by Bill Evans
Finally, my advice:
Walk softly, and carry a big umbrella….
Section
1
Tornadoes
What Is a Tornado?
Tornadoes are the most destructive force on the face of the Earth. In terms of “extreme weather,” tornadoes are the most awesome, devastating force when you consider how they are created, their various ways of destruction, and the surprising way in which they attack. No other weather phenomenon destroys more property or causes more fatalities than a tornado. Derived from the Spanish word torna, meaning “to turn,” a tornado is defined as an intense, rotating column of air extending from the base of a thunderstorm cloud to the ground. Tornadoes can strike at any time, appear from any direction, and have hit every state in the United States, including Alaska and Hawaii.
The swirling debris field of a tornado has been clocked to move as fast as nearly 300 miles per hour. Tornado winds have destroyed wind-measuring instruments, caused pine needles to penetrate bricks, driven straw into trees, and stripped pavement off roads. Tornadoes have caused the roofs of large buildings to be lifted from their frames. The building’s walls then fall outward and the roof is dropped back onto the structure, crushing it.
National Oceanic and Atmospheric Administration/Department of Commerce; Photographer: Joey Ketcham
National Oceanic and Atmospheric Administration/Department of Commerce
Wood, bricks, pipes, steel, tin siding, cars, tanker trucks, train cars, and even livestock have all been swept up into the storm and carried for miles. There’s even one story of the Worcester, Massachusetts, tornado of June 9, 1953, when mattress pieces were carried high into the thunderstorm, when they were coated with ice, before they fell into Boston Harbor.
Unfortunately, people are injured and sometimes killed by tornadoes. In isolated cases, people have been carried as far as a mile from their homes. However, thanks to improved weather forecasting, the use of Doppler radar, and better public awareness, the number of deaths due to tornadoes is fortunately going down.
Another interesting phenomenon of the tornado is its ability to pass over ponds, lakes, or swamps and literally suck up the contents of these bodies of water. At least fifty times a year there are reports of fish, frogs, tadpoles, snakes, eels, or snails dropping from the sky, often landing on unsuspecting people. The debris field is sometimes carried quite high up, into an area where the weather is sunny and nice. As the storm weakens and dissipates, the storm can no longer support the debris, and it will fall to the ground in an area where there is no sign of bad weather!
There have even been reports of frozen fish falling from the sky! In those cases, the tornado passed over a pond and picked up its contents, including the fish. The water and fish were lifted way up into the atmosphere, to a height where the air can be as cold as -90 degrees Fahrenheit (F)/-68 degrees Celsius (C). The fish became encased in ice balls, and then dropped from the sky. Once a fish went right through the windshield of an automobile! It was because of this extreme weather phenomena that I originally wanted to name this book Did You Know That Fish Could Fly…and Other Acts of Cod.
More tornadoes occur in the United States than in any other country on Earth. On average, nearly 1,000 tornadoes are reported each year. What actually causes a tornado to drop from a thunderstorm still remains unknown. Even when all the ingredients for a tornado are present, the storm may never form. This is a great mystery to the scientists and meteorologists who study tornadoes.
How Are Tornadoes Born?
To understand how a tornado works, we must first understand where it comes from. Tornadoes are born from thunderstorms. Thunderstorms form in unstable air when there is warm air at the ground and cold air aloft. Due to this temperature profile, most thunderstorms occur in spring and summer, rather than in fall and winter. In spring, the sun warms the air at ground level, which rises into the cold winter air that still remains high above in the atmosphere at that time of year. This mix of warm and cold can cause violent thunderstorms that create not only tornadoes, but also destructive straight-line winds, damaging hail, and extensive flooding from heavy rains. In fall, the air at ground level is also warmed, but it rises into the warm summer air that remains aloft, so thunderstorms, and tornadoes, form less often.
National Oceanic and Atmospheric Administration/Department of Commerce
Frank Picini
National Oceanic and Atmospheric Administration/Department of Commerce
Frank Picini
Frank Picini
The Life of a Thunderstorm
Updrafts—warm, humid air rises from the ground (Towering Cumulus Stage).
As the air cools to its dew point, it forms a cloud.
Ice crystals or water drops grow big enough to overcome the updraft, they begin falling, dragging down the air (Mature Stage).
The falling precipitation and air being dragged down form downdrafts.
Updrafts continue feeding warm, humid air into the storm. Existence of both updrafts and downdrafts make this the storm’s most violent stage.
Downdrafts grow, choking off updrafts (Dissipating Stage); with its supply of humid air cut, the storm begins dying and rain tapers off.
What Are the Best Conditions for a Tornado?
Extreme thunderstorms, ones that create deadly tornadoes, have special ingredients. The most deadly and destructive tornadoes form from super cells—which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone (which we will discuss later). This type of storm is rare and only occurs in the United States. Characteristics of tornado-producing storms include:
Unstable air that’s forced upward
An upper-air disturbance in the flow
A fresh supply of warm, humid air flowing near the ground with high-altitude cold air flowing in
Wind speeds that increase with altitude, like the jet stream. This interacts with the updrafts, promoting rotation in the storm. Upper-level winds promote and sustain the life of the storm, making it, in most cases, even stronger and thus a super storm.
Downbursts that can be invisible clean air, or lashing or sideways rains. Their abrupt change in wind direction poses a threat to airplanes that are landing and taking off. Doppler radar at airports detects and warns pilots of dangerous downbursts.
National Oceanic and Atmospheric Administration/Department of Commerce
Colored Tornadoes, Rope Tornadoes, and Other Varietals
Most tornadoes are dark because of the debris, dust, and dirt the storm has sucked up from the ground. Tornadoes can be many colors. They can be red from red dirt. Tornadoes can be white from water or from picking up snow in mountainous areas.
Tornadoes tend to look darkest to people who are on the east side of the storm. The tornado is often silhouetted in front of the brighter skies to the west of the thunderstorm. If there is heavy rain behind the tornado, it may look dark gray, blue, or even white—depending on where most of the daylight is coming from. Tornadoes wrapped in rain may exhibit varieties of gray shades on gray.
There are slang terms for tornadoes like “a wedge tornado” or “a rope tornado,” which are basically used by tornado spotters to describe a tornado’s shape and appearance. This does not by any means, however, say anything certain about a tornado’s strength! Wedge tornadoes simply appear as wide as they are tall. Rope tornadoes are very narrow and often snakelike in appearance.
National Oceanic and Atmospheric Administration/Department of Commerce
A Few More Tornado Facts
Nearly 60 percent of tornadoes occur between noon and sunset. This is the time when the sun most heats the atmosphere, creating unstable conditions. The jet stream also plays an important role as it enhances super-storm cell development.
Some tornadoes can have wind speeds of up to 300 mph. A small storm could be as slow as 50 mph and last a few minutes. A large monster can last for several hours and travel as far as 250 miles.
On average, sixty people
are killed by tornadoes each year. Boards, roof shingles, glass, stones, and bricks turn into missiles and cause most tornado-related deaths.
There is a major difference between a tornado and what’s called a “funnel cloud.” In a tornado, the damaging circulation is on the ground—whether or not the cloud is. A true funnel cloud rotates, but has no ground contact. However, a funnel cloud is still dangerous and you should still avoid it! Some funnels may never touch the ground. As the funnel descends, the rotation speed increases. It becomes a tornado when it touches the ground and dramatically increases in speed and power as it feeds on the warmer surface air. If you see one, you should report it to either the local police, fire, or sheriff’s department, as well as your local office of the National Weather Service.
Where Is Tornado Alley?
National Oceanic and Atmospheric Administration/Department of Commerce
Even though we know most tornadoes occur during the spring and summer, there is no “tornado season” (though there is a hurricane season). Still, there are certain times and areas where tornadoes are most likely to occur. From Texas to Oklahoma, through Arkansas to Illinois, through Nebraska to the Dakotas is the area that is called tornado alley, where most tornadoes are likely to occur during any given year. Looking at the map, however, you can see that if you live anywhere from South Dakota to most of Texas, from eastern Colorado to the eastern edges of the Midwest, you have a greater chance of experiencing a tornado than if you live in the Northeast, or mid-Atlantic, or on the Pacific coast. It’s all but guaranteed that if you live in Oklahoma or Texas you will experience at least one tornado every one or two years.
Tornado Swarms
It is the rarest of storm types, but the super cell is the most dangerous because of the extreme weather it can generate. Since super cells can travel as far as 300 miles, multiple numbers of tornadoes can be created. This doesn’t always occur, but when it does, these are called “swarms” of tornadoes. Super cells can be regenerated over a period of days along a cold front or what is called a squall line (a moving line of severe weather across a major portion of the country).
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Ten Greatest Tornado Swarms
NUMBER OF TORNADOES
DATE
DEATHS
148
April 3–4, 1974
315
111
September 19–23, 1967
5
99
May 26–27, 1973
22
95
November 21–23, 1992
26
94
May 4–5, 2003
37
85
May 22–23, 2004
1
80
February 5–6, 2008
60
80
May 18–19, 1995
4
78
May 3–4, 1999
46
70
May 11–12, 1982
2
67
April 26–27, 1994
3
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Top Ten: U.S. Cities Most at Risk for Tornadoes
1. Oklahoma City, Oklahoma
2. Dallas–Ft. Worth, Texas
3. Lubbock, Texas
4. Kansas City, Missouri
5. Indianapolis, Indiana
6. St. Louis, Missouri
7. Jackson, Mississippi, and Birmingham, Alabama
8. Little Rock, Arkansas
9. Omaha, Nebraska
10. Chicago, Illinois
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Most Tornado-Prone Areas in the United States
Highest Absolute Number of Tornado Fatalities
Texas
Mississippi
Alabama
Highest Absolute Number of Tornado Occurrences
Texas
Oklahoma
Florida
Highest Absolute Number of Killer Tornado Occurrences
Texas
Oklahoma
Arkansas
Most Tornadoes in Any Month
The record for the most tornadoes in any month (since modern tornado record keeping began in 1950) was set in April 2011 with 875 tornadoes. This easily broke the old mark of 543, set in May 2003.
Most Tornadoes in Any Month Since 1950
1. April 2011
875
2. May 2003
543
3. June 1992
399
4. May 1995
391
5. June 1998
376
6. May 1991
335
7. May 1982
329
8. June 1990
329
9. June 1993
313
10. May 1998
310
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Ten Deadliest Single Tornadoes
NOAA ONLY STARTED tracking tornado fatalities in 1950. Prior to 1950, information about tornado fatalities was gathered by an independent research group, The Tornado Project. Data in this table is drawn from both NOAA and The Tornado Project.
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Biggest Tornado Ever!
The biggest known tornado is the Hallam, Nebraska, F4 tornado of May 22, 2004. Its peak width was nearly two and a half miles. This is probably close to the maximum size for tornadoes.
Strongest Tornado Ever!
We don’t know how fast winds can go inside a tornado. Tornado wind speeds have only been directly recorded in weaker storms, because strong and violent tornadoes destroy weather instruments. Mobile Doppler radars (which we will discuss later), such as the Oklahoma University “Doppler on Wheels,” have remotely sensed tornado wind speeds above ground level as high as about 302 mph (on May 3, 1999, near Bridge Creek, Oklahoma; that tornado caused F5 damage—this was before the introduction of the EF scale). These are the highest winds ever found on the Earth’s surface, even faster than hurricane winds. But ground-level wind speeds in the most violent tornadoes have never been directly measured.
The Enhanced Fujita Damage Scale
The Enhanced Fujita Scale, or EF Scale, is the scale for rating the strength of tornadoes in the United States, estimated by the damage they cause. The original “F” scale was invented in 1971 by University of Chicago physicist Theodore Fujita, the first and foremost tornado scientist.
The new, enhanced Fujita (EF) scale went into operation on February 1, 2007. The scale was revised to reflect current, better examinations of tornado damage surveys, and to align wind speeds more closely with associated storm damage. The new scale takes into account more types of structures as well as vegetation, expands degrees of damage, and better accounts for variables such as the differences in construction of buildings and structures.
The EF scale is not intended to assess wind speed as much as it is to derive what the wind speeds were in a particular tornado as determined by the type of damage that was left behind by the storm. The new scale takes into account quality of construction and standardizes different kinds of structures. The wind speeds on the original scale were deemed too high by meteorologists and engineers, and engineering studies indicated that slower winds than initially estimated caused the respective degrees of damage.
The following graphic shows the damage assessment from the Enhanced Fujita Scale and what the corresponding tornado might look like. Images are courtesy of the National Severe Storms Lab.
National Oceanic and Atmospheric Administration/Department of Commerce
National Oceanic and Atmospheric Administration/Department of Commerce
National Oceanic and Atmospheric Administration/Department of Commerce
What Is a Derecho?
Thunderstorms not only destroy lives and property by spawning tornadoes, they contain another type of wind that can be just as destructive. A prolonged windstorm that produces straight-line winds rather than or in addition to a tornado is called a derecho. Derecho (pronounced deh-RAY-cho) is the Spanish word
for “direct” or “straight ahead” and describes the winds that can be produced by thunderstorms. Straight-line winds that come from a thunderstorm can be as fast as 100 miles per hour.
Super cell: King of Thunderstorms
Common thunderstorms are born and rain themselves out within a very short period of time, say an hour or less. Super cells are the most dangerous because they can develop the strongest tornadoes. They can travel more than 300 miles across the country as well and cause heavy, flooding rain; large, damaging hail; and straight-line (or derecho) winds in excess of 100 mph. Super cells need humid, unstable air, and some lifting force (like the jet stream) to move the air upward. Super cells can last as long as 5 to 8 hours.
Frank Picini
National Oceanic and Atmospheric Administration/Department of Commerce
