It's Raining Fish and Spiders (4 page)

Biggest Tornado Ever!

The biggest known tornado is the Hallam, Nebraska, F4 tornado of May 22, 2004. Its peak width was nearly two and a half miles. This is probably close to the maximum size for tornadoes.

Strongest Tornado Ever!

We don't know how fast winds can go inside a tornado. Tornado wind speeds have only been directly recorded in weaker storms, because strong and violent tornadoes destroy weather instruments. Mobile Doppler radars (which we will discuss later), such as the Oklahoma University “Doppler on Wheels,” have remotely sensed tornado wind speeds above ground level as high as about 302 mph (on May 3, 1999, near Bridge Creek, Oklahoma; that tornado caused F5 damage—this was before the introduction of the EF scale). These are the highest winds ever found on the Earth's surface, even faster than hurricane winds. But ground-level wind speeds in the most violent tornadoes have never been directly measured.

The Enhanced Fujita Damage Scale

The Enhanced Fujita Scale, or EF Scale, is the scale for rating the strength of tornadoes in the United States, estimated by the damage they cause. The original “F” scale was invented in 1971 by University of Chicago physicist Theodore Fujita, the first and foremost tornado scientist.

The new, enhanced Fujita (EF) scale went into operation on February 1, 2007. The scale was revised to reflect current, better examinations of tornado damage surveys, and to align wind speeds more closely with associated storm damage. The new scale takes into account more types of structures as well as vegetation, expands degrees of damage, and better accounts for variables such as the differences in construction of buildings and structures.

The EF scale is not intended to assess wind speed as much as it is to derive what the wind speeds were in a particular tornado as determined by the type of damage that was left behind by the storm. The new scale takes into account quality of construction and standardizes different kinds of structures. The wind speeds on the original scale were deemed too high by meteorologists and engineers, and engineering studies indicated that slower winds than initially estimated caused the respective degrees of damage.

The following graphic shows the damage assessment from the Enhanced Fujita Scale and what the corresponding tornado might look like. Images are courtesy of the National Severe Storms Lab.

National Oceanic and Atmospheric Administration/Department of Commerce

National Oceanic and Atmospheric Administration/Department of Commerce

National Oceanic and Atmospheric Administration/Department of Commerce

What Is a Derecho?

Thunderstorms not only destroy lives and property by spawning tornadoes, they contain another type of wind that can be just as destructive. A prolonged windstorm that produces straight-line winds rather than or in addition to a tornado is called a
derecho
. Derecho (pronounced deh-RAY-cho) is the Spanish word for “direct” or “straight ahead” and describes the winds that can be produced by thunderstorms. Straight-line winds that come from a thunderstorm can be as fast as 100 miles per hour.

Super cell: King of Thunderstorms

Common thunderstorms are born and rain themselves out within a very short period of time, say an hour or less. Super cells are the most dangerous because they can develop the strongest tornadoes. They can travel more than 300 miles across the country as well and cause heavy, flooding rain; large, damaging hail; and straight-line (or derecho) winds in excess of 100 mph. Super cells need humid, unstable air, and some lifting force (like the jet stream) to move the air upward. Super cells can last as long as 5 to 8 hours.

Frank Picini

National Oceanic and Atmospheric Administration/Department of Commerce

A rotating column of rising air, called a
mesocyclone
, is the key to the long life of a super cell. It gathers in the warm, humid air near the ground and mixes it with the cool, dry air from above. The mesocyclone supplies the spin that strong tornadoes require.

Most Damaging Tornadoes (F5 and EF5) in the U.S. since 1950

This is a list of tornadoes since 1950 which the National Weather Service has rated F5 (before 2007) or EF5 (2007 on—the most intense damage category on the Fujita and Enhanced Fujita damage scales. The tornadoes are numbered in the order they happened since 1950; so the numbers run from the bottom up.