Tornadoes are violently rotating columns of air that are produced from thunderstorms. They are among the most destructive types of storms.
What is a Tornado?
A tornado is a rapidly rotating column of air that is formed beneath a cumulonimbus cloud in a thunderstorm. To be classified as a tornado, the vortex of air must make contact with the ground.
While the size, speed and other factors vary widely between tornadoes, here are some average tornado facts:
- Diameter 500-2000 feet (150-600 meters)
- Speed 30 miles (45 km) per hour
- Length of path 6 miles (10 km) long
Tornadoes can happen at any time of the year, but they occur most often between April and June, and are least likely to occur during December and January.
How Tornadoes Form
Exactly what causes a tornado to form is not completely understood, but conditions that produce tornadoes include strong updrafts and heavy winds. Less than 1% of thunderstorms produce tornadoes, and the most severe tornadoes are formed from gigantic thunderstorms known as supercells.
Tornadoes are generally preceded by the formation of a mesocyclone, a large horizontal cylinder of spinning air between 2 and 6 miles (3-10 km) across. Mesocyclones are created when there is a difference in wind speed between the wind closer to the ground and the wind higher up in the atmosphere. This difference in wind speed causes a condition called wind shear, and produces a horizontal rolling motion of moving air. Strong updrafts from the thunderstorm can then tilt the rolling cylinder of air up into a vertical position and create a funnel cloud.
The air pressure in the center of the tornado is lower that that of the surrounding air. Since air naturally moves from areas of high pressure toward areas of low pressure, this causes the air surrounding the storm to be pulled in toward it, adding to the intensity of the spinning vortex.
Tornado Intensity and Damage
The intensity of tornadoes is categorized by the Fujita Intensity Scale, also known as the F-scale. The scale ranges from F0, with a wind speed of less than 72 miles (116 km) per hour and light damage, up to F5, with a wind speed exceeding 260 miles (419 km) per hour and incredible damage.
Of the approximately 800 tornadoes that occur each year in the United States, over 60% of them are relatively weak and short-lived, lasting only a few minutes and classified as F0 or F1. Violent tornadoes classified as F4 and F5 account for less than 2% of the total number of tornadoes, although they are responsible for 70% of tornado-related fatalities.
The most destructive tornado ever recorded in the US was the Tri-State tornado, which struck on March 18, 1925. Originating in southeastern Missouri, it traveled 219 miles (352 km), moving across southern Illinois and finally ending up in Indiana. It resulted in 695 deaths and 2027 injured, and caused severe damage, nearly completely destroying a number of small towns.
Edward J. Tarbuck and Frederick K. Lutgens, Earth Science, 11th ed. Prentice Hall, 2006.
Image souce: http://commons.wikimedia.org/wiki/File:Union_City_Oklahoma_Tornado_%28mature%29.jpg