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Tornadoes

Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. Both have similar topography with mountains helping catch low-level moisture from over Brazil (Argentina) or from the Indian Ocean (Bangladesh). About 1,000 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades.

 A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes 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.

Tornado season usually refers to the time of year where the U.S. sees the most tornadoes. The peak “tornado season” for the southern plains -- often referred to as Tornado Alley -- is during May into early June. On the Gulf coast, it is earlier during the spring. In the northern plains and upper Midwest, tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day, but most tornadoes occur between 4-9 p.m.

• A tornado is a violently rotating column of air extending from a thunderstorm to the ground.
• Tornadoes cause an average of 70 fatalities and 1,500 injuries in the U.S. each year..
• The strongest tornadoes have rotating winds of more than 250 mph.
• Tornadoes can be one mile wide and stay on the ground over 50 miles.
• Tornadoes may appear nearly transparent until dust and debris are picked up or a cloud forms within the funnel. The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction.
• The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.
• Waterspouts are tornadoes which form over warm water. They can move onshore and cause damage to coastal areas.

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.

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 mayform as the dryline moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorableconditions exist, these thunderstorms can produce tornadoes.

Tornadoes occasionally 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 onshore.

Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year. The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado. The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."

 The dryline is a boundary separating hot, dry air to the west from warm, moist air to the east. You can see it on a weather map by looking for sharp changes in dew point temperatures. Between adjacent weather stations the differences in dew point can vary by as much as 40 degrees or more. The dryline is usually found along the western high plains. Air moving down the eastern slopes of the Rockies warms and dries as it sinks onto the plains, creating a hot, dry, cloud-free zone. During the day, it moves eastward mixing up the warm moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form - because the dryline is the "push" the air needs to start moving up! During the evening, the dryline "retreats" and drifts back to the west. The next day the cycle can start all over again, until a larger weather system pushes through and washes it away.

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, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

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 ft. mountain.

MYTH: The low pressure with a tornado causes buildings to "explode" as the tornado passes overhead.
FACT: Violent winds and debris slamminginto buildings cause 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. Leave the windows alone; instead, immediately go toa safe place.

What does a tornadic storm look like?

Forecasters and storm spotters have learned to recognize certain thunderstorm features and structure that make tornado formation more likely. Some of these are visual cues, like the rear-flank downdraft, and others are particular patterns in radar images, like the tornadic vortex signature (TVS).

VISUAL EVIDENCE

The most reliable evidence of a tornado is for someone who has been trained to recognize tornadoes to actually see one. Storm spotters report what they see to the National Weather Service and provide important information to warning forecasters who must make critical warning decisions. Storm spotters can be emergency managers or even local people with a keen interest in severe weather who have undergone formal storm spotter training in their community. Some of the features storm spotters and forecasters look for in tornadic storms include:

• Inflow Bands
  Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south. The presence of inflow bands suggests that the storm is gathering low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.
• Beaver's tail
  The beaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. It also suggests the presence of rotation.
• Wall Cloud
  A wall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is usually to the rear of the visible precipitation area. Wall clouds are about two miles in diameter and mark the area of strongest updraft in the storm.
  
  As the storm intensifies, the updraft draws in low-level air from several miles around. Some low-level air is pulled into the updraft from the rain area. This rain-cooled air is very humid; the moisture in the rain-cooled air quickly condenses below the rain-free base to form the wall cloud.
  
  A wall cloud that may produce a tornado usually exists for 10-20 minutes before a tornado appears. A wall cloud may also persistently rotate (often visibly), have strong surface winds flowing into it, and may have rapid vertical motion indicated by small cloud elements quickly rising into the rain-free base.
• Rear Flank Downdraft
  The rear flank downdraft is a downward rush of air on the back side of the storm that descends along with the tornado. The RFD looks like a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. It can also look like curtains of rain wrapping around the cloud base circulation. Eventually, the tornado and RFD will reach the ground within a few minutes of each other. The RFD causes gusty surface winds that occasionally have embedded downbursts The rear flank downdraft is the motion in the storm that causes the hook echo feature on radar.

The Fujita Tornado Scale, usually referred to as the F-Scale, classifies tornadoes based on the resulting damage. This scale was developed by Dr. T. Theodore Fujita (University of Chicago) in 1971.

F-SCALE	WINDS	TYPE OF DAMAGE	FREQUENCY
F0	40-72 mph 64-116 km/h	MINIMAL DAMAGE: Some damage to chimneys, TV antennas, roof shingles, trees, and windows.	29%
F1	73-112 mph 117-180 km/h	MODERATE DAMAGE: Automobiles overturned, carports destroyed, trees uprooted.	40%
F2	113-157 mph 181-253 km/h	MAJOR DAMAGE: Roofs blown off homes, sheds and outbuildings demolished, mobile homes overturned.	24%
F3	158-206 mph 254-332 km/h	SEVERE DAMAGE: Exterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests and farmland flattened.	6%
F4	207-260 mph 333-418 km/h	DEVASTATING DAMAGE: Few walls, if any, standing in well-built homes. Large steel and concrete missiles thrown far distances.	2%
F5	261-318 mph 419-512 km/h	INCREDIBLE DAMAGE: Homes leveled with all debris removed. Schools, motels, and other larger structures have considerable damage with exterior walls and roofs gone. Top stories demolished.	less than 1%

*** 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 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 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.

If you hear a "Tornado Warning" seek safety immediately.

Indoors:

• Abandon mobile homes — they are not safe even when tied down. Go to a designated shelter
• Go to a basement or interior room on the lowest floor (bathroom or closet without windows, under stairs). Get under a sturdy piece of furniture.
• Cover yourself with a mattress or blanket
• Put bicycle helmets on kids
• Put on sturdy shoes
• Put infants in car seats (indoors!)
• If you have time, gather prescription medications, wallet and keys.
• DO NOT open your windows!

In a vehicle:

• Leave the vehicle for sturdy shelter or drive out of the tornado’s path
• DO NOT hide under overpasses — they provide no shelter
• Last resort actions - stay in your vehicle or abandon for a roadside ditch

Outdoors:

• Find a culvert or cave.
• Find something to hang onto.
• Lie flat in a ditch.
• Cover your head.

• April 26-28, 2011, Tornado Outbreak Statistics

NOAA, National Geographic, ABC, The Weather Channel

Data compiled from The British Antarctic Study, NASA, Environment Canada, UNEP, EPA and other sources as stated and credited Researched by Charles Welch-Updated daily This Website is a project of the The Ozone Hole Inc. a 501(c)(3) Nonprofit Organization http://www.theozonehole.com

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