Tornadoes - How Tornadoes Form

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A tornado is a violent column of rotating air made visible as they pick up debris on the ground or in the air. A tornado is usually visible, but not always. The important aspect of the definition is that the tornado or funnel cloud is in contact with the ground. The funnel clouds appear to extend downward from cumulonimbus clouds. A point to keep in mind is that this definition is not a truly accepted definition. According to Charles A. Doswell III of the Cooperative Institute for Mesoscale Meteorological Studies, there is actually no real definition of a tornado that has been universally accepted and peer-reviewed by the scientific community.

One idea that is generally accepted is that tornadoes are one of the worst, and most violent, of all the types of severe weather. Tornadoes can be considered billion-dollar storms if the storm lasts sufficiently long enough, and has enough wind speed to do maximum property damage. Fortunately, most tornadoes are short-lived, lasting for only about 5-7 minutes on average.

Tornado Rotation

Most tornadoes in the Northern Hemisphere rotate counter-clockwise or cyclonically. Only about 5% of tornadoes in the Northern Hemisphere rotate clockwise or anticyclonically. While at first it would seem this is a consequence of the Coriolis effect, tornadoes are over almost as quickly as they start. Therefore, the influence of the Coriolis effect on rotation is negligible.

So why do tornadoes tend to rotate counter-clockwise? The answer is that the storm moves in the same general direction as the low pressure systems that spawn them. Since low pressure systems rotate counterclockwise (and this is due to the Coriolis effect), tornado rotation also tends to be inherited from the low pressure systems. As winds get pushed upwards in the updraft, the prevailing direction of rotation is counterclockwise.

Tornado Locations tornado alley

. In the United States, a unique combination of factors including local geology, proximity to water, and movement of frontal systems make the United States a prime location for the formation of tornadoes. In fact, there are 5 key reasons the US is the hardest hit with tornadoes.

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What Causes Tornadoes?

 

Basics of Tornado Formation

Tornadoes are produced when two differing air masses meet. When cooler polar air masses meet warm and moist tropical air masses, the potential for severe weather is created. In tornado alley, air masses to the west are typically continental air masses meaning there is little moisture in the air. This warm, dry air meets the warm, moist air in the Central Plains creating a dryline. It is a well-known fact that tornadoes and severe thunderstorms often form along drylines.

Most tornadoes form during supercell thunderstorms from an intensely rotating updraft. It is believed that differences in vertical wind shear are contributors to the rotation of a tornado. The larger scale rotation inside the severe thunderstorm is known as a mesocyclone and a tornado is one extension of that mesocyclone. An excellent flash animation of tornado formation is available from USA Today.

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Tornado Season

Tornado season is a term used only as a guide for when most tornadoes occur in an area. In reality, a tornado can strike at any time of the year. In fact, the Super Tuesday tornado hit on February 5th and 6th, 2008.

Tornado season and the frequency of tornadoes migrates with the sun. As the seasons change, so does the position of the sun in the sky. The later in the spring season a tornado occurs, the more likely the tornado will be located more northward. According to the American Meteorological Society, maximum tornado frequency follows the sun, the mid-latitude jet stream, and northward pushing maritime tropical air.

In other words, in early spring, expect tornadoes in the more Southern Gulf states. As spring progresses, you can expect a greater maximum frequency of tornadoes to the more Northern Central Plains states.

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TYPES OF TORNADOES

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Waterspouts

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Although most people think of tornadoes as the violent rotating columns of air on land, tornadoes can also occur on water. A waterspout is a type of tornado that forms over water. These tornadoes are usually weak, but can cause damage to boats and recreational vehicles. Sometimes, these tornadoes can move onto land causing other significant damage.

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Supercell Tornadoes

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Tornadoes that originate from a supercell thunderstorm are usually the strongest and most significant types of tornadoes. Most all of the large hail and extremely violent tornadoes are as a result of a supercell thunderstorm. These storms often feature wall clouds and mammatus clouds.

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Dust Devils

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While a dust devil is not a tornado in the strictest sense of the term, it is a type of vortex. They are not caused by thunderstorms and are therefore not a true tornado. A dust devil results when the sun heats dry land surfaces forming a twisting column of air. The storms may look like a tornado, but are not. The storms are generally very weak and do not cause much damage. In Australia, a dust devil is called a willy willy. In the United States, these storms are defined as a tropical cyclone.

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Gustnado

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As a thunderstorm forms and dissipates, a gustnado (sometimes called a gustinado) forms from the outflow in the downdrafts from the storm. These storms are not real tornadoes either, although they are associated with thunderstorms, unlike a dust devil. The clouds are not connected to the cloud base, meaning any rotation is classified as non-tornadic.

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Derechos

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Derechos are thunderstorm wind events, but are not tornadoes. These storms produce strong straight-line winds and can cause damages similar to a tornado.

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