Nature's Attack

by SN creations

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Tornadoes

Tornadoes are vertical funnels of rapidly spinning air. Their winds may top 250 miles (400 kilometers) an hour and can clear-cut a pathway a mile (1.6 kilometers) wide and 50 miles (80 kilometers) long.

Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year. "Tornado Alley," a region that includes eastern South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.

A tornado forms when changes in wind speed and direction create a horizontal spinning effect within a storm cell. This effect is then tipped vertical by rising air moving up through the thunderclouds.

The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in late afternoon, when thunderstorms are common, and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.

Tornadoes' distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm's moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.

Tornadoes move at speeds of about 10 to 20 miles (16 to 32 kilometers) per hour, although they've been clocked in bursts up to 70 miles (113 kilometers) per hour. Most don't get very far though. They rarely travel more than about six miles (ten kilometers) in their short lifetimes.

Tornadoes are classified as weak, strong, or violent storms. Violent tornadoes comprise only about two percent of all tornadoes, but they cause 70 percent of all tornado deaths and may last an hour or more.

People, cars, and even buildings may be hurled aloft by tornado-force winds—or simply blown away. Most injuries and deaths are caused by flying debris.

Tornado forecasters can't provide the same kind of warning that hurricane watchers can, but they can do enough to save lives. Today the average warning time for a tornado alert is 13 minutes. Tornadoes can also be identified by warning signs that include a dark, greenish sky, large hail, and a powerful train-like roar.

 

Causes :

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 may form 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 favorable conditions 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.


Colorado Tornado (David Blanchard)


Dr. Joseph Golden (NOAA)


Tornado Variations

  • Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is most common along the front range of the Rocky Mountains, the Plains, and the Western States.
  • Tornadoes may appear nearly transparent until dust and debris are picked up.
  • Occasionally, two or more tornadoes may occur at the same time.

Waterspout

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

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


Woodward OK (Ron Przybylinski)

 A lower cloud base in the center of the photograph identifies an area of rotation known as a rotating wall cloud. This area is often nearly rain-free. Note rain in the background.


Woodward OK (Ron Przybylinski)

 Moments later a strong tornado develops in this area. Softball-size hail and damaging "straight-line" winds also occurred with this storm.

 

Tornadoes Take Many Shapes and Sizes

Weak Tornadoes

  • 69% of all tornadoes
  • Less than 5% of tornado deaths
  • Lifetime 1-10+ minutes
  • Winds less than 110 mph

Weak tornado

Strong Tornadoes

  • 29% of all tornadoes
  • Nearly 30% of all tornado deaths
  • May last 20 minutes or longer
  • Winds 110-205 mph

strong tornado

Violent Tornadoes

  • Only 2% of all tornadoes
  • 70% of all tornado deaths
  • Lifetime can exceed 1 hour
  • Lifetime can exceed 1 hour

violent tornado

a tornado forms within a severe thunderstorm (NOAA)

 

Tornadoes form in unusually violent thunderstorms when there is sufficient (1) instability, and (2) wind shear present in the lower atmosphere. Instability refers to warmer and more humid than usual conditions in the lower atmosphere, and possibly cooler than usual conditions in the upper atmosphere. Wind shear in this case refers to the wind direction changing, and the wind speed increasing, with height. An example would be a southerly wind of 15 mph at the surface, changing to a southwesterly or westerly wind of 50 mph at 5,000 feet altitude.

 

This kind of wind shear and instability is usually exists only ahead of a cold front and low pressure system. The intense spinning of a tornado is partly the result of the updrafts and downdrafts in the thunderstorm (caused by the unstable air) interacting with the wind shear, causing a tilting of the wind shear to form and upright tornado vortex. Helping the process along, cyclonically flowing air around the cyclone, already slowly spinning in a counter-clockwise direction (in the Northern Hemisphere), converges inward toward the thunderstorm, causing it to spin faster. This is the same process that causes an ice skater to spin faster when she pulls her arms in toward her body.

 

Other processes can enhance the chances for tornado formation. For instance, dry air in the middle atmosphere can be rapidly cooled by rain in the thunderstorm, strengthening the downdrafts that are needed for tornado formation. Notice that, in virtually every picture you see of a tornado, the tornado has formed on the boundary between dark clouds (the storm updraft region) and bright clouds (the downdraft region), evidence for the importance of updrafts and downdrafts to tornado formation. Also, an isolated strong thunderstorm just ahead of a squall line that then merges with the squall line often becomes tornadic; isolated storms or more likely to form tornadoes than squall lines, since an isolated storm can form a more symmetric flow pattern around it and also have less competition for unstable air "fuel" than if it were part of a solid line of storms.

 

Because both instability and wind shear are necessary for tornado information, sometimes weak tornadoes can occur when the wind shear conditions are strong, but the atmosphere is not very unstable. This sometimes happens in southern California in the winter, for instance, when a strong low pressure system comes ashore. Similarly, weak tornadoes can occur when the airmass is very unstable, but has little wind shear. For instance, Florida has many weaker tornadoes of this variety. Of course, the most violent tornadoes occur when both strong instability and strong wind shear are present.

 

Interesting facts:

TORNADO COUNTRY The United States experiences more tornadoes than anyplace else in the world. A few other countries that experience especially violent tornadoes include India, Bangladesh, and Argentina. It is estimated that over 95% of all tornadoes spin cyclonically (counter-clockwise). Oddly, the very first recorded film of a tornado was of one that was spinning in the other direction(!)

 

More About Tornadoes :

Interactive: Forces of Nature
Article: Chasing Tornadoes
Article: Tornado Safety Tips
Photo Gallery: Tornadoes
Photo Gallery: Tornado Destruction
Video: Tornado Montage
Video: Tornado Turnpike