A tornado is defined as a violently
rotating column of air extending from a
thunderstorm to the ground. The most violent
tornadoes generate wind speeds of 250 mph or
more. Damage paths can be in excess of one mile
wide and 50 miles long.
Tornadoes occur in many parts
of the world, but are most common in the United
States east of the Rocky Mountains during the
spring and summer months.
Thunderstorms develop in warm,
moist air in advance of eastward-moving cold
fronts. 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
Tornadoes may also 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
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.
within the thunderstorm updraft tilts the
rotating air from horizontal to vertical.
An area of
rotation 2 to 6 miles wide now extends
through much of the storm. Most strong
and violent tornadoes form within this
area of strong rotation.
The lower cloud base in the center of the
photograph above identifies an area of rotation
known as a wall cloud. This area is often nearly
rain-free, and most tornadoes come out of this
region of the thunderstorm.
Types of Tornadoes
69% of all tornadoes are weak, with winds of
less than 110 mph. They tend to last less than 10
minutes, and account for less than 5% of tornado
29% of all tornadoes are strong, with winds of
110 to 205 mph. Lasting 20 minutes or longer,
they account for nearly 30% of all tornado
Violent tornadoes have wind speeds in excess
of 205 mph. Although only 2% of all tornadoes
fall into this category, they account for 70% of
all tornado deaths. These tornadoes can stay on
the ground for one hour or more, may be a mile or
more in diameter, and may leave a damage path
dozens of miles long. However, a violent tornado
may be relatively small and stay on the ground
for only a short period of time. In addition, it
is not uncommon for a tornado to vary in
intensity during its "lifetime," doing
relatively little damage at some points along its
path only to completely destroy structures at
Waterspouts are weak tornadoes that form over
water. They are most common along the Gulf Coast
and in the southeastern states. In the western
United States, they occur with cold late fall or
late winter storms, during a time when tornadoes
are least expected to form. Waterspouts can move
inland and become tornadoes, often with
The Fujita Scale is
used to rate the intensity of a tornado by
examing the damage caused by the tornado after it
has passed over a man-made structure.
||Type of Damage
||Some damage to
chimneys; breaks branches off trees;
pushes over shallow-rooted trees; damages
||Peels surface off
roofs; mobile homes pushed off
foundations or overturned; moving autors
pushed off the roads; attached garages
may be destroyed.
||Roofs torn off frame
houses; mobile homes demolished; boxcars
pushed over; large trees snapped or
uprooted; light object missles generated.
||Roof and some walls
torn off well constructed houses; trains
overturned; most trees in forest
houses leveled; structures with weak
foundations blown off some distance; cars
thrown and large missiles generated.
||Strong frame houses
lifted off foundations and carried
considerable distances; automobile-sized
missiles fly through the air in excess of
100 meters; trees debarked; steel
reinforced concrete structures badly
||These winds are very
unlikely. Missiles, such as cars and
refrigerators would do serious secondary
damage that could not be directly
identified as F6 damage. If this level is
ever achieved, evidence for it might only
be found in some manner of ground swirl
pattern, for it may never be identifiable
through engineering studies.
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-foot mountain.
MYTH: The low pressure within a tornado causes
buildings to "explode" as the tornado
FACT: Violent winds and debris slamming into
buildings cause the most structural damage.
MYTH: Windows should be opened before a
tornado approaches to equalize pressure and
FACT: Opening windows allows damaging winds to
enter the structure and does absolutely nothing
to equalize pressure. In addition, the time you
spend opening windows may cost you your life, and
will do nothing to save to your house.
National Oceanic and Atmospheric
The Tornado Project www.tornadoproject.com
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