Tornadoes start from super cell thunderstorms that have very special characteristics. They have a cyclonic, usually clockwise turning, updraft which results in the storm growing a vertically. According to the National Weather Service, there are two types of super cells, low precipitation and high precipitation. The low precipitation super cell is the one that people observe the funnel cloud hitting the ground but there is little or no rain. The high precipitation super cell is where there are large amounts of rain and hail; most times people don’t even observe the tornado because it is hidden by the precipitation. Both types have very strong vertical wind shear.
When a thunderstorm becomes a tornado it will have cyclonic and anti-cyclonic rotations happening at the same time. This is what creates the hook of a tornado. Although you would think that air temperature differences is a factor that causes tornadoes this has not been proved and in fact, on May 3, 1999 Oklahoma had a very destructive tornado and there were no extreme temperature variations observed in the air masses. The funnel cloud of a tornado is made visible because dirt and dust rise as the wind spins and it adheres to the water droplets in the cloud. The funnel cloud color will depend on the color of the dirt in the area where it moves. If the dirt is red the funnel cloud will appear red, if the dirt is black the funnel cloud will appear black. It is interesting to note that the size of the funnel cloud does not indicate the strength and damage potential of a tornado. Large width tornadoes can do very little damage while small width tornadoes can do excessive damage.
The EF Scale (Enhanced Fujita Scale) has been used since 1971 to assign tornado ratings. An EF0 is a weak tornado with wind speeds of 65 to 86 mph (10537 kph) and considered Gale force winds. An EF3 is a strong tornado with wind speeds of 136 to 165 mph (218-266 kph) and the winds are considered severe. EF5 is the highest class for tornadoes and are classed as violent storms with winds exceeding 200 mph (322 kph) and are considered incredible storms. EF5 storms are the most destructive of the group.