Tornadoes are characterized by rapidly rotating columns of air hanging from cumulonimbus clouds.



These are a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. The clouds can form alone, in clusters, or along a cold front in a squall line.

Causes

Formed by an instability in the atmosphere.
Usually form during thunderstorms.
A downward flow of cold air from clouds meets a rising flow of warm air from the ground.
When the conditions are just right, a tornado will start.



You'll notice the spiraling effect of the air from a the warm air rising to meet the cold air that is descending.



The following map (below) shows the frequency in which tornadoes occur in the United States.

Tornado alley is more like a blob. A big, nasty blob. If you live in this area you are likely to see 6 or more tornadoes per year via the news or in person.

Most tornadoes have wind speeds between 40 mph (64 km/h) and 110 mph (177 km/h), are approximately 250 feet (75 m) across, and travel a few miles (several kilometers) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).



- Referenced from Wikipedia

True tornadoes

Multiple vortex tornado
A multiple vortex tornado is a type of tornado in which two or more columns of spinning air rotate around a common center. Multivortex structure can occur in almost any circulation, but is very often observed in intense tornadoes. These vortexes often create small areas of heavier damage along the main tornado path.




Satellite tornado
A satellite tornado is a term for a weaker tornado which forms very near a large, strong tornado contained within the same mesocyclone. The satellite tornado may appear to "orbit" the larger tornado (hence the name), giving the appearance of one, large multi-vortex tornado. However, a satellite tornado is a distinct funnel, and is much smaller than the main funnel.

Waterspout
A waterspout is defined by the National Weather Service simply as a tornado over water. However, researchers typically distinguish "fair weather" waterspouts from tornadic waterspouts.








* Fair weather waterspouts are less severe but far more common, and are similar in dynamics to dust devils and landspouts. They form at the bases of cumulus congestus cloud towers in tropical and semitropical waters. They have relatively weak winds, smooth laminar walls, and typically travel very slowly, if at all. They occur most commonly in the Florida Keys and in the northern Adriatic Sea.


* Tornadic waterspouts are more literally "tornadoes over water". They can form over water like mesocyclonic tornadoes, or be a land tornado which crosses onto water. Since they form from severe thunderstorms and can be far more intense, faster, and longer-lived than fair weather waterspouts, they are considered far more dangerous.

Landspout
Landspout (officially known as a dust-tube tornado) is a tornado not associated with a mesocyclone. The name stems from their characterization as essentially a "fair weather waterspout on land". Waterspouts and landspouts share many defining characteristics, including relative weakness, short lifespan, and a small, smooth condensation funnel which often does not reach the ground. Landspouts also create a distinctively laminar cloud of dust when they make contact with the ground, due to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they still produce strong winds and may cause serious damage.

Tornado-like circulations

Gustnado
A gustnado (gust front tornado) is a small, vertical swirl associated with a gust front or downburst. Because they are technically not associated with the cloud base, there is some debate as to whether or not gustnadoes are actually tornadoes. They are formed when fast moving cold, dry outflow air from a thunderstorm is blown through a mass of stationary, warm, moist air near the outflow boundary, resulting in a "rolling" effect (often exemplified through a roll cloud). If low level wind shear is strong enough, the rotation can be turned horizontally (or diagonally) and make contact with the ground. The result is a gustnado. They usually cause small areas of heavier rotational wind damage among areas of straight-line wind damage. It is also worth noting that since they are absent of any Coriolis influence from a mesocyclone, they seem to be alternately cyclonic and anticyclonic without preference.


Dust devil
A dust devil resembles a tornado in that it is a vertical swirling column of air. However, they form under clear skies and are rarely as strong as even the weakest tornadoes. They form when a strong convective updraft is formed near the ground on a hot day. If there is enough low level wind shear, the column of hot, rising air can develop a small cyclonic motion that can be seen near the ground. They are not considered tornadoes because they form during fair weather and are not associated with any actual cloud. However, they can, on occasion, result in major damage, especially in arid areas.


Fire whirl
Tornado-like circulations occasionally occur near large, intense wildfires and are called fire whirls. They are not considered tornadoes except in the rare case where they connect to a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as strong as tornadoes associated with thunderstorms. However, they can produce significant damage.




Steam devil
A steam devil is a term describing a rotating updraft that involves steam or smoke. A steam devil is very rare, but they mainly form from smoke emitting from a power plant smokestack. Hot springs and deserts may also be suitable locations for a steam devil to form. There have also been reports of cold air steam devils as well.

- From Wikipedia

Prevention and practice before the storm:

Make sure you have a plan for you and your family

Keep an emergency supply of water, a charged flashlight, a battery powered radio, and any needed medicine or other supplies available in your safety area.

In a house with a basement:
+ Avoid windows
+ Know where heavy objects are in the house (refrigerators, pianos, etc) and directly above you on the above level. The floor may be weakened and the heavy object could fall through and onto you.
+ Find a sturdy table to crawl under or mattress and cover yourself with it.

In a house without a basement:
+ Avoid windows
+ Go to the lowest floor, small center room (like a bathroom or closet), under a stairwell, or in an interior hallway.

For more helpful advice, especially for other scenarios, please go to Government Tornado Safety