Tornadoes invoke both fear and fascination in their victims, but how are they formed? There are a few ways; however, the largest and most violent are spawned by supercell thunderstorms. Here is a breakdown of tornado formation in supercells:
Stage 1: A rising column of warm, moist air called an updraft enters a thunderstorm and begins to rotate, spun by storm winds. Called a mesocyclone, this rotating column of warm air gives the supercell stability and a long life.
Stage 2: Extremely strong vertical winds, termed vertical wind shear by meteorologists, create a horizontally rotating funnel of air which begins to move faster as it shrinks in diameter. The narrowing column of air stretches downward and creates a wall cloud below the supercell.
Stage 3: Strong updrafts tilt the rotating air vertically. This is a narrow, rapidly spinning column of air is a funnel cloud; the first visible sign of the tornado.
Stage 4: The funnel cloud stretches to the ground and touches down and officially becomes a tornado. At its center is a point of extremely low pressure; because winds always move from an area of high pressure to one of low pressure, the tornado continues to power itself until the pressure equalizes and the funnel decays.
Supercells are the most common cause of tornadoes, but there are two others: non-supercell and dryline. Non-supercell formation occurs when winds from two thunderstorms collide and begin to rotate, causing a less powerful funnel than supercells. Dryline formation happens when thunderstorms form on the boundaries between a front of dry air and one of humid air. These latter are most common during spring and summer in the southern Great Plains.
Tornadoes in the United States happen most frequently in the area between the Rocky and Appalachian mountain ranges from late spring to early summer. The average tornado is 50 yards wide, has wind speeds of 100 miles per hour, and stays on the ground for no more than a few miles. By contrast, the most violent tornadoes can be over a mile in diameter with winds that exceed 300 miles per hour, remaining on the ground for almost 50 miles.
Though natural disasters such as hurricanes, floods, and earthquakes frequently take a higher toll on property and human lives, tornadoes are considered more severe. In other words, they affect a much smaller area and last only moments, but can kill more people and cause more damage than a hurricane that lasts for days. Simply stated, tornadoes are the most destructive natural force in existence.
Sources:
http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tornado/formation.html
http://www.weather.com/encyclopedia/tornado/form.html
http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761554892
http://orca.rsmas.miami.edu/classes/msc118/tornadoes.pdf
