Monday, March 22, 2010


This is the second part of a two part post. For the first part see here.

Simple thunderstorms are simply clouds developing at a frontal boundary, and these are the most common, but other, more complicated types also exist.

Although some thunderstorms from exclusively on the frontal boundary, many spawn other thunderstorms miles, or even tens of miles away whenever there is warm, humid air present. This is because thunderstorms cause downdrafts, or cool air to flow down from upper altitudes. When this air hits the ground, it spreads out and flows parallel to the ground. This moving air can help to create new thunderstorms in two different ways. Either the cold air pushes the warm air up, and as we already know, a warm updraft causes thunderstorms, or, a cool wind gust can encounter another one, and they push each other up, creating additional thunderstorms.

In many cases, thunderstorms are composed of individual units, called cells. These cells form and die in minutes, but as one dies, its downdrafts produces another cell, and on. The progression of these cells tend to move because cells form in humid air at the boundary of air masses, and as the boundary moves, the area of formation moves. However, there are cells that form in a more independent way then this.

The most powerful type of cell is known (rather appropriately) as the supercell. Like a normal cell, the supercell has an updraft fueling its development: a mesocyclone. A mesocyclone is a rotating column of air that feeds the storm with the moisture and rotation it needs to be powerful. A mesocylone is typically between 1 and 5 miles in diameter and can extend up to 10 miles in to the sky. It forms when strong high altitude winds and weak ground winds blow past each other, creating a tube of air. An updraft lifts this tube, and it eventually breaks into two tubes, one spinning clockwise, the other counterclockwise. Due to the Coriolis Effect, the clockwise column is neutralized in the northern hemisphere and the counterclockwise column is strengthened, while this is reversed in the southern hemisphere.

Once the mesocyclone has formed, if there is a frontal boundary present, a colossal anvil shaped cumulonimbus cloud is formed, often stretching over 60,000 feet into the air! Warm middle layer winds feed the cyclone, while upper level winds spark fierce downdrafts. In addition, the supercell creates such a temperature difference in the air, that its downdrafts create their very own mini-frontal boundary. All this, coupled with the powerful mesocyclone at the center, brings heavy rain or hail for hours, and a supercell can travel over 300 miles before dying out.

As if this wasn't enough, sudden huge downdrafts called microbursts can also form during the duration of a supercell. Often, this downdraft pushes down another coil of air spinning parallel to the ground, again forming two columns. The counterclockwise swirl near the ground is expanded by updrafts to form one huge rotating air mass, from the ground way into the clouds. This is the tornado.

Tornadoes are immensely powerful and are actually, contrary as it is to common belief, invisible. Only when debris is picked up by the tornado does it assume a color and this depends totally upon the terrain. Tornadoes range from virtually harmless to incredibly deadly and can last from seconds to hours. Also, multiple tornadoes can come from one mesocyclone and often orbit around each other within it, assuming the form of a funnel cloud, which appears to be one tornado and can be largely hidden among clouds until it hits.

A funnel cloud hidden in the atmosphere. There is a vague suggestion of a rotation in the cloud structure, but there is no obvious tornado, like there is above.

Although a majority of tornadoes are spawned from supercells, some do not. It is possible for a weak tornado to develop from a regular thunderstorm, but this is uncommon, and a tornado in this situation is usually so weak that it is virtually undetectable. In some cases, tornadoes also from over water, forming what is known as a waterspout. As with land tornadoes, the waterspout assumes the color of water, and in most cases, appears blue. Some waterspouts have been known to develop mysteriously in fair weather, but most, known as tornadic waterspouts, form in thunderstorms. Hurricanes can be a source of tornadoes and waterspouts, when circulations form in their thunderstorm bands, and some hurricanes have caused over 100 tornadoes! Three Atlantic hurricanes have attained this: Hurricane Beulah in 1967, with about 115, Hurricane Frances in 2004, with 103, and Hurricane Ivan in 2004, with 117. Of Frances's tornadoes, 6 were considered significant, or with winds exceeding 110 mph, and Ivan had 19 significant tornadoes.

A waterspout off Florida's coast.

Also, dust devils, or swirling gusts of wind, can form in a dry environment, usually a desert. One last type of rotating mesocyclonic structure is a "fire whirl" or "steam devil". If there is a significantly heated environment, as in a fire, or heated water, the updrafts caused by the heated air can be strong enough to start a mesocyclone. The latter types of rotating mesocyclonic structures are not associated with thunderstorms, but are related since they are similar to tornadoes. They are usually weak in comparison to a real tornado.

A dust devil that formed in Nevada. Note the fair weather conditions.

Until recently, most areas used the Fujita scale to measure tornado strength. However, the U.S. thought the categories and their respective damages educated guesses at best, and the scale was changed into what is now known as the Enhanced Fujita Scale.

A comparison of the original Fujita scale introduced in 1971, and the new Enhanced Fujita Scale, which was first used in 2007. Although the Fujita scale covers a higher range of wind speeds, the Enhanced version gives more accurate damage information, and considers that anything under 65 mph is not a tornado at all, but perhaps a spinning funnel cloud that doesn't reach the ground.

The most powerful tornado recorded had peak winds around 300 mph, and wreaked havoc in Oklahoma City in 1999. This tornado also caused the second most damage of any single one, with adjusted damages of $2 billion. However, a tornado in 1896 has the greatest adjusted damage value, at a stunning $2.9 billion. In terms of width, one tornado had a damage path width of 2.5 miles in 2004. In terms of path length, the infamous Tri-State Tornado is the record holder, traveling 219 miles over three states, Missouri, Illinois, and Indiana. It also holds the record for the deadliest U.S. tornado with 695 deaths recorded. The huge F5 tornado wreaked havoc in 1925. In the world, the deadliest tornado probably belongs to the Bangladesh 1989 tornado which took an astounding 1300 lives. Finally, the largest tornado outbreak occurred in 1974, on April 3 and 4 of that year. An extremely powerful system spawned an amazing 148 tornadoes recorded and probably more weak ones going unrecorded. Among these, 23 were F4 and 6 were F5! The number of major tornadoes was simply unheard of, and this outbreak is still, after over 35 years, the largest one in history.

All of the tracks of the tornadoes in the Super Outbreak of 1974. They are numbered in order of development, from 1 to 148.

There have been many exceptional tornado oddities, most involving debris and people being picked up and placed back down unharmed. There have been many claims of this type and some have been disputed. For more information, see here.

Tornadoes are among the most violent weather events on the Earth, and they are the most deadly, there being no accurate way to predict them. Although tornado watches and warnings are put into effect when there is a risk or a sighting of a tornado, these can only be given a few minutes in advance. Climatology wise, tornadoes occur in various parts of the world. As well as the U.S. southern Canada and northern Mexico, much of Europe, South Africa, areas of Argentina, Australia, and Eastern Asia also experience tornadoes regularly.

Currently, Earth is the only celestial body on which tornadoes have been detected. There are rotating systems on other planets, such as Jupiter (the Great Red Spot and various other systems), and many of these may feature mesocyclones, but no real evidence for extraterrestial tornadoes has been collected.

Sources: The Weather Book by Jack Williams,,, National Hurricane Center

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