Tornado
Nature's Most Violent Storm
Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the U.S. east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries.
A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long.
What causes tornadoes?
Thunderstorms develop in warm, moist air most often in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Occasionally, large outbreaks of tornadoes occur along strong frontal systems that form in the Central States and move east. Several states may be affected by numerous severe thunderstorms and tornadoes.
Tornadoes occasionally accompany tropical storms and hurricanes that move over land. They are most common to the right and ahead of the path of the storm center as it comes ashore. Before thunderstorms develop, a change in wind direction and an ../increase in wind speed with ../increasing height create an invisible, horizontal spinning effect in the lower atmosphere.
Rising air within the thunderstorm updraft then tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
Tornadoes take many shapes and sizes:
- Weak Tornadoes - Account for 69% of all tornadoes and less than 5% of the deaths. Their lifetime is usually less than 10 minutes with winds less than 110 mph.
- Strong Tornadoes - Account for 29% of all tornadoes and nearly 29% of the deaths. They last 20 minutes or longer with winds of 110-205 mph.
- Violent Tornadoes - Account for only 2% of all tornadoes and 70% of the deaths. They can last over one hour with winds greater than 205 mph.
- Waterspouts - Weak tornadoes that form over warm water and are most common along the Gulf Coast. Waterspouts occasionally move inland becoming tornadoes and causing damage and injuries.
Know the yerms!
Tornado Watch - Issued when conditions are favorable for tornadoes to develop. You should monitor local radio or TV stations to stay informed and to know immediately if a Tornado Warning is issued.
Tornado Warning - Issued when a tornado has been sighted in the area. Take shelter immediately! Radar technology known as DOPPLER has the ability to detect wind directions that may indicate a tornado and a tornado warning may be issued before one is actually sighted by the public.
NOTE: In this area, tornadoes form quickly and seldom last very long. Warning for these types of tornadoes is often not possible. Remain alert for signs of an approaching tornado. Many people have stated that a tornado "sounds like a train". Tornadoes are usually not detected until they have picked up visible dust and debris.
Tornado Safety Brochure (PDF)
What to do if a warning is issued
In a building, move to a pre-determined shelter such as a basement. If underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Stay away from windows. Get under something sturdy such as a heavy table. If you can, cover yourself with a blanket or sleeping bag. In a high-rise building, use the stairs to go to the designated shelter area or an interior room on the lowest floor possible. Get out of automobiles. Do not try to outrun a tornado in your car. If caught outside, lie flat in a nearby ditch or depression in the ground. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned.
Weather radios are the main source of emergency weather notification for the county. Stay informed, in tune and in touch - a NOAA weather radio is your best option for instant local severe weather notifications. Read more. (link to http://www.weather.gov/nwr/) Buy one. ( link to http://www.google.com/products?q=weather+radio&btnG=Search+Products)
Tornado safety and preparedness
It is important to remain alert to signs of an approaching tornado and seek shelter if threatening conditions exist. Look for environmental clues ../including a dark sky, large hail or a loud roar.
If a warning is issued, move to a pre-designated shelter such as a basement; stay away from windows; get out of automobiles and lie flat in a ditch or depression; do not try to outrun a tornado in your car.
The high-band FM frequencies commonly referred to as the "weather band" carry NOAA Weather Radio broadcast from National Weather Service Forecast offices. Special NOAA Weather Radio receivers, available at most electronics stores for about the cost of a pair of shoes, can be set to turn on only if a severe weather warning is sent out from an NWS office. A special frequency tone is picked up by the radio, the tone turns the radio on and sounds a brief alarm, followed by the warning information. NOAA Weather Radios are becoming standard equipment in more and more schools, hospitals, nursing homes, churches and synagogues and other public gathering places in tornado alley and throughout the country.
Fujita Scale
IMPORTANT NOTE ABOUT ENHANCED F-SCALE WINDS: The Enhanced F-scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of eight levels of damage to many indicators. These estimates vary with height and exposure. The three second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed.