Tornadoes - How Tornadoes Form

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What Is a Tornado?

Virginia Tornado 4/29/08
Local residents check out the damage of vehicles at a mall after it was damaged by a tornado April 29, 2008 in the King's Fork area of Suffolk, Virginia. Three Tornadoes touched down in central and southeastern Virginia injuring at least 200 people. Photo by Alex Wong/Getty Images

A tornado is a violent column of rotating air made visible as they pick up debris on the ground or in the air. A tornado is usually visible, but not always. The important aspect of the definition is that the tornado or funnel cloud is in contact with the ground. The funnel clouds appear to extend downward from cumulonimbus clouds. A point to keep in mind is that this definition is not a truly accepted definition. According to Charles A. Doswell III of the Cooperative Institute for Mesoscale Meteorological Studies, there is actually no real definition of a tornado that has been universally accepted and peer-reviewed by the scientific community.

One idea that is generally accepted is that tornadoes are one of the worst, and most violent, of all the types of severe weather. Tornadoes can be considered billion-dollar storms if the storm lasts sufficiently long enough, and has enough wind speed to do maximum property damage. Fortunately, most tornadoes are short-lived, lasting for only about 5-7 minutes on average.

Tornado Rotation

Most tornadoes in the Northern Hemisphere rotate counter-clockwise or cyclonically. Only about 5% of tornadoes in the Northern Hemisphere rotate clockwise or anticyclonically. While at first it would seem this is a consequence of the Coriolis effect, tornadoes are over almost as quickly as they start. Therefore, the influence of the Coriolis effect on rotation is negligible.

So why do tornadoes tend to rotate counter-clockwise? The answer is that the storm moves in the same general direction as the low pressure systems that spawn them. Since low pressure systems rotate counterclockwise (and this is due to the Coriolis effect), tornado rotation also tends to be inherited from the low pressure systems. As winds get pushed upwards in the updraft, the prevailing direction of rotation is counterclockwise.

Tornado Locations

. In the United States, a unique combination of factors including local geology, proximity to water, and movement of frontal systems make the United States a prime location for the formation of tornadoes. In fact, there are 5 key reasons the US is the hardest hit with tornadoes.

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What Causes Tornadoes?

Basics of Tornado Formation

Tornadoes are produced when two differing air masses meet. When cooler polar air masses meet warm and moist tropical air masses, the potential for severe weather is created. In tornado alley, air masses to the west are typically continental air masses meaning there is little moisture in the air. This warm, dry air meets the warm, moist air in the Central Plains creating a dryline. It is a well-known fact that tornadoes and severe thunderstorms often form along drylines.

Most tornadoes form during supercell thunderstorms from an intensely rotating updraft. It is believed that differences in vertical wind shear are contributors to the rotation of a tornado. The larger scale rotation inside the severe thunderstorm is known as a mesocyclone and a tornado is one extension of that mesocyclone. An excellent flash animation of tornado formation is available from USA Today.

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Tornado Season and Time of Day

Peak Tornado Months by State
Each state has a peak time for the chance of a tornado. NOAA National Severe Storms Laboratory
The Time of Day for a Tornado

Tornadoes commonly occur in the daytime, as reported on the news, but night tornadoes also occur. Any time there is a severe thunderstorm, there is the potential to have a tornado. Night tornadoes can be especially dangerous because they are hard to see.

Tornado Season

Tornado season is a term used only as a guide for when most tornadoes occur in an area. In reality, a tornado can strike at any time of the year. In fact, the Super Tuesday tornado hit on February 5th and 6th, 2008.

Tornado season and the frequency of tornadoes migrates with the sun. As the seasons change, so does the position of the sun in the sky. The later in the spring season a tornado occurs, the more likely the tornado will be located more northward. According to the American Meteorological Society, maximum tornado frequency follows the sun, the mid-latitude jet stream, and northward pushing maritime tropical air.

In other words, in early spring, expect tornadoes in the more Southern Gulf states. As spring progresses, you can expect a greater maximum frequency of tornadoes to the more Northern Central Plains states.

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Types of Tornadoes

Waterspouts

Although most people think of tornadoes as the violent rotating columns of air on land, tornadoes can also occur on water. A waterspout is a type of tornado that forms over water. These tornadoes are usually weak, but can cause damage to boats and recreational vehicles. Sometimes, these tornadoes can move onto land causing other significant damage.

Supercell Tornadoes

Tornadoes that originate from a supercell thunderstorm are usually the strongest and most significant types of tornadoes. Most all of the large hail and extremely violent tornadoes are as a result of a supercell thunderstorm. These storms often feature wall clouds and mammatus clouds.

Dust Devils

While a dust devil is not a tornado in the strictest sense of the term, it is a type of vortex. They are not caused by thunderstorms and are therefore not a true tornado. A dust devil results when the sun heats dry land surfaces forming a twisting column of air. The storms may look like a tornado, but are not. The storms are generally very weak and do not cause much damage. In Australia, a dust devil is called a willy willy. In the United States, these storms are defined as a tropical cyclone.

Gustnado

As a thunderstorm forms and dissipates, a gustnado (sometimes called a gustinado) forms from the outflow in the downdrafts from the storm. These storms are not real tornadoes either, although they are associated with thunderstorms, unlike a dust devil. The clouds are not connected to the cloud base, meaning any rotation is classified as non-tornadic.

Derechos

Derechos are thunderstorm wind events, but are not tornadoes. These storms produce strong straight-line winds and can cause damages similar to a tornado.

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How Tornadoes are Studied - Tornado Forecasts

Dorothy and the Twister Movie
This is "Dorothy" from the movie "Twister". Chris Caldwell, all rights reserved, used with permission

Tornadoes have been studied for years. One of the oldest photos of a tornado ever taken was taken in South Dakota in 1884. So although large systematic studies did not begin until the 20th century, tornadoes have been a source of fascination since ancient times.

Need proof? People are both scared and fscinated by tornadoes. Just think of the popularity of the 1996 hit movie Twister starring Bill Paxton and Helen Hunt. In an ironic twist, the farm that was filmed in the movie near the end is owned by J. Berry Harrison Sr. The farm is located in Fairfax about 120 miles northeast of Oklahoma City. According to the Associated Press, a real tornado hit the farm in May 2010 when a half dozen twisters touched down during storms in Oklahoma.

If you have ever seen the movie Twister, you will surely remember Dorothy and DOT3 which were the sensor packs used to place in front of a tornado. Although the movie was fiction, much of the science of the movie Twister was not too far off base. In fact, a similar project, appropriately called TOTO (Totable Tornado Observatory) was a relatively unsuccessful experimental venture created by the NSSL to study tornadoes. Another notable project was the original VORTEX project.

Tornado Forecasting

The forecasting of tornadoes is extremely difficult. Meteorologists must gather weather data from a variety of sources and interpret the results with a high degree of efficiency. in other words, they need to be right about the location and possibility of a tornado in order to save lives. But a fine balance needs to be struck so that too many warnings, leading to unnecessary panics, are not issued. Teams of meteorologists gather weather data through a network of mobile technologies including the mobile mesonet, Doppler-on-wheels (DOW), mobile balloon soundings, and more.

In order to understand the formation of tornadoes through data, meteorologists must understand fully how, when, and where tornadoes form. The VORTEX-2 (Verification of the Origins of Rotation in Tornadoes Experiment – 2), set for May 10 – June 15 of 2009 and 2010, was designed just for that purpose. In a 2009 experiment, a tornado intercepted in LaGrange, Wyoming on June 5, 2009 became the most intensely examined tornado in history.

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Tornado Classification – The Enhanced Fujita Scale

Virginia Tornado 4/29/08
Local residents check out the damage of vehicles at a mall after it was damaged by a tornado April 29, 2008 in the King's Fork area of Suffolk, Virginia. Three Tornadoes touched down in central and southeastern Virginia injuring at least 200 people. Photo by Alex Wong/Getty Images

Tornadoes used to be classified according to the Fujita Scale. Developed by Ted Fujita and his wife in 1971, the scale has been a famous general marker for how intense a tornado can be. Recently, the Enhanced Fujita scale was developed in order to further classify a storm based on damages.

Famous Tornadoes

There are many different tornadoes that have been infamous in the lives of those most affected by the storms. Several have had notoriety for other reasons. While not named like hurricanes, tornadoes will often get a colloquial name based on their location or damage patterns. Here are just a few:

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Tornado Statistics

10th Deadliest Tornado Year
NOAA Storm Prediction Center

There are literally millions of pieces of data about tornadoes. What I have done here is to collect a common list of tornado facts. Each fact has been reviewed for accuracy. References for these statistics are available on the last page of this document. Most statistics come directly from the NSSL and the National Weather Service.

  • How many tornadoes hit the United States each year?
  • How long does a tornado last?
  • Besides the United States, what other locations get a lot of tornadoes?
  • Can hurricanes cause tornadoes?
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Tornado Myths

Should I Open My Windows During a Tornado?

Decreasing the air pressure in a house by opening a window does nothing to decrease the DAMAGE. Even the strongest tornadoes (EF5 of the Enhanced Fujita scale) do not reduce the air pressure low enough to cause a house to "explode". Leave the windows alone. The tornado will open them for you.

Should I Stay to the South in My House?

The southwest corner of a basement is not the safest place to be in a tornado. Actually, the worst place to be is on the side from which the tornado is approaching...usually the south or southwest.

Are tornadoes the worst type of severe weather?

Tornadoes, while dangerous, are not the worst kind of severe weather. Hurricanes and flooding tend to cause more widespread damage and leave more people dead in their wake. Surprisingly, the worst type of severe weather event in terms of money is often the least expected - It is the drought. Droughts, followed closely by floods, are some of the costliest weather events in the world. Droughts are often so slow in their onset that their damage economically can be hard to quantify.

Are bridges and overpasses safe shelters in a tornado?

The short answer is NO. You are safer outside of your automobile than inside, but an overpass is also not safe. Bridges and overpasses are not safe places to be in a tornado. You are higher above the ground, in the stronger wind, and are in the path where most flying debris occurs.

Do tornadoes target mobile homes?

Tornadoes don’t hit big towns and cities

Tornadoes bounce

Anyone can be a storm chaser

Weather radar always see a tornado

Tornadoes do not hit the same place twice

References
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Where Tornadoes Form

Tornado Alley. NWS

Tornado Alley is a nickname given to the unique location in the United States where tornadoes are most likely to hit. Tornado Alley is located in the Central Plains and includes Texas, Oklahoma, Kansas, and Nebraska. Also included are Iowa, South Dakota, Minnesota, and portions of the other surrounding states. There are 5 main reasons the United States has the ideal conditions for tornado development.

  1. The central plains are a perfect flat alleyway between the Rockies and the Appalachians creating a straight shot for cold polar air to clash with moist warm air from the gulf region.
  2. Other countries are shielded by mountainous or geographic boundaries on the shorelines which prevent severe storms such as hurricanes from coming ashore easily.
  3. The size of the United States is very large, making it a large target for severe weather.
  4. The large amount of shoreline in the Atlantic and Gulf Coast regions allows for massive storms that form in the Atlantic to come ashore in coastal regions, often producing tornadoes spawned from hurricanes.
  5. The North Equatorial Current and Gulf Stream are aimed at the United States, bringing in more severe weather.
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Teaching About Tornadoes

The following lesson plans are great resources for teaching about tornadoes.

If you have any other ideas or lessons you would like to have posted, be sure to contact me. I would be happy to post your original lessons.