tornado

*Where do they come from?
Tornadoes come from the energy released in a thunderstorm. As powerful as they are, tornadoes account for only a tiny fraction of the energy in a thunderstorm. What makes them dangerous is that their energy is concentrated in a small area, perhaps only a hundred yards across. Not all tornadoes are the same, of course, and science does not yet completely understand how part of a thunderstorm's energy sometimes gets focused into something as small as a tornado.

*Where do they occur?
Whenever and wherever conditions are right, tornadoes are possible, but they are most common in the central plains of North America, east of the Rocky Mountains and west of the Appalachian Mountains. They occur mostly during the spring and summer; the tornado season comes early in the south and later in the north because spring comes later in the year as one moves northward. They usually occur during the late afternoon and early evening. However, they 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.

*What type of damage can they do?
The damage from tornadoes comes from the strong winds they contain. It is generally believed that tornadic wind speeds can be as high as 300 mph in the most violent tornadoes. Wind speeds that high can cause automobiles to become airborne, rip ordinary homes to shreds, and turn broken glass and other debris into lethal missiles. The biggest threat to living creatures (including humans) from tornadoes is from flying debris and from being tossed about in the wind. It used to be believed that the low pressure in a tornado contributed to the damage by making buildings "explode" but this is no longer believed to be true.

*How are they detected?
Today, 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 out for approaching tornadoes, so that those communities can take appropriate action in the event of a tornado. Spotter information is relayed to the National Weather Service, which operates the Doppler radars and which issues warnings (usually relayed to the public by radio and TV) for communities ahead of the storms, using all the information they can obtain from weather maps, modern weather radars, storm spotters, monitoring power line breaks, and so on.

*Can they be predicted?
Yes, but only to a limited extent. Although the process by which tornadoes form is not completely understood, scientific research has revealed that tornadoes usually form under certain types of atmospheric conditions. Those conditions can be predicted, but not perfectly. When forecasters see those conditions, they can predict that tornadoes are likely to occur. However, it is not yet possible to predict in advance exactly when and where they will develop, how strong they will be, or precisely what path they will follow. There are some "surprises" every year, when tornadoes form in situations that do not look like the right conditions in advance, but these are becoming less frequent. Once a tornado is formed and has been detected, warnings can be issued based on the path of the storm producing the tornado, but even these cannot be perfectly precise about who will or will not be struck.

*How can I keep myself safe?
The main point is to be protected from flying and falling debris. A storm shelter is your best choice, if you have one. If you have a basement, go there and get under something sturdy to shelter you from falling debris (for instance, a strong workbench, or a staircase).
If you have no basement or storm shelter, go to an interior room without windows on the first floor of your home. Bathrooms are a good choice because the plumbing reinforces the walls. Closets are also a good choice since they normally have no windows. The idea is to put as many walls between you and an approaching tornado as possible; flying debris can penetrate exterior walls.
Stay away from windows entirely! It used to be thought that opening windows would reduce a tornado's damage to a home. This is no longer considered to be good advice: leave windows alone completely and get to a safe place immediately.
If you live in a mobile home, it should be abandoned; seek shelter in a neighboring frame home or, better yet, a storm shelter. If you are in a motor vehicle in an populated area, do not try to drive away from a tornado; abandon your vehicle and seek shelter nearby.
If you are traveling in a rural area, drive away from the tornado at a right angle to its path. If caught in the open, get down in a sheltered low spot, and cover your head with your arms; hang on to something if it is available.
If you are at school or work, your school or workplace should have an approved tornado safety plan, which you should follow. If no such plan exists, complain to someone that a plan needs to be developed!! Knowing what to do in a specific situation means you are less likely to panic and do something dangerous without realizing it. Have a plan within your family about what you would do, and follow it exactly, including where you will go if you are separated.
If you hear a tornado warning, you should seek immediate shelter and not waste precious seconds to see if it is true for you; wasting a few minutes in a safe place once in awhile is better than becoming another tornado statistic!

*Should I seek shelter under a bridge over-pass during a tornado?
It is not advisable to seek shelter under a bridge. This is due to increased wind speeds that result from a phenomenon known as Bernoulli's principle, which states that air moving through a constriction or over a curved surface will undergo a pressure drop and an increase in wind speed. This translates to higher potential for injury from flying tornado debris.
To illustrate this principle, think of what happens when you constrict the flow of water through a garden hose. The water speeds up. Likewise, the air speeds up when forced through a narrow overpass.

*What is the smallest, largest, average size?
The answer to this depends on what is being measured ... the easiest way to answer this is by the size of the damage path. Also, it is important to note that the "average" can be misleading, since most tornadoes are small ... a simple average puts too much emphasis on the infrequent large events. A way around this is to use what is called the "median" to represent what is typical. The typical tornado damage path is about one or two miles, with a width of about 50 yards. The largest tornado path widths can exceed one mile, and the smallest widths can be less than 10 yards. Widths can vary considerably during a single tornado, because the size of the tornado can change considerably during its lifetime. Path lengths can vary from what is basically a single point to more than 100 miles. Note that tornado intensity (the peak wind speeds) is not necessarily related to the tornado size ... bigger is not necessarily stronger!

*How long is a tornado usually on the ground?
Detailed statistics about the time a tornado is on the ground are not available. This time can range from an instant to several hours ... what is typical is roughly 5 minutes or so.

*How fast do tornadoes move?
As with tornado duration, detailed statistics about forward speed are not available. Movement can range from virtually stationary to more than 60 miles per hour ... what is typical is roughly 10-20 miles per hour.

*How do I know if it is a tornado or just a funnel?
The definition of a tornado means that the vortex of rapidly rotating air must be in contact with the ground. This means that to be a tornado, the swirling winds must be at the surface, capable of doing damage. If you see debris (dust and other objects swirling in the winds), it is definitely a tornado, even if there is no visible funnel cloud. If you can't see debris with a funnel cloud, then it might be a tornado but you can not be certain that it is (or is not). A tornado can move over a surface with few objects to be picked up and swirled about, or you may not be able to see all the way to the surface beneath a funnel cloud because of intervening hills, trees, or buildings. All funnel clouds should be treated as if they are tornadoes, unless you can be certain that they will not touch down...and being certain about such things is difficult. Even if the funnel is not in contact with the surface when you first see it, that situation can change quite rapidly!