All About Tornadoes

What is a tornado?

A tornado is a violent rotating column of air extending from a thunderstorm to the ground. The strongest twisters can produce damage paths more than a mile wide and 50 miles long. The average forward speed of a tornado is 30 mph, but it may vary from stationary to 70 mph.

How do tornadoes form?

There are three basic ingredients necessary for development of a tornado:

1. Moist, unstable air at the surface.
2. Cold air aloft.
3. Change in wind direction and/or speed with height know as Vertical wind shear.

What happens:

Before thunderstorms develop, a change in wind direction along with an increase of wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. The area of rotation, 2-6 miles wide, now extends through much of the storm. This rotating column of air begins to descend from the base of the storm, forming a funnel. Once the funnel touches the ground it becomes a tornado.

Since the center of the funnel is a low pressure area, air rushes into the column and rises. The air is cooled as it rises and water vapor condenses to form the familiar funnel shaped cloud. As the rotating winds begin to pick up dirt and debris from the ground, the funnel will darken.

How do we classify tornadoes?

Tornadoes are classified by the damage done based on wind speed. The traditional Fujita Scale (F-scale) used to classify storms has been phased out by the National Weather Service. A new scale developed to better reflect tornado damage, called the Enhanced Fujita Scale (EF-Scale) went into effect on Feb. 1, 2007. The new EF Scale is:

EF0: Winds Less than 85 mph, Light Damage
EF1: Winds 86-110 mph, Moderate Damage
EF2: Winds 111-135 mph, Considerable Damage
EF3: Winds 136-165 mph, Severe Damage
EF4: Winds 166-200 mph, Devastating Damage
EF5: Winds 200+ mph, Incredible Damage

Where and when do tornadoes occur?

Although tornadoes occur in many parts of the world, they are found most frequently in the United States east of the Rockies during the spring and summer months. In an average year, 800 to 1,200 tornadoes are reported nationwide, resulting in dozens of deaths and more than 1,500 injuries.

There are certain areas that are favored for tornado formation at different times of the year. In the southern states and southern Plains, peak tornado season is March through May when warm, moist air from the Gulf of Mexico mixes with cooler air masses that extend into the South. In the northern states the peak tornado season is in the summer, when the warm, moist air spreads northward. In a few areas along the Gulf Coast there is a second peak season in the fall.

How can I tell if a tornado is about to occur or occurring?

In addition to keeping abreast of the latest watches and warnings issued by the National Weather Service there are certain environmental clues that Mother Nature provides us. Dark, often greenish skies, a wall cloud, large hail, and a loud roar-- similar to a freight train-- are all signs that a storm can or is producing a tornado.

Credit: FEMA and the Office of Meteorology, NOAA