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
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.
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.
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
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.
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.
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."
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.
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.
air within the thunderstorm updraft tilts the rotating air from
horizontal to vertical.
area of rotation, 2-6 miles wide, now extends through much of the
storm. Most strong and violent tornadoes form within this area of
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.
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.
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
What does a tornadic storm
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).
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
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.
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.
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.
TYPE OF DAMAGE
DAMAGE: Some damage to chimneys, TV antennas, roof shingles,
trees, and windows.
DAMAGE: Automobiles overturned, carports destroyed, trees
DAMAGE: Roofs blown off homes, sheds and outbuildings demolished,
mobile homes overturned.
DAMAGE: Exterior walls and roofs blown off homes. Metal buildings
collapsed or are severely damaged. Forests and farmland flattened.
DAMAGE: Few walls, if any, standing in well-built homes. Large
steel and concrete missiles thrown far distances.
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.
DERIVED EF SCALE
Fastest 1/4-mile (mph)
3 Second Gust (mph)
3 Second Gust (mph)
3 Second Gust (mph)
*** 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.
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
Put on sturdy shoes
Put infants in car seats
If you have time, gather
prescription medications, wallet and keys.
DO NOT open your
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
Geographic, ABC, The Weather Channel
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