Resource Section  for Meteorology

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 | The Basic about Tornadoes | Tornadoes Forecasting |Tornado Damage | Tornado safety | Historic tornadoes | Tornado Climatology | Quiz | Tornadoes :Can you be safe ? | Credits |

The Basic about Tornadoes :

What is a tornado ?

A tornado is defined as a violently rotating column of air in contact with the ground and pendent from a cumulonimbus cloud.

They can be categorized as "weak", "strong", and "violent"; with weak tornadoes often having a thin, rope-like appearance, as exhibited by this tornado near Dawn, Texas. About 7 in 10 tornadoes are weak, with rotating wind speeds no greater than about 110 MPH. (looking west from about 1 mile.)

The typical strong tornado often has what is popularly considered a more "classic" funnel-shaped cloud associated with the whirling updraft. Rotating wind speeds vary from 110 to 200 MPH.

Nearly 3 in 10 tornadoes are strong, such as this twister on the plains of North Dakota. Looking northeast (from about 2 miles), note the spiraling inflow cloud, probably a tail cloud, feeding into the tornado. An important safety consideration is that weak and strong tornadoes by definition do not level well-built homes. Thus, a secure home will offer shelter from almost 100 percent of all direct tornado strikes.

Only violent tornadoes are capable of leveling a well-anchored, solidly constructed home. Fortunately, less than 2 percent of all tornadoes reach the 200+ MPH violent category. Furthermore, most violent tornadoes only produce home-leveling damage within a very small portion of their overall damage swath. Less than 5 percent of the 5,000 affected homes in Wichita Falls, Texas were leveled by this massive 1979 tornado. (Looking south from 5 miles).

Note the huge, circular wall cloud above the tornado. This feature is probably close both in size and location to the parent rotating updraft (called a mesocyclone) which has spawned the violent tornado. Strong and violent tornadoes usually form in association with mesocyclones, which tend to occur with the most intense events in the thunderstorm spectrum.

How do tornadoes form ?

"warm moist Gulf air meets cold Canadian air and dry air from the Rockies" -- is a gross oversimplification. Many thunderstorms form under those conditions (near warm fronts, cold fronts and drylines respectively), which never even come close to producing tornadoes. Even when the large-scale environment is extremely favorable for tornadic thunderstorms, as in an SPC "High Risk" outlook, not every thunderstorm spawns a tornado. 

What direction do tornadoes come from ?

 
Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked. [A tornado can double back suddenly, for example, when its bottom is hit by outflow winds from a thunderstorm's core.] Some areas of the US tend to have more paths from a specific direction, such as northwest in Minnesota or southeast in coastal south Texas. This is because of an increased frequency of certain tornado-producing weather patterns (say, hurricanes in south Texas, or northwest-flow weather systems in the upper Midwest).

Does hail always come before the tornado? Rain? Lightning? Utter silence?

Not necessarily, for any of those. Rain, wind, lightning, and hail characteristics vary from storm to storm, from one hour to the next, and even with the direction the storm is moving with respect to the observer. While large hail can indicate the presence of an unusually dangerous thunderstorm, and can happen before a tornado, don't depend on it. Hail, or any particular pattern of rain, lightning or calmness, is not a reliable predictor of tornado threat.

 How long does a tornado last?

Tornadoes can last from several seconds to more than an hour. The longest-lived tornado in history is really unknown, because so many of the long-lived tornadoes reported from the early 1900s and before are believed to be tornado series instead. Most tornadoes last less than 10 minutes.

What is the F- scale ?

Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and Pearson 1973) for winds, including tornadoes, which is supposed to relate the degree of damage to the intensity of the wind. This scale was the result. The F-scale should be used with great caution. Tornado wind speeds are still largely unknown; and the wind speeds on the F-scale have never been scientifically tested and proven. Different winds may be needed to cause the same damage depending on how well-built a structure is, wind direction, wind duration, battering by flying debris, and a bunch of other factors. Also, the procDess of rating the damage itself is largely a judgment call -- quite inconsistent and arbitrary (Doswell and Burgess, 1988). Even meteorologists and engineers highly experienced in damage survey techniques may come up with different F-scale ratings for the same damage. Even with all its flaws, the F-scale is the only widely used tornado rating method, and probably will remain so until ground-level winds can be measured in most tornadoes.

*** 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.

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What is the Enhanced F-Scale ?

The Enhanced F-scale will be a much more precise way to rank tornado damage than the original, because it will classify damage F0-F5 calibrated by engineers across over 20 different types of buildings. A team of meteorologists and engineers has worked on this for several years. The idea is that a "one size fits all" approach just doesn't work in rating tornado damage, and that a tornado scale needs to take into account the typical strengths and weaknesses of different types of construction. This is because the same wind does different things to different kinds of buildings. In the Enhanced F-scale, there will be different, customized standards for assigning any given F rating to a well built, well anchored wood-frame house compared to a garage, school, skyscraper, unanchored house, barn, factory, utility pole or other type of structure. In a real-life tornado track, these ratings can be mapped together more smoothly to make a damage analysis. Of course, there still will be gaps and weaknesses on a track where there was little or nothing to damage, but such problems will be less common than under the original F-scale. More details on the final version will be posted here when available. Until then, this PDF document by J. McDonald of Texas Tech provides a more thorough overview of the idea.

What is a "significant" Tornado?

 
A tornado is classified as "significant" if it does F2 or greater damage on the F scale. Grazulis (1993) also included killer tornadoes of any damage scale in his significant tornado database. It is important to know that those definitions are arbitrary, for scientific research. No tornado is necessarily insignificant. Any tornado can kill or cause damage; and some tornadoes rated less than F2 probably could do F2 or greater damage if they hit a well-built house during peak intensity.

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Tornadoes Forecasting :

Who forecasts tornadoes?

In the U.S, only the National Weather Service (NWS) issues tornado forecasts nation  wide. Warnings come from each local NWS office. The Storm Prediction Center issues watches, general severe weather outlooks, and mesoscale discussions.

How do you forecast tornadoes?

This is a  simple question with no simple answer! Here is a very generalized view from the perspective of a severe weather forecaster: When predicting severe weather (including tornadoes) a day or two in advance, we look for the development of temperature and wind flow patterns in the atmosphere which can cause enough moisture, instability, lift, and wind shear for tornadic thunderstorms. Those are the four needed ingredients. But it is not as easy as it sounds. "How much is enough" of those is not a hard fast number, but varies a lot from situation to situation -- and sometimes is unknown! A large variety of weather patterns can lead to tornadoes; and often, similar patterns may produce no severe weather at all. To further complicate it, the various computer models we use days in advance can have major biases and flaws when the forecaster tries to interpret them on the scale of thunderstorms. As the event gets closer, the forecast usually (but not always) loses some uncertainty and narrows down to a more precise threat area. [At SPC, this is the transition from outlook to mesoscale discussion to watch.] Real-time weather observations -- from satellites, weather stations, balloon packages, airplanes, wind profilers and radar-derived winds -- become more and more critical the sooner the thunderstorms are expected; and the models become less important. To figure out where the thunderstorms will form, we must do some hard, short-fuse detective work: Find out the location, strength and movement of the fronts, drylines, outflows, and other boundaries between air masses which tend to provide lift. Figure out the moisture and temperatures -- both near ground and aloft -- which will help storms form and stay alive in this situation. Find the wind structures in the atmosphere which can make a thunderstorm rotate as a supercell, then produce tornadoes. [Many supercells never spawn a tornado!] Make an educated guess where the most favorable combination of ingredients will be and when; then draw the areas and type the forecast.

What is the role of Doppler Radar in tornado forecasting?
Each NWS forecast office uses output from at least one Doppler radar in the area to help to determine if a warning is needed. Doppler radar signatures can tell warning meteorologists a great deal about a thunderstorm's structure, but usually can't see the tornado itself. This is because the radar beam gets too wide to resolve even the biggest tornadoes within a few tens of miles after leaving the transmitter. Instead, a radar indicates strong winds blowing toward and away from it in a way that tells forecasters, "An intense circulation probably exists in this storm and a tornado is possible." Possible doesn't mean certain, though. That is why local forecasters must also depend on spotter reports, SPC forecast guidance on the general severe weather threat, and in-house analysis of the weather situation over the region containing thunderstorms, to make the best-informed warning decisions

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Tornado Damage :

How is tornado damage rated ?

The most widely used method worldwide is the F-scale developed by Dr. T. Theodore Fujita. In Britain, there is a similar scale with more divisions; for more info, go to the TORRO scale website. In both cases, the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically verified in real tornadoes. Because:

Who surveys tornado damage ?

This varies from place to place; and there is no rigid criteria. The responsibility for damage survey decisions at each NWS office usually falls on the Warning-Coordination Meteorologist (WCM) and/or the Meteorologist in Charge (MIC). Budget constraints keep every tornado path from having a direct ground survey by NWS personnel; so spotter, chaser and news accounts may be used to rate relatively weak, remote or brief tornadoes. Killer tornadoes, those striking densely populated areas, or those generating reports of exceptional damage are given highest priority for ground surveys. Most ground surveys involve the WCM and/or forecasters not having shift responsibility the day of the survey. For outbreaks and unusually destructive events -- usually only a few times a year -- the NWS may support involvement by highly experienced damage survey experts and wind engineers from elsewhere in the country. Aerial surveys are expensive and usually reserved for tornado events with multiple casualties and/or massive degrees of damage. Sometimes, local NWS offices may have a cooperative agreement with local media or police to use their helicopters during surveys

How can a tornadoes destroy one house and leave the next one almost unscratched?

Most of the time, this happens either with multiple-vortex tornadoes or very small, intense single-vortex tornadoes. The winds in most of a multivortex tornado may only be strong enough to do minor damage to a particular house. But one of the smaller embedded subvortices, perhaps only a few dozen feet across, may strike the house next door with winds over 200 mph, causing complete destruction. Also, there can be great differences in construction from one building to the next, so that even in the same wind speed, one may be flattened while the other is barely nicked. For example, a flimsy, unanchored mobile home may be obliterated while all surrounding objects suffer little or no damage.
 

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Tornado Safety

There is no such thing as guaranteed safety inside a tornado. Freak accidents happen; and the most violent tornadoes can level and blow away almost any house and its occupants. Extremely violent F5 tornadoes are very rare, though. Most tornadoes are actually much weaker and can be survived using these safety ideas...

 Prevention and practice before the storm: At home, have a family tornado plan in place, based on the kind of dwelling you live in and the safety tips below. Know where you can take shelter in a matter of seconds, and practice a family tornado drill at least once a year. Have a pre-determined place to meet after a disaster. Flying debris is the greatest danger in tornadoes; so store protective coverings (e.g., mattress, sleeping bags, thick blankets, etc) in or next to your shelter space, ready to use on a few seconds' notice. When a tornado watch is issued, think about the drill and check to make sure all your safety supplies are handy. Turn on local TV, radio or NOAA Weather Radio and stay alert for warnings. Forget about the old notion of opening windows to equalize pressure; the tornado will blast open the windows for you! If you shop frequently at certain stores, learn where there are bathrooms, storage rooms or other interior shelter areas away from windows, and the shortest ways to get there. All administrators of schools, shopping centers, nursing homes, hospitals, sports arenas, stadiums, mobile home communities and offices should have a tornado safety plan in place, with easy-to-read signs posted to direct everyone to a safe, closeby shelter area. Schools and office building managers should regularly run well-coordinated drills. If you are planning to build a house, especially east of the Rockies, consider an underground tornado shelter or an interior "safe room".

 Know the signs of a tornado: Weather forecasting science is not perfect and some tornadoes do occur without a tornado warning. There is no substitute for staying alert to the sky. Besides an obviously visible tornado, here are some things to look and listen for:

Strong, persistent rotation in the cloud base.

Whirling dust or debris on the ground under a cloud base -- tornadoes sometimes have no funnel!

Hail or heavy rain followed by either dead calm or a fast, intense wind shift. Many tornadoes are wrapped in heavy precipitation and can't be seen.

Day or night - Loud, continuous roar or rumble, which doesn't fade in a few seconds like thunder.

Night - Small, bright, blue-green to white flashes at ground level near a thunderstorm (as opposed to silvery lightning up in the clouds). These mean power lines are being snapped by very strong wind, maybe a tornado.

Night - Persistent lowering from the cloud base, illuminated or silhouetted by lightning -- especially if it is on the ground or there is a blue-green-white power flash underneath.

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Historical Tornadoes

What were the deadliest U.S tornadoes?

The "Tri-state" tornado of 18 March 1925 killed 695 people as it raced along at 60-73 mph in a 219 mile long track across parts of Missouri, Illinois and Indiana, producing F5 damage. The death toll is an estimate based on the work of Grazulis (1993); older references have different counts. This event also holds the known record for most tornado fatalities in a single city or town: at least 234 at Murphysboro IL. The 25 deadliest tornadoes on record are listed here. We also have web links related to this and other major tornado events.
 

What were the deadliest U.S tornadoes days ?

On 3 April 1974, the main day of the two-day "Super Outbreak," tornadoes killed 308 people. The next deadliest day for tornadoes was 11 April 1965, the original "Palm Sunday Outbreak," where 260 perished. A list is online of top 20 deadliest tornado days since detailed record keeping began in 1950.
 

What was the biggest out break of tornadoes?

147 tornadoes touched down in 13 U.S. states on 3 and 4 April, 1974. Here is a map of them , with F-scale damage plotted beside each. [One more tornado touched down in Canada at Windsor ON, then lifted as it entered MI, for a total of 148. Since it did no damage in the U.S., it is not counted in the U.S. tornado database used to plot our map.] The outbreak killed 310 in the U.S., 8 in Canada, with 5454 U.S. injuries and 23 hurt in Canada. 48 of the tornadoes were killers. Seven produced damage rated F5 -- the maximum possible -- and 23 more were rated F4. This was one of only two outbreaks with over 100 confirmed tornadoes, the other being with Hurricane Beulah in 1967 (115 tornadoes). In 1999, NOAA Public Affairs created a large website on the 1974 super-outbreak in commemoration of its 25th anniversary. SPC also has a list of web links devoted to this and other major tornado events.

What was the biggest known tornado?

The Hallam, Nebraska F4 tornado of 22 May 2004 is the newest record-holder for peak width, at nearly two and a half miles, as surveyed by Brian Smith of NWS Omaha. This is probably close to the maximum size for tornadoes; but it is possible that larger, unrecorded ones have occurred.

What was the strongest tornado?

Tornado wind speeds have only been directly recorded in the weaker ones, because strong and violent tornadoes destroy weather instruments. Mobile Doppler radars such as the OU Doppler on Wheels have remotely sensed tornado wind speeds above ground level as high as 318 mph (3 May 1999 near Bridge Creek OK) -- the highest winds ever found near earth's surface by any means. [That tornado caused F5 damage.] But ground-level wind speeds in the most violent tornadoes have never been directly measured.
 

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Tornado Climatology

How many tornadoes hit the U.S yearly ?

About one thousand. The actual average is unknown, because tornado spotting and reporting methods have changed so much in the last several decades that the officially recorded tornado climatologies are believed to be incomplete. Also, in the course of recording thousands of tornadoes, errors are bound to occur. Events can be missed or mis-classified; and some non-damaging tornadoes in remote areas could still be unreported.

How many tornadoes have there been in the US this year and how does it compare to previous year ?

Such tornado report totals are in an online table of monthly tornado stastistics at the SPC. Remember, those are preliminary numbers which may be amended at any time.

What is tornado season ?

Tornado season usually means the peak period for historical tornado reports in an area, when averaged over the history of reports. There is a general northward shift in "tornado season" in the U.S. from late winter through mid summer. The peak period for tornadoes in the southern plains, for example, is during May into early June. On the Gulf coast, it is earlier during the spring; in the northern plains and upper Midwest, it is June or July. Remember: tornadoes can happen any time of year if the conditions are right! If you want to know the tornado peak periods for your area, Harold Brooks of NSSL has prepared numerous tornado probability graphics, which include distribution during the year.
 

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 Created by Niru Khanna