Tornadoes

Why Talk About Tornadoes?

Tornadoes have been reported in every state, and though they generally occur during spring and summer, they can happen any time of the year. While tornadoes can occur at any time of the day or night, they are most likely to occur between 3:00 and 9:00 p.m. There are no areas immune to tornadoes; they have been reported in mountains and valleys, over deserts and swamps, from the Gulf Coast into Canada, in Hawaii and even Alaska. Regardless of the location or time of year, if conditions are right, a tornado can happen. Over 1,000 tornadoes are reported annually nationwide, and as our tornado detection systems improve, more are being reported each year. However, sometimes tornadoes will develop in areas in which no tornado watch or warning is in effect, so stay alert for changing weather conditions.

Over 1,000 tornadoes are reported annually nationwide, and as our tornado detection systems improve, more are being reported each year.

What Are Tornadoes, and What Causes Them?

A tornado is a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes have rotating winds of 250 miles per hour or more. They are capable of causing extreme destruction, including uprooting trees and well-made structures, and turning normally harmless objects into deadly missiles. Most tornadoes are just a few dozen yards wide and only briefly touch down, but highly destructive violent tornadoes may carve out paths over a mile wide and more than 50 miles long. Although violent tornadoes comprise only 2 percent of all tornadoes, they are responsible for nearly 70 percent of tornado-related fatalities.

Tornadoes develop from severe thunderstorms in warm, moist, unstable air along and ahead of cold fronts. Such thunderstorms also may generate large hail and damaging winds. When intense springtime storm systems produce large, persistent areas that support tornado development, major outbreaks can occur. During the late spring, tornadic thunderstorms can develop in the southern High Plains along a "dry line," the interface between warm, moist air to the east and hot, dry air to the west. From the front range of the Rocky Mountains southward into the Texas Panhandle, slope flow of unstable air can cause tornadic thunderstorms to develop. While generally smaller and not as frequent, tornadoes occurring west of the Rocky Mountains of the United States also cause damage and threaten lives annually.

Land falling tropical storms and hurricanes also generate tornadoes. Such tornadoes are most common to the right and ahead of the storm path or the storm center as it comes ashore. In 1967, Hurricane Beulah produced 148 tornadoes as it made landfall in south Texas.

While tornadoes can be highly destructive and are potentially deadly, timely precautions can save lives and reduce property damage. During active weather, stay alert of the forecast by listening to radio or television or by using a NOAA Weather Radio. Contact your local National Weather Service (NWS) office, emergency management agency, or American Red Cross chapter for more information about your risk from tornadoes.

Awareness Information

• A National Weather Service WATCH is a message indicating that conditions favor the occurrence of a certain type of hazardous weather. For example, a severe thunderstorm watch means that a severe thunderstorm is expected in the next six hours or so within an area approximately 120 to 150 miles wide and 300 to 400 miles long (36,000 to 60,000 square miles). The NWS Storm Prediction Center issues such watches. Local NWS forecast offices issue other watches (flash flood, winter weather, etc.) 12 to 36 hours in advance of a possible hazardous-weather or flooding event. Each local forecast office usually covers a state or a portion of a state.
• An NWS WARNING indicates that a hazardous event is occurring or is imminent in about 30 minutes to an hour. Local NWS forecast offices issue warnings on a county-by-county basis.

Tornadoes may appear nearly transparent until dust and debris are picked up. Stay alert for high winds even if you do not "see" a tornado.

Tornadoes often occur when it is not raining. In fact, in the Great Plains and other semiarid regions, that scenario is the rule rather than the exception. Tornadoes are associated with a powerful updraft, so rain does not fall in or next to a tornado. Very large hail, however, does fall in the immediate area of the tornado. In humid environments, rain often tends to wrap around the tornado, being pulled from the main precipitation area around the outside of the rotating updraft. The rain could make it difficult to see the tornado.

Waterspouts are weak tornadoes that form over warm water and are most common along the Gulf Coast and southeastern states. In the western United States, waterspouts occur with cold late fall or late winter storms, during a time when you least expect tornado development. Waterspouts, which are tornadoes over a body of water, occasionally move inland becoming tornadoes and causing damage and injuries.

Damage happens when wind gets inside a home through a broken window, door, or damaged roof. Keep windows closed. Houses do not explode due to air pressure differences. Stay away from windows during severe storms. Flying debris could shatter the glass and cause injury.

In the past, it was advised to go to the southwest corner for safety; however, the southwest corner of a house is no safer than any other corner. Historical information has shown that any corner on the lowest level away from windows is as safe as any other corner. If tornado winds enter the room, debris has a tendency to collect in corners. When selecting a tornado "safe place," look for a place on the lowest level and away from windows, preferably in a small room (closet or bathroom) in the center of the house. Closer walls will help provide more support to the roof, and each wall between you and the outside will provide further protection.

Folklore passed down through the generations used to advise opening windows in case of a tornado because air pressure differences would cause a house to explode. This information is not true. Air pressure differences in a tornado are not strong enough to cause a house to explode; houses are damaged by the violent winds associated with a tornado and from the debris blown at high velocities by tornado winds.

Folklore also used to advise that if you are driving and a tornado is suspected or sighted, you should turn and drive at right angles to the storm. This advice is not recommended because tornadoes do not necessarily travel in straight lines; you cannot always tell the direction the storm is coming from; the road you turn onto may curve and head into the storm, rather than away from it; and there may be more than one tornado associated with a strong storm system, but you may not see it because visibility is diminished by heavy rain and wind-blown debris. The safest thing to do is go to a nearby sturdy building and go inside to an area on the lowest level, without windows. If a sturdy building is not available, then get out of the vehicle and lay down in a low spot on the ground not subject to flooding, protecting the head and neck.