A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of up to 300 mph. They can destroy large buildings, uproot trees and hurl vehicles hundreds of yards. They can also drive straw into trees. Damage paths can be in excess of one mile wide to 50 miles long.
In an average year, 1000 tornadoes are reported nationwide. Of the 1000 tornadoes that occur each year, about 2% of them are rated EF4 or EF5. That means that there are as many as 20 devastating tornadoes each year. It is possible that meteorologists have underestimated the number of violent tornadoes that occur each year. Tornadoes are rated only by the damage they do to man-made structures. Therefore, if a tornado doesn't hit a structure of some kind, we cannot estimate its strength. Also, a tornado varies in strength during its lifetime and could be its strongest while between areas of houses or other buildings.
Below is video of the strong 1 mile-wide EF5 tornado that moved through Tuscaloosa, Alabama on April 27th, 2011.
Before a tornado reaches the ground, its called a funnel cloud. A funnel cloud is a rotating cone-shaped column of air extending downward from the base of a thunderstorm, but not touching the ground. When it reaches the ground it is then called a tornado.
Most tornadoes form from thunderstorms. You need warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere. A change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
It is not fully understood about how exactly tornadoes form, grow and die. Tornado researchers are still trying to solve the tornado puzzle, but for every piece that seems to fit they often uncover new pieces that need to be studied.
They can happen at any time of the year and at any time of the day. In the southern states, peak tornado season is from March through May. Peak times for tornadoes in the northern states are during the summer. A few southern states have a second peak time for tornado outbreaks in the fall. They are most likely to occur between 3 p.m. and 9 p.m. Tornadoes have been known to occur in every state in the United States, on any day of the year, and at any hour. They also occur in many other parts of the world, including Australia, Europe, Africa, Asia, and South America.
The geography of the central part of the United States, known as the Great Plains, is suited to bring all of the ingredients together to forms tornadoes. More than 500 tornadoes typically occur in this area every year and is why it is commonly known as "Tornado Alley".
Most tornadoes are associated with a wall cloud. A wall cloud is an abrupt lowering of a rain-free cumulonimbus base into a low-hanging accessory cloud. A wall cloud is usually situated in the southwest portion of the storm. A rotating wall cloud usually develops before tornadoes or funnel clouds.
A supercell thunderstorm is a long-lived thunderstorm whose updrafts and downdrafts are in near balance. These storms have the greatest tendency to produce tornadoes that stay on the ground for long periods of time. Supercell thunderstorms can produce violent tornadoes with winds exceeding 200 mph. A mesocyclone is a rotating vortex of air within a supercell thunderstorm. Though, mesocyclones do not always produce tornadoes.
Tornadoes often produce hail. Hail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted higher and higher into the sky until they freeze into ice. Once they become heavy, they will start to fall. If the smaller hailstones get caught in the updraft again, they will get more water on them and get lifted higher in the sky and get bigger. Once they get lifted again, they freeze and fall. This happens over and over again until the hailstone is too heavy and then falls to the ground. According to the National Weather Service, the largest hailstone is 8 inches in diameter and weights approximately 2 pounds. It fell in Vivian, South Dakota on July 23, 2010.
Tornadoes can also form over water. These are called waterspouts. They are most common along the Gulf Coast and are usually weak. Waterspouts can sometimes move inland, becoming tornadoes causing damage and injuries. A landspout is a very weak tornado that is not associated with a wall cloud or a mesocyclone. It is the land equivalent of a waterspout.
Gustnadoes can also be mistaken for tornadoes. A gustnado is a short-lived, relatively weak whirlwind that forms along a gust front. A gust front is the surge of very gusty winds at the leading edge of a thunderstorm's outflow of air. Gustnadoes are not tornadoes. They do not connect with any cloud-base rotation. But because gustnadoes often have a spinning dust cloud at ground level, they are sometimes wrongly reported as tornadoes. Gustnadoes can do minor damage.
The development of Doppler radar has made it possible, under certain circumstances, to detect a tornado's winds with a radar. However, human beings remain an important part of the system to detect tornadoes, because not all tornadoes occur in situations where the radar can "see" them. Ordinary citizen volunteers make up what is called the SKYWARN network of storm spotters, who work with their local communities to watch for approaching tornadoes, so those communities can take appropriate action in the event of a tornado. Spotter information is relayed to the National Weather Service, which operates the national Doppler radar network and which issues warnings to the public by radio, TV, and NOAA Weather Radio, using information obtained from weather maps, weather radars, and local storm spotters.
Fujita Scale of Tornado Intensity
BEFORE A TORNADO: Have a
disaster plan. Make sure everyone knows where to go in case a tornado
threatens. Make sure you know which county or parish you live in. Prepare an
emergency kit for your home. Have enough food and water for at least 3 days.