Tornadoes – Types and Classifications

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Supercell Thunderstorm[2]
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Tornadoes form mainly from two types of thunderstorms: supercell and non-supercell.

  • Tornadoes that come from a supercell thunderstorm are the most common, and often the most dangerous. A rotating updraft is a key to the development of a supercell, and eventually a tornado.
  • Non-supercell tornadoes are circulations that do not form from organized storm-scale rotation. These tornadoes form from a vertically spinning parcel of air already occurring near the ground caused by wind shear from a warm, cold, or sea breeze front, or a dryline. When an updraft moves over the spinning, and stretches it, a tornado can form.[1]

Tornadoes are classified by type, but the more practical method is by intensity.

Tornadoes Classified by Type

Below are the main types of tornadoes that can form:

Multiple Vortex[4]

Multivortex (a.k.a. multiple-vortex) tornadoes contain two or more small, intense subvortices orbiting the center of the larger tornado circulation. These vortices may form and die within a few seconds, sometimes appearing to train through the same part of the tornado one after another. They can happen in all sorts of tornado sizes, from huge "wedge" tornadoes to narrow "rope" tornadoes.

Subvortices are the cause of most of the narrow, short, extreme swaths of damage that sometimes arc through tornado tracks. From the air, they can preferentially mow down crops and stack the stubble, leaving cycloidal marks in fields.

Multivortex tornadoes are the source of most of the old stories from newspapers and other media before the late 20th century which told of several tornadoes seen together at once.[3]

 

Water Spout[6]

A waterspout is a tornado over water--usually meaning non-supercell tornadoes over water. Waterspouts are common along the southeast U. S. coast--especially off southern Florida and the Keys--and can happen over seas, bays and lakes worldwide.

Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people.

The National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.[5]

A landspout is storm-chaser slang for a non-supercell tornado. So-called "landspouts" resemble waterspouts in that way, and also in their typically small size and weakness compared to the most intense tornadoes. But "landspouts" are tornadoes by definition; and they are capable of doing significant damage and killing people.[7]

A gustnado is a small and usually weak whirlwind which forms as an eddy in thunderstorm outflows. They do not connect with any cloud-base rotation and are not tornadoes. But because gustnadoes often have a spinning dust cloud at ground level, they are sometimes wrongly reported as tornadoes. Gustnadoes can do minor damage (e.g., break windows and tree limbs, overturn trash cans and toss lawn furniture), and should be avoided.[8]

A dust devil is a small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons.[9]

 

Tornadoes Classified by Intensity (EF Scale)

The most common and practical way to determine the strength of a tornado is to look at the damage it caused. From the damage, we can estimate the wind speeds. An “Enhanced Fujita Scale” was implemented by the National Weather Service in 2007 to rate tornadoes in a more consistent and accurate manner.

The Enhanced Fujita Scale, which went into effect on February 1, 2007, is an updated version of the original Fujita Scale that was developed by Ted Fujita with Allen Pearson, assigns a numerical rating from EF0 to EF5 to rate the damage intensity of tornadoes. EF0 and EF1 tornadoes are considered “weak” tornadoes, EF2 and EF3 are classified as “strong” tornadoes, with winds of at least major hurricane force, where EF4 and EF5 are categorized as “violent” tornadoes, with winds corresponding to category 5 hurricane winds and rising to match or exceed the strongest tropical cyclones on record.

The EF-Scale takes into account more variables than the original Fujita Scale (F-Scale) when assigning a wind speed rating to a tornado, incorporating 28 damage indicators such as building type, structures and trees. For each damage indicator, there are 8 degrees of damage ranging from the beginning of visible damage to complete destruction of the damage indicator.[10] [11]

Below are the levels of the new Enhanced Fujita Scale – in order of severity – and the damage descriptions as they relate to the original Fujita Tornado Damage Scale:[12] [13]

EF0 Damage[14]

EF0

Wind Speed: 65–85 mph (105–137 km/h)

Damage: Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.

 

 

 

 

EF1 Damage[15]

EF1

Wind Speed: 86-110 mph (138-178 km/h)

Damage: Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.

 

 

 

 

EF2 Damage[16]

EF2

Wind Speed: 111-135 mph (179-218 km/h)

Damage: Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

 

 

 

 

EF3 Damage[17]

EF3

Wind Speed: 136-165 mph (219-266 km/h)

Damage: Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.

 

 

 

 

EF4 Damage[18]

EF4

Wind Speed: 166-200 mph (267-322 km/h)

Damage: Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.

 

 

 

 

EF5 Damage[19]

EF5

Wind Speed: >200 mph (>322 km/h)

Damage: Explosive damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (300 ft); steel reinforced concrete structure badly damaged; high-rise buildings have significant structural deformation; incredible phenomena will occur.

 

 

 

For the original F-Scale, Fujita plotted hypothetical winds higher than F5. On the Enhanced F-scale, there is no such thing as "EF6" or higher. Damage--no matter how "incredible" or how strong the wind--maxes out at EF-5.

The 1999 Moore, OK tornado had doppler-indicated winds of 318 mph. Even if the winds measured by portable Doppler radar (32 meters above ground level, roughly 302 mph) had been over 318 mph, the tornado still would have been rated "only" F5, since that is the most intense possible damage level.[20]

 

 


References:

  1. NOAA – National Severe Storms Laboratory (NSSL) – Severe Weather 101 – Tornado Types: https://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/
  2. Image Source: http://www.erh.noaa.gov/rnk/SKYWARNonline_files/800×600/slide44.html [Accessed May 15, 2014]
  3. NOAA – The Online Tornado FAQs: http://www.spc.noaa.gov/faq/tornado/#multivortex1
  4. Image Source: http://www.srh.noaa.gov/oun/?n=events-19790410-tornado-wfalls [Accessed May 15, 2014]
  5. NOAA – The Online Tornado FAQs: http://www.spc.noaa.gov/faq/tornado/#waterspout1
  6. Image Source: http://www.vos.noaa.gov/MWL/dec_04/waterspout.shtml [Accessed May 15, 2014]
  7. NOAA – The Online Tornado FAQs: http://www.spc.noaa.gov/faq/tornado/#landspout1
  8. NOAA – The Online Tornado FAQs: http://www.spc.noaa.gov/faq/tornado/#gustnado1
  9. NOAA – NWS Glossary: http://forecast.weather.gov/glossary.php?word=DUST%20DEVIL
  10. NOAA – Storm Prediction Center – Enhanced F Scale for Tornado Damage: http://www.spc.noaa.gov/efscale/ef-scale.html
  11. National Weather Service – Enhanced Fujita Scale: http://www.spc.noaa.gov/faq/tornado/ef-ttu.pdf
  12. NOAA – Fujita Tornado Scale: http://www.spc.noaa.gov/faq/tornado/f-scale.html
  13. NOAA – Enhanced F Scale for Tornado Damage: http://www.spc.noaa.gov/efscale/ef-scale.html
  14. Image Source: http://www.crh.noaa.gov/lsx/?n=05_13_2009 [Accessed May 15, 2014]
  15. Image Source: http://www.srh.noaa.gov/ffc/?n=torfotos51108e [Accessed May 15, 2014]
  16. Image Source: http://www.srh.noaa.gov/ffc/?n=torfotos51108e [Accessed May 15, 2014]
  17. Image Source: http://www.srh.noaa.gov/ffc/?n=torfotos51108e [Accessed May 15, 2014]
  18. Image Source: http://www.erh.noaa.gov/cle/wx_events/2010/June/Jun5-6/survey.php [Accessed May 15, 2014]
  19. Image Source: http://www.srh.noaa.gov/srh/srnews/stories/2011/outbreak_smithvilleEF5.htm [Accessed May 15, 2014]
  20. NOAA – The Online Tornado FAQs: http://www.spc.noaa.gov/faq/tornado/#f-scale3

 

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