Basic Facts About Tornadoes

A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground.  The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more.  Damage paths can be in excess of one mile wide and 50 miles long.  Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!

The United States reports in excess of 1,000 tornadoes per year; more than any other country in the world.  It also reports more violent (F-4 and F-5) tornadoes than any other country.  Tornadoes are common in many states but are most common to the west of the Appalachian Mountains and to the east of the Rockies.  Therefore, this region is often referred to as “tornado alley”.  

The Atlantic seaboard states - North Carolina, South Carolina, Georgia and Virginia - are also vulnerable, as well as Florida.  The areas most vulnerable to tornadoes are the Southern Plains and Florida, though most Florida tornadoes are relatively weak.  The Southern United States region is one of the worst affected regions in terms of casualties.

Tornado reports have only been officially collated since 1950.  Prior to then, the Weather Bureau could not mention tornadoes in forecasts for fear of causing panic!  Reports of tornadoes since 1950 have been gathered by the National Climatic Data Center (NCDC), based in Asheville, North Carolina.  It is important to note that sometimes a tornado can be reported more than once (e.g. - when such a storm crosses a county line, reports are made from both counties).

The most active tornado region of all is the southern part of The Great Plains.  According to figures from the NCDC, Texas had 7,565 tornado reports for the period 1950 to 2006 inclusive, followed by Kansas (3,298) and Oklahoma (3, 263).  Oklahoma and Florida vie with each other for the most tornadoes per unit area but Florida tornadoes only rarely approach the intensity of the Plains storms. 

Oklahoma has reported ninety eight F-4 / F-5 tornadoes over the same period; Texas reported eighty four and Kansas seventy four.  Florida - by contrast - has only reported four and none of these were F-5 category.  Kansas has reported more F-5 (or eF-5) tornadoes over the period 1950 - 2007 than any other state.  Other notable tornadoes to ravage the Southern Plains include the:

1.     Glazier-Higgins-Woodward Tornadoes in April, 1947

2.     Goliad Tornado in May, 1902

3.     Waco Tornado in May, 1953 the Wichita Falls Tornado in April, 1979

4.     Andover, Kansas Tornado Outbreak in April, 1991 and

5.     Jarrell Tornado of May, 1997.

The Oklahoma Tornado Outbreak of May, 1999, was the most costly in U.S. history.  Texas has reported more tornado fatalities than any other state, although the sheer size of the state should be taken into account here.

The Midwest and the states around the Great Lakes are also tornado prone.   Nebraska is fifth overall for sheer numbers of tornadoes, while Indiana has had eighty-eight (88) 'violent' tornado reports from the 1950 - 2006 period, more than any state except Oklahoma.   Iowa reported eighty three, almost as many as Texas.

The entire region is vulnerable - from western Pennsylvania to eastern Wyoming, and from Nebraska to the Canadian border.  The most deadly tornado in US history - the Tri-State Tornado - struck Missouri, Illinois and Indiana in March, 1925.  St. Louis, Missouri and neighboring East St. Louis, Illinois have been hit more than once by violent tornadoes - the most notorious of which was the St. Louis Tornado of May, 1896.

The New Richmond Tornado of May, 1899 and the Flint, Michigan tornado of June, 1953, also rank amongst the most deadly tornadoes in US history.  The region was badly hit by the Palm Sunday Tornado Outbreak in April, 1965 and by the Super Outbreak of April, 1974.

The Southern United States region has suffered more tornado fatalities than any other part of the country.   Part of the reason for this has been that housing in this region is often of a weaker construction than elsewhere in the U.S.  The population density in these southern states - though not particularly high - is nonetheless higher than the very sparsely populated Plains states such as Iowa, Nebraska and the Dakotas.  However, this region also encounters more long - tracked violent tornadoes than any other region.

Some areas are particular Tornado “blackspots”, such as the Tennessee Valley in northern Alabama.  The state of Alabama has reported more F5 tornadoes than any state except for Kansas.  For the period 1950 to 2006, three hundred and fifty eight people were killed by tornadoes in Alabama, ranking the state third nationwide behind Texas (521) and neighboring Mississippi (404).  Fourth is Arkansas (336) and fifth is Tennessee with 271 fatalities.  

Some people mistakenly believe that tornadoes only occur out in the countryside. This is hardly the case.  While it is true that the plains states are the most tornado-prone places in the nation, it should be noted that tornadoes have been reported in every U.S. state, including Alaska and Hawaii.

One likely reason why tornadoes are so common in the central U.S. is because this is where Arctic air first collides with warm tropical air from the Gulf of Mexico where the cold front has not been "weakened" yet.   As it heads further east, however, it is possible for the front to lose its strength as it travels over warmer air.  Therefore, tornadoes are not as common on the East Coast as they are in the Midwest.   However, they have occurred on the East Coast on rare occasion, such as the F-2 twister that struck the northern suburbs of New York City on July 12, 2006, 

or the F-2 twister in parts of Brooklyn, New York on August 8, 2007.

Tornadoes can occur west of the continental divide but they are infrequent and usually relatively weak with a short duration.  Recently tornadoes have struck the Pacific coast town of Lincoln City, Oregon (1996); Sunnyvale, California (1998); and downtown Salt Lake City, Utah (1999).

The California Central Valley is an area of some frequency for tornadoes, albeit of weak intensity.  More tornadoes occur in Texas than in any other US state.  The state which has the highest number of tornadoes per unit area is Florida, although most of the tornadoes in Florida are weak tornadoes of F-0 or F-1 intensity.  A number of Florida's tornadoes occur along the edge of hurricanes that strike the state.

The state with the highest number of strong tornadoes per unit area is Oklahoma. The neighboring state of Kansas is another particularly notorious tornado state.   It records the most F-4 and F-5 tornadoes in the country.  It should be mentioned that states such as Oklahoma and Kansas have much lower population densities than Florida and that some tornadoes may go unreported.

How Tornadoes Form

Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most commonly and frequently to the right and ahead of the path of the storm center as they come onshore.

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts.  These thunderstorms often produce large hail, strong winds, and tornadoes.  Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east.

Occasionally, large outbreaks of tornadoes occur with strong and eastwardly moving weather patterns.  Several states may be affected by numerous severe thunderstorms and tornadoes.

During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west.  Tornado-producing thunderstorms may form as the “dryline” moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain.  If other favorable weather conditions exist, these thunderstorms can produce tornadoes.

Tornado Variations

·         Some tornadoes may form during the early stages of rapidly developing
     thunderstorms.  This type of tornado is most common along the front range of

     the Rocky Mountains, the Plains, and the Western States.  

·         Tornadoes may appear nearly transparent until dust and debris are picked up.
 

·          Occasionally, two or more tornadoes may occur at the same time.

 

Waterspout Variations

·         Waterspouts are weak tornadoes that form over warm water.

·         Waterspouts are most common along the Gulf Coast and southeastern states.  

     In the western United States, they occur with cold late fall or late winter storms,

     during a time when you least expect tornado development.

·         Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.

Tornado Measurement and Intensity Rating

Over time, several scales have been developed and utilized to measure tornado intensity and damage.  The TORRO Scale, the Beaufort Scale and the Fujita Tornado Intensity Scale are the most widely known tornado “scoring or rating” scales of recent times.

 

The most frequently used scale is The Fujita Tornado Intensity Scale and subsequently, the Enhanced Fujita (eFS) Scale.  While the Fujita Scale may not be a perfect system or tool for linking damage to wind speed, it does have distinct advantages over its predecessors and tornado intensity ratings before its inception.   The Fujita Scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure (and/or landscape).  The scale has six (6) categories and groups tornadoes in four (4) “families” as described below:
 

Gale - Moderate Tornadoes (F-0 and F-1 Categories)

1.    Represent 69% of all tornadoes.

2.    Result in less than 5% of all tornado related deaths.

3.    Typically last from 1 – 10+ minutes.

4.    Gale category tornadoes have winds of 40 – 72 mph.

5.    Moderate tornadoes have winds reaching 73 - 112 mph.

                                                                                              

Significant – Severe Tornadoes (F-2 and F-3 Categories)

1.    Represents 29% of all tornadoes.

2.    Result in approximately 30% of all tornado related deaths.

3.    Typically last up to 20 minutes or longer.

4.    Significant tornadoes have winds reaching 113 – 157 mph.

5.    Severe tornadoes have winds reaching 158 - 206 mph.

 

Devastating - Incredible Tornadoes (F-4 and F-5 Categories)

1.     Represent only 2% of all tornadoes.

2.     Result in approximately 70% of all tornado deaths.

3.     Can last up to 20 minutes or longer.

4.     Typically span miles of geography and have very unpredictable movement patterns.

5.     Devastating tornadoes have winds reaching 207 – 260 mph.

6.     Incredible tornadoes have winds reaching 261 - 318 mph.

Inconceivable Tornadoes (F-6 Category)

1.     Generally surrounded by F-4 and F-5 gales; generally not identifiable as a single “swirl” pattern or tornado.

2.     No known and individually recorded F-6 category tornadoes to date.

3.     Wind speed could reach 319 – 379 mph.

 

Understanding and Using the Fujita Scale (F-Scale)

Tornadic activity or storms may be accompanied by complex combinations of strong downbursts and other straight line winds.  Therefore, separating tornado damage from other wind damage makes for a daunting and difficult task for even the most experienced surveyor or climatologist in “rating or scoring” tornadic intensity and damage.

 

Tornadoes are VERY, VERY dangerous events that can be VERY, VERY destructive dependent upon what is in their path!!  Therefore, it is important to understand thattornadoes basically have three (3) factors that influence the Fujita Scale rating [1]:

1.     Loss of life.

2.     Property damage/destruction

3.     Wind speed and size.

 

Implications Of These Factors On F-Scale Ratings

The Seymour, TX tornado in April 1979 is a prime example of how these factors could significantly influence the Fujita Scale rating.  It produced only telephone and tree damage as it passed through rural areas with few buildings or man-made structures in its path.  Based on this, the tornado could not be rated higher than an F-2 category event.  However, this tornado had an intensity sufficient to have been rated as an F-4 category had it passed through a town.

 

The Wichita Falls, TX tornado stayed on the ground an hour and traveled 47 miles as it wiped out a fifth of the city and damaged even more areas before dissipating in Oklahoma.  It was rated an F-4, with winds from 207-260 mph.  In many cases, the center room of a house or business - even a bank vault - was the only thing standing after a tornado up to a mile-and-a-half wide churned through town, hitting 5,000 houses and several apartments.  Some 20,000 people - nearly one-fourth of the city's population - were homeless, while 42 were killed and 1,700 injured.

 

The Seymour tornado was in the same family (considering the wind speed and size factors) as the devastating Wichita Falls, TX tornado (which occurred on the same day). The Wichita Falls tornado remains one of, if not the most damaging tornadoes in U.S. history and the fifth (5^th) deadliest in the state’s history.
 

The Most Deadly U.S. Tornadoes

The United States gets about 1,000 recorded tornadoes every year.  Today, only a few are killers, but that has not always been so.  About 200 US tornadoes have killed 18 or more people.  Of those, about 150 occurred in the 70 year period between 1879 and 1949.  

 

There have been about 45 tornadoes since 1950 that have killed 18 or more people.  In the 1950s, there were 18 tornadoes that killed 18 or more people.  In the 1960s, there were 12 tornadoes that killed 18 or more people.  In the 1970s, there were 11 tornadoes that killed 18 or more people.  And in the 1980s, there were only 2 tornadoes that killed more than 18 people.  In spite of an ever-burgeoning population, death figures continue to go down as improved forecasting, detection, communications, and public awareness increase.

 

 

 

The table below lists the Top Ten U.S. Killer Tornadoes.