Tornado
A violently rotating column of air
(vortex), hanging from a cumulonimbus cloud, with circulation that touches the
surface of the earth
Supercell Storm
•
Severe
weather occurs as strong downbursts…large hail…occasional flash floods and weak
to violent tornadoes
•
Severe
event almost always occur near the updraft interface typically in the rear
(southwest) storm flank. Some of the supercells have the interface on the front
of the southeast flank
•
High
predictability of occurrence of severe events once a storm is identified as a
supercell
•
Extremely
dangerous to public
Extremely
dangerous to aviation
Tornado Fact
•
Tornados
can occur almost anywhere in the world
•
Duration:
a few minutes
•
Diameter
(Avg.): 0.4 km
•
Length
of path (Avg.): 6 km
•
Funnel
can travel from 0 mph up to ~70 mph, usually travels at 30 mph
•
99%
of all tornados in Northern Hemisphere rotate counterclockwise
•
Texas
is #1 for frequency of tornados per year
•
Between
1950 and 1995 Texas had 5,722 recorded tornados
•
Risk
of death in a tornado in Texas: 1 in 1,054,267
Texas cost
per person per year for tornados: $3.94
Tornado Oddities
•
Tornados
are reported to routinely carry objects many miles and have:
•
sucked the frogs out of a pond and dropped
them on a town
•
carried
a necktie rack with 10 ties attached 40 miles
•
carried
a flour sack 110 miles from a mill
•
Tornados also drive objects into other objects
and have:
•
Driven
splinters into an iron fire hydrant
•
Driven
straw and grass into telephone poles
Tornado Wind Speed
In 1971, Dr. Fujita developed a way of measuring the winds of
a tornado. He reasoned that there was a link between wind speed and the damage caused by a tornado. There are 6
categories of tornados (F0 – F5)
F0 Category
•
(Weak) winds (40-72) mph , little damage
•
Damage:
tree branches snapped, chimneys toppled, signs torn down
F3 Category
•
(Strong)
winds: (158-206) mph, severe damage
•
Damage:
most trees uprooted, trains overturned, roofs torn off, walls demolished
F5 Category
•
(Violent)
winds: (261- 319) mph, incredible damage; rare
Damage: bark
peeled off trees, houses lifted off foundations, vehicles travel greater than
100 m through the air
Tornado
Forecasting
Meteorologists
who predict tornado development analyze the current atmospheric conditions such
as: air temp., barometric pressure, the locations of fronts, wind velocities,
convection, etc. Probably the most useful tool a meteorologist can use to
identify tornados is radar, specifically Doppler radar (WSR-88D)
The Life
cycle of a Tornado
•
The
“Life Cycle” of a tornado consists of four distinct stages.
•
These
stages were first determined during the Union City, OK tornado of 1973
•
Most
tornadic events are difficult to classify and may not exhibit stages that are
obvious to the observer
•
Sometimes
events become unclear: one tornado weakens another appears, or single event?
Tornados do
not “skip” – gaps in damage path may indicate a temporary weakening in
intensity
How tornadoes
form?
Most tornadoes form from
thunderstorms. You need warm, moist air from the Gulf Of Mexico and cool,
dry air from Canada. When these two air masses meet, they create instability in
the atmosphere. 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 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.
What are some other factors for
tornadoes to form?
Several conditions are required for the development of tornadoes and the
thunderstorm clouds with which most tornadoes are associated. Abundant low
level moisture is necessary to contribute to the development of a thunderstorm,
and a "trigger" (perhaps a cold front or other low level zone of
converging winds) is needed to lift the moist air aloft. Once the air begins
to rise and becomes saturated, it will continue rising to great heights to
produce a thunderstorm cloud, if the atmosphere is unstable. An unstable
atmosphere is one where the temperature decreases rapidly with height.
Atmospheric instability can also occur when dry air overlays moist air near the
earth's surface. Finally, tornadoes usually form in areas where winds at all
levels of the atmosphere are not only strong, but also turn with height in a
clockwise or veering direction.
What do tornadoes look like?
Tornadoes can appear as a traditional funnel shape, or in a slender rope-like
form. Some have a churning, smoky look to them, and other contain
"multiple vortices", which are small, individual tornadoes rotating
around a common center. Even others may be nearly invisible, with only swirling
dust or deribs at ground levels as the only indication of the tornado's
presence.
