THE BASICS ABOUT TORNADOES
What is a tornado? According to the Glossary of Meteorology (AMS 2000), a tornado is "a violently rotating
column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not
always) visible as a funnel cloud." Literally, in order for a vortex to be classified as a tornado, it must be in
contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice,
however, to classify and define tornadoes. For example, the difference is unclear between an strong
mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also
disgreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. It is
well-known that a tornado may not have a visible funnel. Also, at what wind speed of the cloud-to-ground
vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify
as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.
How do tornadoes form? The classic answer -- "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. The truth is that we don't fully understand.
The most destructive and deadly tornadoes occur from supercells -- which are rotating thunderstorms with
a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe
non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be
dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent
theories and results from the VORTEX program suggest that once a mesocyclone is underway, tornado
development is related to the temperature differences across the edge of downdraft air wrapping around
the mesocyclone (the occlusion downdraft). Mathematical modelling studies of tornado formation also
indicate that it can happen without such temperature patterns; and in fact, very little temperature variation
was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind
these theories are given in several of the Scientific References accompanying this FAQ.
What direction do tornadoes come from? Does the region of the US play a role in path direction?
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 do tornadoes dissipate? The details are still debated by tornado scientists. We do know tornadoes
need a source of instability (heat, moisture, etc.) and a larger-scale property of rotation (vorticity) to keep
going. There are a lot of processes around a thunderstorm which can possibly rob the area around a
tornado of either instability or vorticity. One is relatively cold outflow -- the flow of wind out of the precipitation
area of a shower or thunderstorm. Many tornadoes have been observed to go away soon after being hit by
outflow. For decades, storm observers have documented the death of numerous tornadoes when their
parent circulations ( mesocyclones) weaken after they become wrapped in outflow air -- either from the
same thunderstorm or a different one. The irony is that some kinds of thunderstorm outflow may help to
cause tornadoes, while other forms of outflow may kill tornadoes.
Do tornadoes really skip? Not in a literal sense, despite what you may have read in many older
references, news stories, or even damage survey reports. By definition (above), a tornado must be in
contact with the ground. There is disagreement in meteorology over whether or not multiple touchdowns of
the same vortex or funnel cloud mean different tornadoes (a strict interpretation). In either event, stories of
skipping tornadoes usually mean
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.
How close to a tornado does the barometer drop? And how far does it drop ? It varies. A barometer can
start dropping many hours or even days in advance of a tornado if there is low pressure on a broad scale
moving into the area. Strong pressure falls will often happen as the mesocyclone (parent circulation in the
thunderstorm) moves overhead or nearby. The biggest drop will be in the tornado itself, of course. It is very
hard to measure pressure in tornadoes since most weather instruments can't survive. A few low-lying,
armored probes called "turtles" have been placed successfully in tornadoes. This includes one
deployment on 15 May 2003 by engineer/storm chaser Tim Samaras, who recorded pressure fall of over
40 millibars through an unusually large tornado. On 24 June 2003, another of Tim's probes recorded a 100
millibar pressure plunge in a violent tornado near Manchester, SD. Despite those spectacyular results, and
a few fortuitous passes over barometers through history, we still do not have a database of tornado
pressures big enough to say much about average tornado pressures or other barometric characterstics.
What is a waterspout? 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 small boats, damage 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.
How are tornadoes in the northern hemisphere different from tornadoes in the southern hemisphere?
The sense of rotation is usually the opposite. Most tornadoes -- but not all! -- rotate cyclonically, which is
counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes
(clockwise-spinning in the northern hemisphere) have been observed, however -- usually in the form of
waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell's
mesocyclone. There have been several documented cases of cyclonic and anticyclonic tornadoes under
the same thunderstorm at the same time. Anticyclonically rotating supercells with tornadoes are extremely
rare; but one struck near Sunnyvale, CA, in 1998. Remember, "cyclonic" tornadoes spin counter-clockwise
in the northern hemisphere, and clockwise.
What is a multivortex tornado? Multivortex (a.k.a. multiple-vortex) tornadoes contain two or more small,
intense subvortices orbiting the center of the larger tornado circulation. When a tornado doesn't contain too
much dust and debris, they can sometimes be spectacularly visible. 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.
What is the original F-scale? Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and
Pearson 1973) for winds, including tornadoes, which was supposed to relate the degree of damage to the
intensity of the wind. This scale was the result. The original F-scale should not be used anymore, because
it has been replaced by an enhanced version. Tornado wind speeds are still largely unknown; and the wind
speeds on the original 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 process 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 often came up with
different F-scale ratings for the same damage. Even with all its flaws, the original F-scale was the only
widely used tornado rating method for over three decades. The enhanced F-scale takes effect 1 February
2007.
I heard the Oklahoma City tornado was almost "F6." Is that a real level on the original F-scale? Only in
untested theory. Fujita plotted hypothetical winds higher than F5; but as mentioned in the previous answer
above, they were only guesses. Even if a winds measured by portable Doppler radar (slightly above ground
level) had been over 318 mph, the tornado would still be rated "only" F5 since F5 is the most intense
possible damage level. On the Enhanced F-scale, there is no such thing as "F6."
What is a "significant" tornado? A tornado is classified as "significant" if it does F2 or greater damage on
the Enhanced 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.
Big fat tornadoes are the strongest ones, right? Not necessarily. There is a statistical trend (as
documented by NSSL's Harold Brooks) toward wide tornadoes having higher F-scale damage. This can be
out of more strength or out of greater opportunity for targets to damage -- or some blend of both. However,
the size or shape of any particular tornado does not say anything conclusive about its strength. Some small
"rope" tornadoes can still do violent damage of F4 or F5; and some very large tornadoes over a quarter-
mile wide have produced only weak damage of F0 to F1.
Can't we weaken or destroy tornadoes somehow, like by bombing them or sucking out their heat with a
bunch of dry ice? The main problem with anything which could realistically stand a chance at affecting a
tornado (e.g., hydrogen bomb) is that it would be even more deadly and destructive than the tornado itself.
Lesser things (like huge piles of dry ice or smaller conventional weaponry) would be too hard to deploy in
the right place fast enough, and would likely not have enough impact to affect the tornado much anyway.
Imagine the legal problems one would face, too, by trying to bomb or ice a tornado, then inadvertantly
hurting someone or destroying private property in the process. In short -- bad idea!
How does cloud seeding affect tornadoes? Nobody knows, for certain. There is no proof that seeding can
or cannot change tornado potential in a thunderstorm. This is because there is no way to know that the
things a thunderstorm does after seeding would not have happened anyway. This includes any presence
or lack of rain, hail, wind gusts or tornadoes. Because the effects of seeding are impossible to prove or
disprove, there is a great deal of controversy in meteorology about whether it works, and if so, under what
conditions, and to what extent.
What does a tornado sound like? That depends on what it is hitting, its size, intensity, closeness and
other factors. The most common tornado sound is a continuous rumble, like a closeby train. Sometimes a
tornado produces a loud whooshing sound, like that of a waterfall or of open car windows while driving very
fast. Tornadoes which are tearing through densely populated areas may be producing all kinds of loud
noises at once, which collectively may make a tremendous roar. Just because you may have heard a loud
roar during a damaging storm does not necessarily mean it was a tornado. Any intense thunderstorm wind
can produce damage and cause a roar.
Do hurricanes and tropical storms produce tornadoes? Often, but not always. There are great differences
from storm to storm, not necessarily related to tropical cyclone size or intensity. Some landfalling
hurricanes in the U.S. fail to produce any known tornadoes, while others cause major outbreaks. The
same hurricane also may have none for awhile, then erupt with tornadoes...or vice versa! Andrew (1992),
for example, spawned several tornadoes across the Deep South after crossing the Gulf, but produced
none during its rampage across South Florida. Katrina (2005) spawned numerous tornadoes after its
devastating LA/MS landfall, but only one in Florida (in the Keys). Though fewer tornadoes tend to occur with
tropical depressions and tropical storms than hurricanes, there are notable exceptions like TS Beryl of
1994 in the Carolinas. Some tropical cyclones even produce two distinct sets of tornadoes -- one around
the time of landfall over Florida or the Gulf Coast, the other when well inland or exiting the Atlantic coast.
What's the nature of tornadoes in hurricanes and tropical storms? Hurricane-spawned tornadoes tend
to occur in small, low-topped supercells within the outer bands, NNW through ESE of the center -- mainly
the northeast quadrant. There, the orientation and speed of the winds create vertical shear profiles
somewhat resembling those around classic Great Plains supercells -- the shear being in a shallower
layer but often stronger. Occasionally a tornado will happen in the inner bands as well, but the large
majority still form outside the hurricane force wind zone. Because tornado-producing circulations in
hurricane supercells tend to be smaller and shorter-lived than their Midwest counterparts, they are harder
to detect on Doppler radar, and more difficult to warn for. But hurricane-spawned tornadoes can still be
quite deadly and destructive, as shown by the F3 tornado from Hurricane Andrew at La Place LA (1992, 2
killed) and an F4 tornado at Galveston TX from Hurricane Carla (1961, 8 killed).
Do tropical cyclones produce waterspouts? Yes. Waterspouts -- tornadoes over water -- have been
observed in tropical systems. We don't know how many of them happen in tropical cyclones, but a majority
probably are from supercells. The similarity in Doppler radar velocity signatures over water to tornado-
producing cells in landfalling hurricanes suggests that it may be common -- and yet another good reason
for ships to steer well clear of tropical cyclones.
Does tropical cyclone strength or size matter for tornadoes? Often, but not always. Relatively weak
hurricanes like Danny (1985) have spawned significant supercell tornadoes well inland, as have larger,
more intense storms like Beulah (1967) and Ivan (2004). In general, the bigger and stronger the wind
fields are with a tropical cyclone, the bigger the area of favorable wind shear for supercells and tornadoes.
But supercell tornadoes (whether or not in tropical cyclones) also depend on instability, lift and moisture.
Surface moisture isn't lacking in a tropical cyclone, but sometimes instability and lift are too weak. This is
why tropical systems tend to produce more tornadoes in the daytime and near any fronts that may get
involved in the cyclone circulation. It is also why SPC won't always have tornado watches out for every
instance of a tropical cyclone affecting land. For more details, there is a set of articles on tropical cyclone
tornadoes listed in the Scientific References section. For more information on hurricanes, go to the
Tropical Cyclone FAQ by Chris Landsea, Neal Dorst and Erica Rule.
Tornado Safety
What should I do in case of a tornado? That depends on where you are. This list of tornado safety tips
covers most situations.
What is a tornado watch? A tornado watch defines an area shaped like a parallelogram, where tornadoes
and other kinds of severe weather are possible in the next several hours. It does not mean tornadoes are
imminent -- just that you need to be alert, and to be prepared to go to safe shelter if tornadoes do happen
or a warning is issued. This is the time to turn on local TV or radio, turn on and set the alarm switch on your
weather radio, make sure you have ready access to safe shelter, and make your friends and family aware
of the potential for tornadoes in the area. The Storm Prediction Center issues tornado and severe
thunderstorm watches; here is an example. For more information on tornado watches and other SPC
bulletins, go here.
What is a tornado warning? A tornado warning means that a tornado has been spotted, or that Doppler
radar indicates a thunderstorm circulation which can spawn a tornado. When a tornado warning is issued
for your town or county, take immediate safety precautions. local NWS offices issue tornado warnings.
Do mobile homes attract tornadoes? Of course not. It may seem that way, considering most tornado
deaths occur in them, and that some of the most graphic reports of tornado damage come from mobile
home communities. The reason for this is that mobile homes are, in general, much easier for a tornado to
damage and destroy than well-built houses and office buildings. A brief, relatively weak tornado which may
have gone undetected in the wilderness -- or misclassified as severe straight-line thunderstorm winds
while doing minor damage to sturdy houses -- can blow a mobile home apart. Historically, mobile home
parks have been reliable indicators, not attractors, of tornadoes.
Long ago, I was told to open windows to equalize pressure. Now I have heard that's a bad thing to do.
Which is right? Opening the windows is absolutely useless, a waste of precious time, and can be very
dangerous. Don't do it. You may be injured by flying glass trying to do it. And if the tornado hits your home, it
will blast the windows open anyway.
I've seen a video of people running under a bridge to ride out a tornado. Is that safe? Absolutely not!
Stopping under a bridge to take shelter from a tornado is a very dangerous idea, for several reasons:
actually not inside the tornado vortex itself, but instead in a surface inflow jet -- a small belt of intense wind
flowing into the base of the tornado a few dozen yards to their south. Even then, flying debris could have
caused serious injury or death. More recently, on 3 May 1999, two people were killed and several others
injured outdoors in Newcastle and Moore OK, when a violent tornado blew them out from under bridges on
I-44 and I-35. Another person was killed that night in his truck, which was parked under a bridge.
So if I'm in a car, which is supposed to be very unsafe, and shouldn't get under a bridge, what can I do?
Vehicles are notorious as death traps in tornadoes, because they are easily tossed and destroyed. Either
leave the vehicle for sturdy shelter or drive out of the tornado's path. When the traffic is jammed or the
tornado is bearing down on you at close range, your only option may be to park safely off the traffic lanes,
get out and find a sturdy building for shelter, if possible. If not, lie flat in a low spot, as far from the road as
possible (to avoid flying vehicles). However, in open country, the best option is to escape if the tornado is
far away. If the traffic allows, and the tornado is distant, you probably have time to drive out of its path. Watch
the tornado closely for a few seconds compared to a fixed object in the foreground (such as a tree, pole, or
other landmark). If it appears to be moving to your right or left, it is not moving toward you. Still, you should
escape at right angles to its track: to your right if it is moving to your left, and vice versa -- just to put more
distance between you and its path. If the tornado appears to stay in the same place, growing larger or
getting closer -- but not moving either right or left -- it is headed right at you. You must take shelter away
from the car or get out of its way fast!
I have a basement, and my friend said to go to the southwest corner in a tornado. Is that good? Not
necessarily. The SW corner is no safer than any other part of the basement, because walls, floors and
furniture can collapse (or be blown) into any corner. The "safe southwest corner" is an old myth based on
the belief that, since tornadoes usually come from the SW, debris will preferentially fall into the NE side of
the basement. There are several problems with this concept, including:
from under heavy furniture or appliances resting on top of the floor above.
What is a safe room? So-called "safe rooms" are reinforced small rooms built in the interior of a home,
which are fortified by concrete and/or steel to offer extra protection against tornadoes, hurricanes and other
severe windstorms. They can be built in a basement, or if no basement is available, on the ground floor. In
existing homes, interior bathrooms or closets can be fortified into "safe rooms" also.
What about community tornado shelters? Community tornado shelters are excellent ideas for apartment
complexes, schools, mobile home parks, factories, office complexes and other facilities where large
groups of people live, work or study. FEMA has some excellent design and construction guidance for these
kinds of shelters; and a licensed engineer can help customize them to the needs of your facility.
What about tornado safety in sports stadiums or outdoor festivals? Excellent question -- and a very, very
disturbing one to many meteorologists. Tornadoes have passed close to such gatherings on a few
occasions, including a horse race in Omaha on 6 May 1975 and a crowded dog track in West Memphis AR
on 14 December 1987. A supercell without a tornado hit a riverside festival in Ft. Worth in 1995, catching
over 10,000 people outdoors and bashing many of them with hail bigger than baseballs. Just in the last
few years, tornadoes have hit the football stadium for the NFL Tennessee Titans, and the basketball arena
for the NBA Utah Jazz. Fortunately, they were both nearly empty of people at the time. There is the potential
for massive death tolls if a stadium or fairground is hit by a tornado during a concert, festival or sporting
event -- even with a warning in effect. Fans may never know about the warning; and even if they do, mass-
panic could ensue and result in casualties even if the tornado doesn't hit. Stadium and festival managers
should work with local emergency management officials to develop a plan for tornado emergencies -- both
for crowd safety during the watch and warning stages, and (similar to a terrorism plan) for dealing with
mass casualties after the tornado.
What would happen if a large, violent tornado hit a major city today? This has happened on several
occasions, including in parts of Oklahoma City on May 3, 1999. Because of excellent, timely watches and
warnings and intense media coverage of the Oklahoma tornado long before it hit, only 36 people were
killed. The damage toll exceeded $1 billion. Still, it did not strike downtown, and passed over many miles of
undeveloped land. Moving the same path north or south in the same area may have led to much greater
death and damage tolls. The threat exists for a far worse disaster! Placing the same tornado outbreak in
the Dallas-Ft. Worth Metroplex, especially during rush hour gridlock (with up to 62,000 vehicles stuck in the
path), the damage could triple what was done in Oklahoma. There could be staggering death tolls in the
hundreds or thousands, and overwhelmed emergency services. Ponder the prospect of such a tornado's
path in downtown Dallas, for example. The North Texas Council of Governments and NWS Ft. Worth has
compiled a very detailed study of several such violent tornado disaster scenarios in the Metroplex, which
could be adapted to other major metro areas as well.
TORNADO DAMAGE
How is tornado damage rated? The most widely used method worldwide, for over three decades, was the
F-scale developed by Dr. T. Theodore Fujita. In the U.S., and probably elsewhere within a few years, the
new Enhanced F-scale is becoming the standard for assessing tornado damage. In the original F-Scale,
the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically
verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are
only judgmental estimates. Because:
meteorologists and wind engineers can and often do disagree among themselves on a tornado's strength.
Who surveys tornado damage? What's the criteria for the National Weather Service to do a survey?
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 tornado 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.
How do tornadoes do some weird things, like drive straw into trees, strip road pavement and drive
splinters into bricks? The list of bizarre things attributed to tornadoes is almost endless. Much of it is
folklore; but there are some weird scenes in tornado damage. Asphalt pavement may strip when tornado
winds sandblast the edges with gravel and other small detritus, eroding the edges and causing chunks to
peel loose from the road base. Storm chasers and damage surveyors have observed this phenomenon
often after the passage of a violent tornado. With a specially designed cannon, wind engineers at Texas
Tech University have fired boards and other objects at over 100 mph into various types of construction
materials, duplicating some of the kinds of "bizarre" effects, such as wood splinters embedded in bricks.
Intense winds can bend a tree or other objects, creating cracks in which which debris (e.g., hay straw)
becomes lodged before the tree straightens and the crack tightens shut again. All bizarre damage effects
have a physical cause inside the roiling maelstrom of tornado winds. We don't fully understand what some
of those causes are yet, however; because much of it is almost impossible to simulate in a lab.
I've heard about tornadoes picking up objects and carrying them for miles. Does this happen? Yes,
numerous tornadoes have lofted (mainly light) debris many miles into the sky, which was then carried by
middle- and upper-atmospheric winds for long distances. The vertical winds in tornadoes can be strong
enough to temporarily levitate even heavy objects if they have a large face to the wind or flat sides (like
roofs, walls, trees and cars), and are strong enough to carry lightweight objects tens of thousands of feet
high. Though the heaviest objects, such as railroad cars, can only be airborne for short distances, stories
of checks and other papers found over 100 miles away are often true. The Worcester MA tornado of 9 June
1953 carried mattress pieces high into the thunderstorm, where they were coated in ice, before they fell
into Boston Harbor. Pilots reported seeing debris fluttering through the air at high altitude near the
thunderstorm which spawned the Ruskin Heights MO tornado of 20 May 1957.
How does the damage from tornadoes compare to that of hurricanes? The differences are in scale.
Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes
typically cause much more damage individually and over a season, and over far bigger areas.
Economically, tornadoes cause about a tenth as much damage per year, on average, as hurricanes.
Hurricanes tend to cause much more overall destruction than tornadoes because of their much larger size,
longer duration and their greater variety of ways to damage property. The destructive core in hurricanes can
be tens of miles across, last many hours and damage structures through storm surge and rainfall-caused
flooding, as well as from wind. Tornadoes, in contrast, tend to be a few hundred yards in diameter, last for
minutes and primarily cause damage from their extreme winds.
What is a tornado? According to the Glossary of Meteorology (AMS 2000), a tornado is "a violently rotating
column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not
always) visible as a funnel cloud." Literally, in order for a vortex to be classified as a tornado, it must be in
contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice,
however, to classify and define tornadoes. For example, the difference is unclear between an strong
mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also
disgreement as to whether separate touchdowns of the same funnel constitute separate tornadoes. It is
well-known that a tornado may not have a visible funnel. Also, at what wind speed of the cloud-to-ground
vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify
as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.
How do tornadoes form? The classic answer -- "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. The truth is that we don't fully understand.
The most destructive and deadly tornadoes occur from supercells -- which are rotating thunderstorms with
a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe
non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be
dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent
theories and results from the VORTEX program suggest that once a mesocyclone is underway, tornado
development is related to the temperature differences across the edge of downdraft air wrapping around
the mesocyclone (the occlusion downdraft). Mathematical modelling studies of tornado formation also
indicate that it can happen without such temperature patterns; and in fact, very little temperature variation
was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind
these theories are given in several of the Scientific References accompanying this FAQ.
What direction do tornadoes come from? Does the region of the US play a role in path direction?
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 do tornadoes dissipate? The details are still debated by tornado scientists. We do know tornadoes
need a source of instability (heat, moisture, etc.) and a larger-scale property of rotation (vorticity) to keep
going. There are a lot of processes around a thunderstorm which can possibly rob the area around a
tornado of either instability or vorticity. One is relatively cold outflow -- the flow of wind out of the precipitation
area of a shower or thunderstorm. Many tornadoes have been observed to go away soon after being hit by
outflow. For decades, storm observers have documented the death of numerous tornadoes when their
parent circulations ( mesocyclones) weaken after they become wrapped in outflow air -- either from the
same thunderstorm or a different one. The irony is that some kinds of thunderstorm outflow may help to
cause tornadoes, while other forms of outflow may kill tornadoes.
Do tornadoes really skip? Not in a literal sense, despite what you may have read in many older
references, news stories, or even damage survey reports. By definition (above), a tornado must be in
contact with the ground. There is disagreement in meteorology over whether or not multiple touchdowns of
the same vortex or funnel cloud mean different tornadoes (a strict interpretation). In either event, stories of
skipping tornadoes usually mean
- There was continuous contact between vortex and ground in the path, but it was too weak to do
damage; - Multiple tornadoes happened; but there was no survey done to precisely separate their paths (very
common before the 1970s); or - There were multiple tornadoes with only short separation, but the survey erroneously classified
them as one tornado.
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.
How close to a tornado does the barometer drop? And how far does it drop ? It varies. A barometer can
start dropping many hours or even days in advance of a tornado if there is low pressure on a broad scale
moving into the area. Strong pressure falls will often happen as the mesocyclone (parent circulation in the
thunderstorm) moves overhead or nearby. The biggest drop will be in the tornado itself, of course. It is very
hard to measure pressure in tornadoes since most weather instruments can't survive. A few low-lying,
armored probes called "turtles" have been placed successfully in tornadoes. This includes one
deployment on 15 May 2003 by engineer/storm chaser Tim Samaras, who recorded pressure fall of over
40 millibars through an unusually large tornado. On 24 June 2003, another of Tim's probes recorded a 100
millibar pressure plunge in a violent tornado near Manchester, SD. Despite those spectacyular results, and
a few fortuitous passes over barometers through history, we still do not have a database of tornado
pressures big enough to say much about average tornado pressures or other barometric characterstics.
What is a waterspout? 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 small boats, damage 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.
How are tornadoes in the northern hemisphere different from tornadoes in the southern hemisphere?
The sense of rotation is usually the opposite. Most tornadoes -- but not all! -- rotate cyclonically, which is
counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes
(clockwise-spinning in the northern hemisphere) have been observed, however -- usually in the form of
waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell's
mesocyclone. There have been several documented cases of cyclonic and anticyclonic tornadoes under
the same thunderstorm at the same time. Anticyclonically rotating supercells with tornadoes are extremely
rare; but one struck near Sunnyvale, CA, in 1998. Remember, "cyclonic" tornadoes spin counter-clockwise
in the northern hemisphere, and clockwise.
What is a multivortex tornado? Multivortex (a.k.a. multiple-vortex) tornadoes contain two or more small,
intense subvortices orbiting the center of the larger tornado circulation. When a tornado doesn't contain too
much dust and debris, they can sometimes be spectacularly visible. 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.
What is the original F-scale? Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and
Pearson 1973) for winds, including tornadoes, which was supposed to relate the degree of damage to the
intensity of the wind. This scale was the result. The original F-scale should not be used anymore, because
it has been replaced by an enhanced version. Tornado wind speeds are still largely unknown; and the wind
speeds on the original 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 process 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 often came up with
different F-scale ratings for the same damage. Even with all its flaws, the original F-scale was the only
widely used tornado rating method for over three decades. The enhanced F-scale takes effect 1 February
2007.
I heard the Oklahoma City tornado was almost "F6." Is that a real level on the original F-scale? Only in
untested theory. Fujita plotted hypothetical winds higher than F5; but as mentioned in the previous answer
above, they were only guesses. Even if a winds measured by portable Doppler radar (slightly above ground
level) had been over 318 mph, the tornado would still be rated "only" F5 since F5 is the most intense
possible damage level. On the Enhanced F-scale, there is no such thing as "F6."
What is a "significant" tornado? A tornado is classified as "significant" if it does F2 or greater damage on
the Enhanced 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.
Big fat tornadoes are the strongest ones, right? Not necessarily. There is a statistical trend (as
documented by NSSL's Harold Brooks) toward wide tornadoes having higher F-scale damage. This can be
out of more strength or out of greater opportunity for targets to damage -- or some blend of both. However,
the size or shape of any particular tornado does not say anything conclusive about its strength. Some small
"rope" tornadoes can still do violent damage of F4 or F5; and some very large tornadoes over a quarter-
mile wide have produced only weak damage of F0 to F1.
Can't we weaken or destroy tornadoes somehow, like by bombing them or sucking out their heat with a
bunch of dry ice? The main problem with anything which could realistically stand a chance at affecting a
tornado (e.g., hydrogen bomb) is that it would be even more deadly and destructive than the tornado itself.
Lesser things (like huge piles of dry ice or smaller conventional weaponry) would be too hard to deploy in
the right place fast enough, and would likely not have enough impact to affect the tornado much anyway.
Imagine the legal problems one would face, too, by trying to bomb or ice a tornado, then inadvertantly
hurting someone or destroying private property in the process. In short -- bad idea!
How does cloud seeding affect tornadoes? Nobody knows, for certain. There is no proof that seeding can
or cannot change tornado potential in a thunderstorm. This is because there is no way to know that the
things a thunderstorm does after seeding would not have happened anyway. This includes any presence
or lack of rain, hail, wind gusts or tornadoes. Because the effects of seeding are impossible to prove or
disprove, there is a great deal of controversy in meteorology about whether it works, and if so, under what
conditions, and to what extent.
What does a tornado sound like? That depends on what it is hitting, its size, intensity, closeness and
other factors. The most common tornado sound is a continuous rumble, like a closeby train. Sometimes a
tornado produces a loud whooshing sound, like that of a waterfall or of open car windows while driving very
fast. Tornadoes which are tearing through densely populated areas may be producing all kinds of loud
noises at once, which collectively may make a tremendous roar. Just because you may have heard a loud
roar during a damaging storm does not necessarily mean it was a tornado. Any intense thunderstorm wind
can produce damage and cause a roar.
Do hurricanes and tropical storms produce tornadoes? Often, but not always. There are great differences
from storm to storm, not necessarily related to tropical cyclone size or intensity. Some landfalling
hurricanes in the U.S. fail to produce any known tornadoes, while others cause major outbreaks. The
same hurricane also may have none for awhile, then erupt with tornadoes...or vice versa! Andrew (1992),
for example, spawned several tornadoes across the Deep South after crossing the Gulf, but produced
none during its rampage across South Florida. Katrina (2005) spawned numerous tornadoes after its
devastating LA/MS landfall, but only one in Florida (in the Keys). Though fewer tornadoes tend to occur with
tropical depressions and tropical storms than hurricanes, there are notable exceptions like TS Beryl of
1994 in the Carolinas. Some tropical cyclones even produce two distinct sets of tornadoes -- one around
the time of landfall over Florida or the Gulf Coast, the other when well inland or exiting the Atlantic coast.
What's the nature of tornadoes in hurricanes and tropical storms? Hurricane-spawned tornadoes tend
to occur in small, low-topped supercells within the outer bands, NNW through ESE of the center -- mainly
the northeast quadrant. There, the orientation and speed of the winds create vertical shear profiles
somewhat resembling those around classic Great Plains supercells -- the shear being in a shallower
layer but often stronger. Occasionally a tornado will happen in the inner bands as well, but the large
majority still form outside the hurricane force wind zone. Because tornado-producing circulations in
hurricane supercells tend to be smaller and shorter-lived than their Midwest counterparts, they are harder
to detect on Doppler radar, and more difficult to warn for. But hurricane-spawned tornadoes can still be
quite deadly and destructive, as shown by the F3 tornado from Hurricane Andrew at La Place LA (1992, 2
killed) and an F4 tornado at Galveston TX from Hurricane Carla (1961, 8 killed).
Do tropical cyclones produce waterspouts? Yes. Waterspouts -- tornadoes over water -- have been
observed in tropical systems. We don't know how many of them happen in tropical cyclones, but a majority
probably are from supercells. The similarity in Doppler radar velocity signatures over water to tornado-
producing cells in landfalling hurricanes suggests that it may be common -- and yet another good reason
for ships to steer well clear of tropical cyclones.
Does tropical cyclone strength or size matter for tornadoes? Often, but not always. Relatively weak
hurricanes like Danny (1985) have spawned significant supercell tornadoes well inland, as have larger,
more intense storms like Beulah (1967) and Ivan (2004). In general, the bigger and stronger the wind
fields are with a tropical cyclone, the bigger the area of favorable wind shear for supercells and tornadoes.
But supercell tornadoes (whether or not in tropical cyclones) also depend on instability, lift and moisture.
Surface moisture isn't lacking in a tropical cyclone, but sometimes instability and lift are too weak. This is
why tropical systems tend to produce more tornadoes in the daytime and near any fronts that may get
involved in the cyclone circulation. It is also why SPC won't always have tornado watches out for every
instance of a tropical cyclone affecting land. For more details, there is a set of articles on tropical cyclone
tornadoes listed in the Scientific References section. For more information on hurricanes, go to the
Tropical Cyclone FAQ by Chris Landsea, Neal Dorst and Erica Rule.
Tornado Safety
What should I do in case of a tornado? That depends on where you are. This list of tornado safety tips
covers most situations.
What is a tornado watch? A tornado watch defines an area shaped like a parallelogram, where tornadoes
and other kinds of severe weather are possible in the next several hours. It does not mean tornadoes are
imminent -- just that you need to be alert, and to be prepared to go to safe shelter if tornadoes do happen
or a warning is issued. This is the time to turn on local TV or radio, turn on and set the alarm switch on your
weather radio, make sure you have ready access to safe shelter, and make your friends and family aware
of the potential for tornadoes in the area. The Storm Prediction Center issues tornado and severe
thunderstorm watches; here is an example. For more information on tornado watches and other SPC
bulletins, go here.
What is a tornado warning? A tornado warning means that a tornado has been spotted, or that Doppler
radar indicates a thunderstorm circulation which can spawn a tornado. When a tornado warning is issued
for your town or county, take immediate safety precautions. local NWS offices issue tornado warnings.
Do mobile homes attract tornadoes? Of course not. It may seem that way, considering most tornado
deaths occur in them, and that some of the most graphic reports of tornado damage come from mobile
home communities. The reason for this is that mobile homes are, in general, much easier for a tornado to
damage and destroy than well-built houses and office buildings. A brief, relatively weak tornado which may
have gone undetected in the wilderness -- or misclassified as severe straight-line thunderstorm winds
while doing minor damage to sturdy houses -- can blow a mobile home apart. Historically, mobile home
parks have been reliable indicators, not attractors, of tornadoes.
Long ago, I was told to open windows to equalize pressure. Now I have heard that's a bad thing to do.
Which is right? Opening the windows is absolutely useless, a waste of precious time, and can be very
dangerous. Don't do it. You may be injured by flying glass trying to do it. And if the tornado hits your home, it
will blast the windows open anyway.
I've seen a video of people running under a bridge to ride out a tornado. Is that safe? Absolutely not!
Stopping under a bridge to take shelter from a tornado is a very dangerous idea, for several reasons:
- Deadly flying debris can still be blasted into the spaces between bridge and grade -- and impaled
in any people hiding there. - Even when strongly gripping the girders (if they exist), people may be blown loose, out from under
the bridge and into the open -- possibly well up into the tornado itself. Chances for survival are not
good if that happens. - The bridge itself may fail, peeling apart and creating large flying objects, or even collapsing down
onto people underneath. The structural integity of many bridges in tornado winds is unknown --
even for those which may look sturdy. - Whether or not the tornado hits, parking on traffic lanes is illegal and dangerous to yourself and
others. It creates a potentially deadly hazard for others, who may plow into your vehicle at full
highway speeds in the rain, hail, and/or dust. Also, it can trap people in the storm's path against
their will, or block emergency vehicles from saving lives.
actually not inside the tornado vortex itself, but instead in a surface inflow jet -- a small belt of intense wind
flowing into the base of the tornado a few dozen yards to their south. Even then, flying debris could have
caused serious injury or death. More recently, on 3 May 1999, two people were killed and several others
injured outdoors in Newcastle and Moore OK, when a violent tornado blew them out from under bridges on
I-44 and I-35. Another person was killed that night in his truck, which was parked under a bridge.
So if I'm in a car, which is supposed to be very unsafe, and shouldn't get under a bridge, what can I do?
Vehicles are notorious as death traps in tornadoes, because they are easily tossed and destroyed. Either
leave the vehicle for sturdy shelter or drive out of the tornado's path. When the traffic is jammed or the
tornado is bearing down on you at close range, your only option may be to park safely off the traffic lanes,
get out and find a sturdy building for shelter, if possible. If not, lie flat in a low spot, as far from the road as
possible (to avoid flying vehicles). However, in open country, the best option is to escape if the tornado is
far away. If the traffic allows, and the tornado is distant, you probably have time to drive out of its path. Watch
the tornado closely for a few seconds compared to a fixed object in the foreground (such as a tree, pole, or
other landmark). If it appears to be moving to your right or left, it is not moving toward you. Still, you should
escape at right angles to its track: to your right if it is moving to your left, and vice versa -- just to put more
distance between you and its path. If the tornado appears to stay in the same place, growing larger or
getting closer -- but not moving either right or left -- it is headed right at you. You must take shelter away
from the car or get out of its way fast!
I have a basement, and my friend said to go to the southwest corner in a tornado. Is that good? Not
necessarily. The SW corner is no safer than any other part of the basement, because walls, floors and
furniture can collapse (or be blown) into any corner. The "safe southwest corner" is an old myth based on
the belief that, since tornadoes usually come from the SW, debris will preferentially fall into the NE side of
the basement. There are several problems with this concept, including:
- Tornadoes are not straight-line winds, even on the scale of a house, so the strongest wind may be
blowing from any direction; and - Tornadoes themselves may arrive from any direction.
from under heavy furniture or appliances resting on top of the floor above.
What is a safe room? So-called "safe rooms" are reinforced small rooms built in the interior of a home,
which are fortified by concrete and/or steel to offer extra protection against tornadoes, hurricanes and other
severe windstorms. They can be built in a basement, or if no basement is available, on the ground floor. In
existing homes, interior bathrooms or closets can be fortified into "safe rooms" also.
What about community tornado shelters? Community tornado shelters are excellent ideas for apartment
complexes, schools, mobile home parks, factories, office complexes and other facilities where large
groups of people live, work or study. FEMA has some excellent design and construction guidance for these
kinds of shelters; and a licensed engineer can help customize them to the needs of your facility.
What about tornado safety in sports stadiums or outdoor festivals? Excellent question -- and a very, very
disturbing one to many meteorologists. Tornadoes have passed close to such gatherings on a few
occasions, including a horse race in Omaha on 6 May 1975 and a crowded dog track in West Memphis AR
on 14 December 1987. A supercell without a tornado hit a riverside festival in Ft. Worth in 1995, catching
over 10,000 people outdoors and bashing many of them with hail bigger than baseballs. Just in the last
few years, tornadoes have hit the football stadium for the NFL Tennessee Titans, and the basketball arena
for the NBA Utah Jazz. Fortunately, they were both nearly empty of people at the time. There is the potential
for massive death tolls if a stadium or fairground is hit by a tornado during a concert, festival or sporting
event -- even with a warning in effect. Fans may never know about the warning; and even if they do, mass-
panic could ensue and result in casualties even if the tornado doesn't hit. Stadium and festival managers
should work with local emergency management officials to develop a plan for tornado emergencies -- both
for crowd safety during the watch and warning stages, and (similar to a terrorism plan) for dealing with
mass casualties after the tornado.
What would happen if a large, violent tornado hit a major city today? This has happened on several
occasions, including in parts of Oklahoma City on May 3, 1999. Because of excellent, timely watches and
warnings and intense media coverage of the Oklahoma tornado long before it hit, only 36 people were
killed. The damage toll exceeded $1 billion. Still, it did not strike downtown, and passed over many miles of
undeveloped land. Moving the same path north or south in the same area may have led to much greater
death and damage tolls. The threat exists for a far worse disaster! Placing the same tornado outbreak in
the Dallas-Ft. Worth Metroplex, especially during rush hour gridlock (with up to 62,000 vehicles stuck in the
path), the damage could triple what was done in Oklahoma. There could be staggering death tolls in the
hundreds or thousands, and overwhelmed emergency services. Ponder the prospect of such a tornado's
path in downtown Dallas, for example. The North Texas Council of Governments and NWS Ft. Worth has
compiled a very detailed study of several such violent tornado disaster scenarios in the Metroplex, which
could be adapted to other major metro areas as well.
TORNADO DAMAGE
How is tornado damage rated? The most widely used method worldwide, for over three decades, was the
F-scale developed by Dr. T. Theodore Fujita. In the U.S., and probably elsewhere within a few years, the
new Enhanced F-scale is becoming the standard for assessing tornado damage. In the original F-Scale,
the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically
verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are
only judgmental estimates. Because:
- Nobody knows the "true" wind speeds at ground level in most tornadoes, and
- The amount of wind needed to do similar-looking damage can vary greatly, even from block to block
or building to building,
meteorologists and wind engineers can and often do disagree among themselves on a tornado's strength.
Who surveys tornado damage? What's the criteria for the National Weather Service to do a survey?
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 tornado 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.
How do tornadoes do some weird things, like drive straw into trees, strip road pavement and drive
splinters into bricks? The list of bizarre things attributed to tornadoes is almost endless. Much of it is
folklore; but there are some weird scenes in tornado damage. Asphalt pavement may strip when tornado
winds sandblast the edges with gravel and other small detritus, eroding the edges and causing chunks to
peel loose from the road base. Storm chasers and damage surveyors have observed this phenomenon
often after the passage of a violent tornado. With a specially designed cannon, wind engineers at Texas
Tech University have fired boards and other objects at over 100 mph into various types of construction
materials, duplicating some of the kinds of "bizarre" effects, such as wood splinters embedded in bricks.
Intense winds can bend a tree or other objects, creating cracks in which which debris (e.g., hay straw)
becomes lodged before the tree straightens and the crack tightens shut again. All bizarre damage effects
have a physical cause inside the roiling maelstrom of tornado winds. We don't fully understand what some
of those causes are yet, however; because much of it is almost impossible to simulate in a lab.
I've heard about tornadoes picking up objects and carrying them for miles. Does this happen? Yes,
numerous tornadoes have lofted (mainly light) debris many miles into the sky, which was then carried by
middle- and upper-atmospheric winds for long distances. The vertical winds in tornadoes can be strong
enough to temporarily levitate even heavy objects if they have a large face to the wind or flat sides (like
roofs, walls, trees and cars), and are strong enough to carry lightweight objects tens of thousands of feet
high. Though the heaviest objects, such as railroad cars, can only be airborne for short distances, stories
of checks and other papers found over 100 miles away are often true. The Worcester MA tornado of 9 June
1953 carried mattress pieces high into the thunderstorm, where they were coated in ice, before they fell
into Boston Harbor. Pilots reported seeing debris fluttering through the air at high altitude near the
thunderstorm which spawned the Ruskin Heights MO tornado of 20 May 1957.
How does the damage from tornadoes compare to that of hurricanes? The differences are in scale.
Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes
typically cause much more damage individually and over a season, and over far bigger areas.
Economically, tornadoes cause about a tenth as much damage per year, on average, as hurricanes.
Hurricanes tend to cause much more overall destruction than tornadoes because of their much larger size,
longer duration and their greater variety of ways to damage property. The destructive core in hurricanes can
be tens of miles across, last many hours and damage structures through storm surge and rainfall-caused
flooding, as well as from wind. Tornadoes, in contrast, tend to be a few hundred yards in diameter, last for
minutes and primarily cause damage from their extreme winds.
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Tornado FAQ