Tornadoes: What and why they occur Kevin Scott & Kevin Fitzgerald

What is a Tornado?  A local storm of short duration.  Violent wind storms that take the form of a rotating column of air extending down from a cumulonimbus cloud.  Produces severe thunderstorms, heavy winds, rainfall, & often damaging hail.

Occurrence & Development  Meteorologists are not sure what triggers tornado formation.  Can form in any situation that produces severe weather.  Important precondition linked to tornado development in severe thunderstorms is the development of a mesocyclone. –Mesocyclone is a vertical cylinder of rotating air, about 2 to 6 miles across, that develops in the updraft of a severe thunderstorm.

Atmospheric Conditions.  Where cold dry continental polar air meets the warm humid tropical air.  The greater the contrast when these air masses meet, the more intense storm.  Tornadoes occur slightly ahead of the cold front, in the zone of southwest winds.

Where Tornadoes Occur  Most tornadoes occur in the central part of the United States known as “Tornado Alley.” –Climatologists can only conclude that the weather conditions in this area are particularly suited for tornado development

Watches, Warnings, & Doppler  Tornado watches –For areas already identified in severe weather outlooks. Watch covers about 25,000 square miles for 4-6 hours.  Tornado warnings –When a tornado has actually been spotted. Last for about minutes.  Doppler radar –Can detect the initial formation and development of a mesocyclone.

Fujita Tornado Damage Scale.  F0 -Light Damage (<73 mph); Some damage to chimneys; branches broken off trees; shallow- rooted trees pushed over; sign boards damaged.  F1 - Moderate Damage ( mph); Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off road.  F2 - Considerable Damage ( mph); Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

Fujita Scale Continued…  F3 - Severe Damage ( mph); Roofs and some walls torn off well-constructed houses, trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.  F4 - Devastating Damage ( mph); Well- constructed houses leveled; structure with weak foundations blown off some distance; cars thrown and large missiles generated.  F5 - Incredible Damage ( mph); Strong frame houses lifted off foundations and swept away; automobile sized missiles fly through the air in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur.

What the Fujita Scale Really Means

Tornadoes  Average of 770 tornadoes reported annually in the United States.  Occur most April through June.  Lowest occurrences December and January.

Where 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  Average tornado has a diameter of 500 – 2000 feet.  Travels at about 30 miles per hour.  On average cuts a path about 6 miles long.  Maximum winds beyond 310 miles per hour.  Typically tornadoes move towards the northeast.

Continued…  63% of tornadoes are weak (F0-F1)  2% of tornadoes are classified as violent (F4-F5) –Account for 70% of tornado related deaths.  Tornadoes most difficult natural phenomena to forecast precisely.

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

A Final Image An F-4 in Kansas