Tornado

Description of event

The American Meteorological Society defines a tornado as “a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.” A tornado is not necessarily visible, but the intense low pressure caused by the high wind speeds and rapid rotation will usually cause the water vapor in the air to condense into a visible funnel-shaped cloud.

The damage from a tornado is a result of both the high wind velocity and wind-blown debris. Tornados can touch the ground with winds greater than 300 miles per hour (mph). Even though winds from the strongest tornados far exceed that from the strongest hurricanes, the latter typically cause much more damage individually and over a season and over far bigger areas. Hurricanes tend to cause much more overall destruction than tornados 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. Tornados, in contrast, tend to be a few hundred yards in diameter, last for minutes to hours, and primarily cause damage from their extreme winds. ^,

The United States averages approximately 1000 tornados per year. “A distant second is Canada, with around 100 per year. Other locations that experience frequent tornado occurrences include northern Europe, western Asia, Bangladesh, South Africa, far eastern Asia and Japan, Argentina, Paraguay and Southern Brazil, Australia and New Zealand. In fact, the United Kingdom has the most tornadoes per land area with an average of about 30 per year. Fortunately, most U.K. tornadoes are relatively weak.”

Tornado damage was previously measured on the Fujita Scale or Fujita-Pearson Tornado Scale (F-Scale) until the National Weather Service (NWS) adopted the use of the Enhanced Fujita Scale (EF-Scale) on February 1, 2007. The F-Scale is named after the most noted researcher of tornados, Dr. Fujita of the University of Chicago. The scale ranges from F0 (very weak) to F12 (at wind speed of Mach 1, an unimaginable force). The strongest tornados observed to date have been F5 (261 to 318 mph). An update to the original F-Scale by a team of meteorologists and engineers was implemented in the United States on February 1, 2007. The EF-Scale is based on 28 damage indicators and is considered more replicable and accurate than the F-Scale. Because no storms greater than F-5 have been recorded, the enhanced F-Scale stops at EF-5. The F-Scale and EF-Scale are shown in Table 96-1 .

Table 96-1

Enhanced Fujita Scale for Tornado Damage * ^,^†

	Fujita scale			Derived enhanced fujita scale		Operational enhanced fujita scale
F Number	Fastest 0.25-Mile kph (mph)	3-Second gust (mph)	Comments	EF Number	3-Second gust (mph)	EF Number	3-Second gust (mph)
0 Weak	65-118 (40-73)	45-78	Damage is light. Chimneys on houses may be damaged; trees have broken branches; shallow-rooted trees pushed over; some windows broken; damage to sign boards.	0	65-85	0	65-85
1 Weak	119-181 (74-112)	79-117	Shingles on roofs blown off; mobile homes pushed off foundations or overturned; moving cars pushed off roads.	1	86-109	1	86-110
2 Strong	182-253 (113-157)	118-161	Considerable damage. Roofs torn off houses; mobile homes destroyed; train boxcars pushed over; large trees snapped or uprooted; light objects thrown like missiles.	2	110-137	2	111-135
3 Strong	254-332 (158-206)	162-209	Damage is severe. Roofs and walls torn off better constructed homes, businesses, and schools; trains overturned; most trees uprooted; heavy cars lifted off ground and thrown some distance.	3	138-167	3	136-165
4 Violent	333-419 (207-260)	210-261	Better constructed homes completely leveled; structures with weak foundation blown off some distance.	4	168-199	4	166-200
5 Violent	420-513 (261-318)	262-317	Better constructed homes lifted off foundations and carried considerable distances where they disintegrate; trees debarked; cars thrown in excess of 100 meters.	5	200-234	5	> 200
6 Unimaginably Violent	Although F6 and above are theoretically possible in the Enhanced Fujita Scale, they are not used.			N/A—The Enhanced Fujita Scale does not provide for greater than F5		N/A

* An update to the original F-Scale by a team of meteorologists and wind engineers was implemented in the United States on February 1, 2007.

† Important note about Enhanced Fujita Scale winds : The Enhanced Fujita Scale still is a set of wind estimates (not measurements) based on damage. It uses 3-second gusts estimated at the point of damage based on a judgment of eight levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important : The 3-second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, “1 minute mile” speed.

Conditions favorable for tornado development often occur over the central U.S. Plains during spring and summer. As the season goes on, tornados are likely farther and farther north on the Plains and in the Midwest, but in April and May tornados are common in both the South and on the Plains and in the Midwest. Often a large storm system can create tornado conditions for several days in a row.

Tornados are most common in spring and least common in winter. Because autumn and spring are transitional periods from warm to cool and vice versa, there are abundant chances of cooler air meeting warmer moist air, creating thunderstorms and increased risk of tornados. ^,

In the United States and Canada, warning of a tornado is given in two phases :

A Tornado Watch is a message that indicates that the conditions are favorable for formation of a tornado. Because tornados are spawned from severe thunderstorms a tornado watch therefore implies that it is also a Severe Thunderstorm Watch. The watch boxes (or weather watches, WWs) are usually issued in the format of x miles north and south, or east and west, or either side of a line from y miles direction of city, state , to z miles another direction of another city, state . For example, “THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 110 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 45 MILES NORTHWEST OF BARTLESVILLE OKLAHOMA TO 50 MILES NORTHEAST OF HARRISON ARKANSAS.” (“Either side” means perpendicular to the center line.) In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch ( Fig. 96-1 ). ^,

When severe hail (at least 0.75-inch diameter) or damaging winds (at least 50 knots or 58 mph) appear imminent, local NWS offices will issue a Severe Thunderstorm Warning. The warning is rapidly disseminated over National Oceanographic and Atmospheric Administration (NOAA) weather radio, commercial radio, and television (TV) stations and news wires, so that people in the warning area can find safe shelter to take cover from the storm.

If SKYWARN watchers sight a tornado or the Doppler radar picture shows the characteristic “hook” of a tornado (threshold strong with a tight rotation signature), then a Tornado Warning is issued. A tornado warning means there is immediate danger for the warned and the immediately surrounding area (because the path may not be completely predictable), if not from the relatively narrow tornado itself, then from the severe thunderstorm. All in the path of such a storm are urged to take cover immediately because it is a life-threatening situation. A tornado warning will also be issued if a tropical cyclone is making landfall with winds in excess of 115 mph (185 kph). When a tropical cyclone makes landfall both the extreme wind and the likelihood of accompanying tornados can cause damage.

In the event that the conditions that lead to a tornado watch are likely to form a major outbreak of tornados along with the thunderstorm’s destructive winds and hail, the tornado or thunderstorm watch may be enhanced with the words “particularly dangerous situation” (PDS) added to the watch.

Pre-Incident actions

The issuing of appropriate warnings and the need for the population to take appropriate action on the basis of those warnings are the most important factors in preventing tornado-related death and injury. In the United States the NWS has acquired sophisticated instrumentation, such as Doppler radar, which permits them to identify conditions conducive to the formation of a tornado and to issue warnings. A tornado watch means that conditions are conducive to tornado formation in a given area, and a tornado warning means that a tornado has been sighted in a given area and those residing in that area should take appropriate shelter, which entails going to the basement if one exists, going to an inside room or closet, or if outside going to a ditch or gully. See Box 96-1 for detailed shelter advice.

Box 96-1

Sheltering Guidelines in Case of Tornado Warning

In General

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Stay away from windows.
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When a warning is issued, move to the safest area immediately.
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If you have access to a motorcycle, bicycle, or climbing helmet, in all cases, put it on. Get under a sturdy piece of furniture and cover yourself with a blanket, pillows, or mattress.

If you are	You should
Outside	Get inside a building. If stuck outside, stay away from cars and trees. Lie down in a ditch or culvert and cover your head and neck with your hands. If you have a motorcycle, bicycle, or climbing helmet, wear it.
In a mobile home	Leave the mobile home. Go to a community home shelter. If no shelter is available, see “Outside” above.
In a car	Move the car off the road. Leave the vehicle. Do not seek shelter under an overpass. If stuck outside, see above.
At work or school	Go to the designated interior room or hallway away from windows, and get under a sturdy piece of furniture.
In a house	Go to the basement. If there is no basement, go to an interior room or hallway away from windows, get under a sturdy piece of furniture, and cover yourself with a blanket. Get into an interior bathroom, and get in the tub as an appropriate alternative.