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Severe Weather and Natural Disaster
Tornadoes

The Basics In-Depth Experiments Witness Account
Words to Know Experts Say Be Ready! See a Tornado

In-Depth

Tornados can cause severe damage like this demolished apartment house in Florida in 1998. (Photo: © J. Pat Carter/AP Wide World)

Keep reading to find out…
Where do tornadoes occur?
How are they measured?
How does the warning system work?

Where do they occur?

The U.S. has by far the most tornadoes in the world. It averages 1,000 tornadoes a year! Tornadoes also occur in other parts of the world, most notably in Australia.

The geography of the central U.S. is uniquely suited to bring together all the ingredients for tornado formation. With the Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east, this area has become know as "Tornado Alley," averaging more than 500 tornadoes annually.

During the spring and summer months, southerly winds originating from the Gulf of Mexico prevail across the plains, providing the warm, humid air needed to fuel severe thunderstorm development. Hot, dry air forms over the higher elevations to the west, and becomes the cap as it spreads eastward over the moist, Gulf air. Where the dry air and the Gulf air meet near the ground, a boundary known as a dry line forms to the west of Oklahoma. A storm system moving out of the southern Rockies may push the dry line eastward, with severe thunderstorms and tornadoes forming along the dry line or in the moist air just ahead of it.

Peak months of tornado activity in the U.S. are April, May, and June. However, tornadoes have occurred in every month and at all times of the day or night. A typical time of occurrence is on an unseasonably warm and humid spring day between 3 p.m. and 9 p.m.

When a storm system high in the atmosphere moves east and begins to lift the layers, it begins to build severe thunderstorms that spawn tornadoes. As it lifts it removes the cap, setting the stage for explosive thunderstorms to develop as strong updrafts form. If the rising air encounters wind shear, it may cause the updraft to begin rotating, and a tornado is born.

"Tornado Alley," or the states at the highest risk of getting a tornado, include Arkansas, Iowa, Kansas, Louisiana, Minnesota, Nebraska, North Dakota, Ohio, Oklahoma, South Dakota, and Texas.

How are they measured?

Dr. T. Theodore Fujita ("Dr. Tornado") was a pioneer in the study of tornadoes and severe thunderstorm phenomena. In 1971, he created the Fujita Tornado Damage Scale to estimate tornado strength based on damage surveys. Since it's extremely difficult to measure tornado winds directly, this is the best way to classify them.

The scale of a tornado is measured from F0 to F5, or lowest danger to highest danger. An F0 tornado reaches winds from 40–72 miles per hour, while an F5 tornado can tear through land at 261–318 mph.

The scale goes up to F5 — or up to 318 mph. It's possible that a tornado could generate winds above the scale, but it has never been recorded. On May 3, 1999, an Oklahoma University Doppler radar remotely sensed tornado wind speeds above ground of 318 mph at Bridge Creek, Oklahoma — the highest winds ever found near Earth's surface, and right at the threshold of being F6 winds.

Fujita Tornado Damage Scale — Developed by "Dr. Tornado", T. Theodore Fujita of the University of Chicago

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Scale Wind Estimate (mph) Typical Damage
F0 < 73 Light damage.
Some damage to chimneys and TV antennas; breaks twigs off trees; pushes over shallow-rooted trees.
F1 73-112 Moderate damage.
Peels surface off roofs; windows broken; light trailer houses pushed or overturned; some trees uprooted or snapped; moving automobiles pushed off the road. The beginning of hurricane wind speed is 74 mph.
F2 113-157 Considerable damage.
Roofs torn off frame houses, leaving strong upright walls; weak buildings in rural areas demolished; trailer houses destroyed; large trees snapped or uprooted; railroad boxcars pushed over; light-object missiles generated; cars blown off highway.
F3 158-206 Severe damage.
Roofs and some walls torn off frame houses; some rural buildings completely demolished; trains overturned; steel-framed hangar-warehouse-type structures torn; cars lifted off the ground; most trees in a forest uprooted, snapped, or leveled.
F4 207-260 Devastating damage.
Whole frame houses leveled, leaving piles of debris; steel structures badly damaged; trees debarked by small flying debris; cars and trains thrown some distances or rolled considerable distances; large missiles generated.
F5 260-318 Incredible damage.
Whole frame houses tossed off foundations; steel-reinforced concrete structures badly damaged; automobile-sized missiles generated; trees debarked; incredible phenomena can occur.
F6-F12 319 to sonic Inconceivable damage.
Should a tornado with the maximum wind speed in excess of F5 occur, the extent and types of damage may not be conceived. A number of missiles such as iceboxes, water heaters, storage tanks, automobiles, etc. will create serious secondary damage on structures.

How does the warning system work?

There are five basic steps in the National Weather Service warning system. Every part of the system has to work for the greatest number of people to get the warning in time.

Step 1. The Tornado Watch
The forecasters at the Storm Prediction Center in Kansas City, Missouri, use satellite pictures, radar reports, numerous weather charts, and other tools to predict where severe thunderstorms and tornadoes are likely to occur. When they determine it's likely that a severe storm may occur, they issue a tornado or severe thunderstorm watch. Watches may be issued hours before a severe storm hits an area.

Step 2. Spotters
Once a tornado watch has been declared, the spotters get busy. Spotters — the National Weather Service's eyes in the field — report critical weather information as it is happens. If a Spotter sees a tornado, she immediately transmits this information to the National Weather Service through the information network known as Skywarn. This changes the tornado status from a watch to a warning. Most tornadoes are detected by the use of Doppler radar, but the spotter system still accounts for reporting many tornadoes. In order to become a Spotter, you need to pass a Skywarn training program. Find out more about Storm Spotters.

Step 3. Civil Defense and the State Police
Once a tornado watch is issued, the National Weather Service alerts the state and local civil defense organizations and the state police. This allows authorities to prepare for a possible emergency.

Step 4. Informing the Public
If it seems likely that a tornado or other weather emergency will hit an area, the National Weather Service has a direct line to local media offices so that they can relay the information to the public as quickly as possible.

Step 5. The Users
Users include everyone within the severe thunderstorm or tornado warning area. The best way to save lives is to reach the greatest number of people possible to tell them that they need to prepare. Even if every other step in the warning system works, it does little good unless the users know what to do, and act. The best way to receive timely tornado warning advisories is through use of a NOAA Weather Radio with a warning alarm and battery backup.