damage path covering more than 2,500 miles. path covering more than 2,500 miles.

A Final ImageA Final ImageAn F-4 in KansasAn F-4 in Kansas