Severe Weather

Tornadoes

Tornadoes are rapidly rotating winds that blow around a small area of intense low pressure. A tornado's circulation is present on the ground either as a funnel-shaped cloud or as a swirling cloud of dust or and debris.

A funnel cloud is a tornado whose circulation has not reached the ground. When viewed from above, the majority of tornadoes rotate counterclockwise. A few have been seen rotating clockwise, but those are rare.

Major tornadoes usually evolve through a series of stages.

• The first stage is the dust-whirl stage, where dust swirling upward from the surface marks the tornado's circulation on the ground and a short funnel often extends downward from the thunderstorm's base.
• The organizing stage finds the tornado increasing in intensity with an overall downward extent of the funnel.
• During the tornado's mature stage, damage normally is most severe as the funnel reaches its greatest width and is almost vertical.
• The shrinking phase is characterized by an overall decrease in the funnel's width, an increase in the funnel's tilt, and a narrowing of the damage swath at the surface.
• The final, or decay stage, usually finds the tornado streched into the shape of a rope. Normally the tornado becomes greatly contorted before it finally dissipates.

Summary of Tornado characteristics

The diameter of most tornadoes is between 100 and 600 m (about 300 to 2000 ft.), although some are just a few meters wide and others have diameters exceeding 1600 m (1 mi).

Most tornadoes last only a few minutes and have an average path length of about 7 km (4 mi). There are cases when they have traveled for hundred of kilometers and have existed for many hours.

Tornadoes that form ahead of an advancing cold front tend to move from the southwest towards the northeast at speeds usually between 20 and 40 knots. However, some have been clocked at speeds greater than 70 knots.

Violent tornadoes may have winds speeds that exceed 250 knots. A violent tornado may actually have smaller whirls (suction vortices) rotating within it.

Each year, tornadoes take the lives of many people. The yearly average is about 100, although over 100 may die in a single day. The deadliest tornadoes are those that occur in families; that is, different tornadoes spawned by the same thunderstorm.

When a large number of tornadoes (tipically 6 or more) forms over a particular region, this constitutes a tornado outbreak. One of the most violent occurred on April 3 and 4, 1974. During a 16 hour period, 148 tornadoes cut through parts of 13 states, killing 307 people, injuring more than 6000, and causing an estimated $600 million in damage.

Tornado Occurrence

Tornadoes occur in many places of the world, but no country experiences more tornadoes than the United States, which averages more than 1000 annually and experienced a record 1424 tornadoes during 1998.

Although tornadoes have occurred in every state the greatest number occur in the tornado belt or tornado alley of the Central Plains, which streches from central Texas to Nebraska.

About three-fourths of all tornadoes in the United States develop from March to July. Although tornadoes have occurred at all times of the day and night, they are most frequent in the late afternoon, when the surface air is most unstable.

Tornado days per year

Significant (F2 or larger Tornado days per century

Violent (F4 or larger) Tornado days per millennium

Tornado Winds

The strong winds of a tornado can destroy buildings, uproot trees, and hurl all sorts of lethal missiles into the air. Our earlier knowledge of the furious winds of a tornado came mainly from observations of the damage done and the analysis of motion pictures.

In the late 1960s, the late Dr. T. Theodore Fujita, a noted authority on tornadoes at the University of Chicago, proposed a scale (called the Fujita scale for classifying tornadoes according to their rotational wind speed based on the damage done by the storm.

Fujita Scale for Damaging Wind

Scale	Category	Mi/Hr	Expected damage
F0	Weak	40-72	light: tree branches broken; sign boards damaged
F1		73-112	moderate: trees snapped, windows broken
F2	Strong	113-157	considerable: large trees uprooted, weak structures destroyed
F3		158-206	severe: trees leveled, cars overturned, walls removed from buildings
F4	Violent	207-260	devastating: frame houses destroyed
F5		261-276	incredible: structures the size of autos moved over 100 meters, steel reinforced structures highly damaged

Tornado Formation

A very comprehensive explanation behind the formation of tornadoes id given in the NOAA web page: tornadoes... Nature's Most Violent Storms A PREPAREDNESS GUIDE Including Safety Information for Schools U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service September 1992 (NOAA, FEMA, The American Red Cross).

Links to the Historical Tornado Data Archive hosted by the STORM PREDICTION CENTER