Tornados

Tornado

A tornado near Seymour, Texas.

A tornado near Seymour, Texas.

A tornado is defined by the Glossary of Meteorology as "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud..."[6]

Condensation funnel

A tornado is not necessarily visible; however, the intense low pressure caused by the fast wind speeds (see Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually causes water vapor in the air to condense into a visible condensation funnel.[4] Strictly, the term tornado refers to the vortex of wind, not the condensation cloud.

A funnel cloud is a visible condensation funnel with no associated strong winds at the surface. Not all funnel clouds evolve into a tornado. However, many tornadoes are preceded by a funnel cloud as the mesocyclonic rotation descends toward the ground. Most tornadoes produce strong winds at the surface while the visible funnel is still above the ground, so it is difficult to tell the difference between a funnel cloud and a tornado from a distance.[3]

The 1997 Miami, Florida tornado

The 1997 Miami, Florida tornado

Tornado family

Occasionally a single storm may produce multiple tornadoes and mesocyclones. This process is known as cyclic tornadogenesis. Tornadoes produced from the same storm are referred to as a tornado family. Sometimes multiple tornadoes from distinct mesocyclones occur simultaneously.[7]

Tornado outbreak

Occasionally, several tornadoes are spawned from the same large-scale storm system. While there is no single agreed upon definition, multiple tornadoes spawned by the same storm system with no break in activity is considered a tornado outbreak. A period of several successive days with tornado outbreaks in the same general area (spawned by multiple weather systems) is a tornado outbreak sequence, occasionally called an extended tornado outbreak.[6][8][9]

[edit] Etymology

The word "tornado" is an altered form of the Spanish word tronada, which means "thunderstorm". This in turn was taken from the Latin tonare, meaning "to thunder". It most likely reached its present form through a combination of the Spanish tronada and tornar ("to turn"); however, this may be a folk etymology.[10][11] Tornadoes are also commonly referred to as twisters.[12]

[edit] Types of tornadoes

A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.

A multiple-vortex tornado outside of Dallas, Texas on April 2, 1957.

A waterspout near the Florida Keys.

A waterspout near the Florida Keys.

A landspout near North Platte, Nebraska on May 22, 2004.

A landspout near North Platte, Nebraska on May 22, 2004.

Dust devil in Johnsonville, South Carolina.

Dust devil in Johnsonville, South Carolina.

A cold-air vortex extending from a generic cumulus cloud.

A cold-air vortex extending from a generic cumulus cloud.

[edit] 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, however it is very often observed in intense tornadoes.

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.[3]

Waterspout

A waterspout is officially defined by the U.S. National Weather Service simply as a tornado over water. Amongst researchers, however, they are typically divided into two categories: "fair weather" waterspouts, and tornadic waterspouts.

* "Fair weather" waterspouts are the less-severe (but far more common) variety, and are similar in dynamics to dust devils and landspouts.[13] They form from the bases of cumulus congestus (also called "convective cumulus") cloud towers in tropical and semitropical waters.[13] They have relatively weak winds (F0 on the Fujita scale), smooth laminar walls, and typically travel very slowly, if at all (since the cloud they are attached to is being formed by convective action instead of the interaction between colliding fronts).[13] They occur more commonly in the Florida Keys than anywhere on Earth.[14]

* Tornadic waterspouts are more literally "tornadoes over water", and form the same way as tornadoes. A tornado which spawns in the traditional manner on land and later crosses into a body of water would also be considered a tornadic waterspout. Since they form from severe thunderstorms and have the capacity to be far more intense, faster, and longer-lived than their fair weather cousins, they are considered to be far more dangerous.

Landspout

A landspout is an unofficial term for a tornado not associated with a mesocyclone. The name stems from their characterization as essentially a "fair weather waterspout on land". They share most of the characteristics with their water-based brethren, 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, owing to their differing mechanics from true mesoform tornadoes. Though usually weaker than classic tornadoes, they still possess strong winds and can cause serious damage.[3][15]

[edit] Tornado-like circulations