A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes generate wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long.

Tornadoes occur in many parts of the world, but are most common in the United States east of the Rocky Mountains during the spring and summer months.

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.

Tornadoes may also accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes ashore.

How Tornadoes Form
Before thunderstorms develop, a change in wind direction and an increase in 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.	An area of rotation 2 to 6 miles wide now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
	The lower cloud base in the center of the photograph above identifies an area of rotation known as a wall cloud. This area is often nearly rain-free, and most tornadoes come out of this region of the thunderstorm.

Types of Tornadoes

69% of all tornadoes are weak, with winds of less than 110 mph. They tend to last less than 10 minutes, and account for less than 5% of tornado deaths.

29% of all tornadoes are strong, with winds of 110 to 205 mph. Lasting 20 minutes or longer, they account for nearly 30% of all tornado deaths.

Violent tornadoes have wind speeds in excess of 205 mph. Although only 2% of all tornadoes fall into this category, they account for 70% of all tornado deaths. These tornadoes can stay on the ground for one hour or more, may be a mile or more in diameter, and may leave a damage path dozens of miles long. However, a violent tornado may be relatively small and stay on the ground for only a short period of time. In addition, it is not uncommon for a tornado to vary in intensity during its "lifetime," doing relatively little damage at some points along its path only to completely destroy structures at other points.

Waterspouts are weak tornadoes that form over water. They are most common along the Gulf Coast and in the southeastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when tornadoes are least expected to form. Waterspouts can move inland and become tornadoes, often with devastating results.

The Fujita Scale is used to rate the intensity of a tornado by examing the damage caused by the tornado after it has passed over a man-made structure.

Tornado Myths

MYTH: Areas near rivers, lakes, and mountains are safe from tornadoes.
FACT: No place is safe from tornadoes. In the late 1980's, a tornado swept through Yellowstone National Park leaving a path of destruction up and down a 10,000-foot mountain.

MYTH: The low pressure with a tornado causes buildings to "explode" as the tornado passes overhead.
FACT: Violent winds and debris slamming into buildings cause the most structural damage.

MYTH: Windows should be opened before a tornado approaches to equalize pressure and minimize damage.
FACT: Opening windows allows damaging winds to enter the structure and does absolutely nothing to equalize pressure. In addition, the time you spend opening windows may cost you your life, and will do nothing to save to your house.

Questions or comments about this page?

National Oceanic and Atmospheric Administration. www.nssl.noaa.gov/edu/safety/tornadoguide.html
The Tornado Project. www.tornadoproject.com

This page was last updated on 02/10/2010.