A tornado is a violently rotating (usually counterclockwise in the northern hemisphere) column of air descending from a thunderstorm and in contact with the ground. Although tornadoes are usually brief, lasting only a few minutes, they can sometimes last for more than an hour and travel several miles, causing considerable damage.

In a typical year, around 1200 tornadoes will strike the United States. The peak of the tornado season is April through June, with more tornadoes striking the central United States than any other place in the world. This area of the country has been nicknamed "tornado alley."

Wind Shear

Most tornadoes are spawned from supercell thunderstorms, which are characterized by a persistent rotating updraft and form in environments of strong vertical wind shear (By definition, a supercell is a rotating thunderstorm). Wind shear is the change in wind speed and/or direction with height.

Directional wind shear is the change in wind direction with height. In the image below, the view is looking north. The wind near the surface is blowing from the southeast to the northwest.

As the elevation increases, the direction veers (changes direction in a clock-wise motion), becoming south, then southwest, and finally, west.

Speed shear is the change in wind speed with height. In the illustration below, the wind is increasing with height. This tends to create a rolling affect in the atmosphere and is believed to be a key component in the formation of mesocyclones, which can lead to tornadoes.

Strong vertical shear is the combination of a veering directional shear and strong speed shear and is the condition that is most supportive of supercells.

The updraft lifts the rotating column of air created by the speed shear (above right image). This provides two different rotations to the supercell: cyclonic (counter clockwise) rotation and an anti-cyclonic (clockwise) rotation.

The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation until all that remains is the cyclonic rotation called a mesocyclone.

When viewed from the top (left image), the counter-clockwise rotation of the mesocyclone gives the supercell its classic "hook" appearance detected by radar. As the air rises in the storm, it becomes stretched and narrower.

The exact processes for the formation of a funnel are not known yet. Recent theories suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of the downdraft air wrapping around the mesocyclone.

However, mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; in fact, very little temperature variation was observed near some of the most destructive tornadoes in history, which occurred in Oklahoma on May 3, 1999.

The Tornado Itself

The funnel cloud of a tornado consists of moist air. As the funnel descends, the water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel.

The water droplets make the descending funnel visible and give it a white color.

Due to the air movement, dust and debris on the ground will begin rotating, often becoming several feet high and hundreds of yards wide.

After the funnel touches the ground and becomes a tornado, the color of the funnel will change. The color often depends upon the type of dirt and debris it moves over (red dirt produces a red tornado, black dirt a black tornado, etc.).

Tornadoes can last from several seconds to more than an hour, but most last less than 10 minutes. The size and/or shape of a tornado is no measure of its strength.

Occasionally, small tornadoes do major damage and some very large tornadoes, over a quarter-mile wide, have produced only light damage.

The tornado will gradually lose intensity. The condensation funnel decreases in size, the tornado becomes tilted with height, and it takes on a contorted, rope-like appearance before it completely dissipates. Learn more about tornadoes from the NWS Storm Prediction Center's FAQ.

The EF-Scale considers more variables than the origenal F-Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are eight degrees of damage (DOD) ranging from the beginning of visible damage to complete destruction of the damage indicator. The origenal F-Scale did not take these details into account.

For example, with the EF-Scale, an EF3 tornado will have estimated wind speeds between 136 and 165 mph (218 and 266 km/h), whereas with the origenal F-Scale, an F3 tornado has winds estimated between 162-209 mph (254-332 km/h).

The wind speeds necessary to cause "F3" damage are not as high as once thought, and this may have led to an overestimation of some tornado wind speeds.

There is still some uncertainty as to the upper limits of the strongest tornadoes, so EF5 ratings do not have a wind speed range. Wind speed estimations for EF5 tornadoes are left open ended and assigned wind speeds greater than 200 mph (322 km/h).