Tornadoes form in unusually violent thunderstorms when there is sufficient instability and wind shear present in the lower atmosphere. Instability refers to when you have warmer and more humid conditions in the lower atmosphere and colder air above it in the mid and upper atmosphere. Wind shear in this care refers to the wind direction changing with speed and height. An example would be a southerly wind at 15 mph at the surface, changing to a southwesterly or westerly wind of 50 mph at 5,000 feet.
This kind of wind shear and instability usually exists only ahead of a cold front and low pressure system. The intense spinning of a tornado is partly the result of the updrafts and downdrafts in the thunderstorm (caused by the unstable air) interacting with the wind shear, causing a tilting of the wind shear to form an upright tornado vortex. Helping the process along, cyclonically flowing air around the cyclone, already slowly spinning in a counter-clockwise direction (in the Northern Hemisphere where we're located), converges inward toward the thunderstorm, causing it to spin faster. This is the same process that causes an ice skater to spin when she or he pulls their arms in toward their body.
Other processes can enhance the chances for tornado formation. For instance, dry air in the middle atmosphere can be rapidly cooled by rain in the thunderstorm, strengthening the downdrafts that are needed for tornado formation. Notice that, in virtually every picture you see of a tornado (like the one above), the tornado has formed on the boundary between dark clouds (the storm updraft region) and the bright clouds (the downdraft region), evidence for the importance of updrafts and downdrafts to tornado formation. Also, an isolated strong thunderstorm just ahead of a squall line that then merges with the squall line often becomes tornadic; isolated storms are more likely to form tornadoes than squall lines, since an isolated storm can form a more symmetric flow pattern around it and also has less competition for unstable air or "fuel" than if it were part of a solid line of storms.
The Unites State experiences more tornadoes than anyplace else in the world. A few other countries that experience especially violent tornadoes include India, Bangladesh and Argentina. It is estimated that more than 95% of all tornadoes spin cyclonically (counter clockwise). Oddly, the very first recorded film of a tornado was of one that was spinning the other direction! Explain that one?
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