What is a Tornado?
How do tornadoes form?
What is a supercell thunderstorm?
What is the difference between a funnel cloud and a tornado?
What is the path length of tornadoes? How long do they last? How fast do they move?
When and where do tornadoes occur?
What causes tornadoes?
What is the Fujita Tornado Damage Scale?
What should I do to prepare myself for a tornado?
What was the deadliest tornado outbreak in the U.S.?
What was the longest path length of a tornado? What was the shortest?
What are the strongest winds in a tornado?
Should I open my windows and doors during a tornado?
Are the number of tornadoes increasing?
How are tornadoes detected?
What is a mesocyclone?
What is a wall cloud?  Why are they important to look for?

What is a tornado?

A tornado is a violently rotating column of air which descends from a thunderstorm. No other weather phenomenon can match the fury and destructive power of tornadoes. Tornadoes can be strong enough to destroy large buildings, leaving only the bare concrete foundation. In addition, they can lift 20-ton railroad cars from their tracks and they can drive straw and blades of grass into tree and telephone poles.

Back to top

How do tornadoes form?

The truth is that scientists don't fully understand how tornadoes form.  Typically, tornadoes develop several thousand feet above the earth's surface inside of a severe rotating thunderstorm. This type of storm is called a supercell thunderstorm. The spinning of these supercell thunderstorms is visible via Doppler radar.

Back to top

What is a supercell thunderstorm?

A supercell is an organized thunderstorm that contains a very strong, rotating updraft. This rotation helps to produce severe weather events such as large hail, strong downbursts, and tornadoes. Supercell storms are usually isolated from other thunderstorms because it allows them to have more energy and moisture from miles around. These storms are rare, but always a threat to life and property.

Back to top

What is the difference between a funnel cloud and a tornado?

A tornado begins as a rotating, funnel-shaped cloud extending from a thunderstorm cloud base. A funnel cloud is made visible by cloud droplets, however, in some cases it can appear to be invisible due to lack of moisture. When the funnel cloud is half-way between the cloud base and the ground, it is called a tornado. The tornado's high-speed winds rotate about a small, relatively calm center, and suck up dust and debris, making the tornado darker and more easily seen.

Back to top

What is the path length of tornadoes? How long do they last? How fast do
they move?

Tornado paths range from 100 yards to one mile wide and are rarely more than 15 miles long. They can last from several seconds to more than an hour, however, most don't exceed 10 minutes. Most tornadoes travel from the southwest to northeast with an average speed of 30 mph, but the speed has been observed to range from almost no motion to 70 mph.

Back to top

When and where do tornadoes occur?

Most tornadoes occur in the deep south and in the broad, relatively flat basin between the Rockies and the Appalachians, but no state is immune. 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 sultry Spring afternoon between 3 p.m. and 9 p.m.

Back to top

What causes tornadoes?

Tornadoes form under a certain set of weather conditions in which three very different types of air come together in a certain way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create what we call instability. A necessary ingredient for tornado formation. The change in wind speed and direction with height is known as wind shear. This wind shear is linked to the eventual development of rotation from which a tornado may form.

A third layer of hot dry air becomes established between the warm moist air at low levels and the cool dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further...making the air even more unstable. Things start to happen when a storm system aloft moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. Complex interactions between the updraft and the surrounding winds may cause the updraft to begin rotating-and a tornado is born.

The Great Plains of the Central United States are uniquely suited to bring all of these ingredients together, and so have become known as "Tornado Alley". The main factors are the Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east.

During the spring and summer months southerly winds prevail across the plains. At the origin of those south winds lie the warm waters of the Gulf of Mexico, which provide plenty of 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.

Back to top

What is the Fujita Tornado Damage Scale?

Dr. T. Theodore Fujita was a pioneer in the study of tornadoes and severe thunderstorm phenomena. In 1971, he proposed the adoption of the Fujita Tornado Damage Scale to provide estimates of tornado strength based on damage surveys. Since it is extremely difficult to make direct measurements of tornado winds, an estimate of the winds based on damage is the best way to classify them. Fujita proposed a range of intensities zero to five, with weak tornadoes being classified as "F0" and the strongest tornadoes as "F5".

Note that it is impossible to have an F6 tornado, since the Fujita Scale is a damage scale, and there is no classification for damage beyond F5 damage (incredible destruction). However, it is theoretically possible to have a tornado with winds in excess of the F5 classification (318 mph). 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.

Back to top

The Fujita Scale for measuring Tornado Speed and Damage Severity
Developed by "Dr. Tornado", T. Theodore Fujita of the University of Chicago

What should I do to prepare myself for a tornado?

Continued vigilance and quick response to tornado watches and warnings are critical, since tornadoes can strike virtually anywhere at any time. Most tornadoes are abrupt at onset, short-lived and often obscured by rain or darkness. The best way to deal with them is preparedness. Every individual and business should have a tornado emergency plan for their homes and places of work, and should learn how to protect themselves in cars, open country, and other situations that may arise.

Remember if a tornado warning is issued for your area, a tornado is imminent. Know what to do--have an emergency plan to protect yourself and those for whom you are responsible. Quick response when a tornado approaches can save many lives. There may be only seconds in which to take action.

There are 5 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

Meteorologists using the latest in computers, radar and satellite data are always monitoring the weather elements. When a high probability of severe weather exists, a tornado or severe thunderstorm watch is issued. Watches may be issued hours before any severe storm hits the area. The forecasters at the National Severe Storms Forecast Center in Kansas City, Missouri use every tool available including satellite pictures, radar reports, and numerous weather charts to predict the areas where severe thunderstorms and tornadoes are likely to occur.

Step 2: Spotters

Severe weather spotters such as amateur radio operators are constantly being trained under the SKYWARN training program. Spotting severe weather is serious business and requires specific training. However, once an observer is trained in severe weather spotting procedures, they are one of the most reliable tools of the National Weather Service meteorologist. Spotters serve as the National Weather Service's eyes in the field.

Step 3: Civil Defense and the State Police

Any information that the National Weather Service issues is relayed to individual cities and towns through state and local civil defense, and the State Police.

Step 4: The Media

The vast majority of people are reached through the cooperation of the media. A direct line between the National Weather Service and local media offices insures that severe weather information is relayed quickly and broadcast within the shortest time possible.

Step 5: The Users

Users include everyone within the severe thunderstorm or tornado warning area. We want to reach the greatest number of people possible and provide a concise, yet persuasive message of necessary action. Even if every other step in the warning system works, it does little good unless the users know what to do, and act.

Back to top

What was the deadliest tornado outbreak in the U.S.?

Most meteorologists consider the deadliest tornado outbreak in U.S. history to have occurred on March 18, 1925.  747 people were killed and 2,027 were injured in Missouri, Illinois, and Indiana when several twisters touched down on this day. The largest of these tornadoes, named the "Tri-State", took 695 lives and was classified as an F5. It moved over 215 miles of land at 60-73 mph.

Back to top

What was the longest path length of a tornado? What was the shortest?

The longest tornado path was reported to have traveled from Missouri to Indiana in 1917. It was on the ground for 7 hours and 20 minutes. Because there was no damage report filed, its difficult to know if this tornado actually stayed on the ground for the entire time, or if it was the result of several tornadoes. The shortest reported tornado was 7 feet long.

Back to top

What are the strongest winds in a tornado?

Mobile Doppler radar reported one twister on May 3, 1999 as having winds as fast as 318 mph. The winds swirling around a tornado vortex have been reported to be on the range of 280-300 mph. Scientists have found that the strongest winds occur about 300 feet above the ground. Most tornadoes, however, fail to have wind speeds in excess of 113 mph.

Back to top

Should I open my windows and doors during a tornado?

NO! Opening your windows and doors may in fact increase the damage to your house and make you susceptible to being struck by flying glass. Instead, use that time to find a safe spot under heavy furniture and away from windows. Any openings, including garage doors, that allow wind to enter a building increases the chance for damage.

Back to top

Are the number of tornadoes increasing?

The number of tornadoes that occur each year is not increasing, but the number of spotted and reported tornadoes is. The reason for this is that more people live in or travel through tornado prone areas than used to. This has led to better communication and reporting of severe weather.

Back to top

How are tornadoes detected?

In addition to the thousands of National Weather Service trained severe weather spotters, NEXRAD Doppler radars detect severe weather. These radars spot large scale rotation from which many tornadoes form. NEXRAD does not detect every tornado, but it is likely that they will provide advance warning for large twisters.

Back to top

What is a mesocyclone?

A mesocyclone is an entire column of circulation within a thunderstorm cell.  Instead of a concentrated circulation (like a tornado) the entire cloud formation swirls around a central point underneath a supercell.  This is one of the most dangerous points of a thunderstorm and can lead to a very volatile tornado.  A mesocyclone is often found in the right rear flank of a supercell (or often on the eastern, or front, flank of a High-Precipitation storm).

Back to top

What is a wall cloud?  Why are they important to look for?

A wall cloud is defined as, "A localized, persistent, often abrupt lowering from a rain-free base". Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate.

Wall clouds are important to look for because a rotating wall cloud usually develops before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion.

Back to top